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Technical Round Materials-JAVA-Free Download

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What makes J2EE suitable for distributed multitiered Applications?
- The J2EE platform uses a multitiered distributed application model. Application logic is divided into components according to function, and the various application components that make up a J2EE application are installed on different machines depending on the tier in the multitiered J2EE environment to which the application component belongs. The J2EE application parts are:

· Client-tier components run on the client machine.

· Web-tier components run on the J2EE server.

· Business-tier components run on the J2EE server.

· Enterprise information system (EIS)-tier software runs on the EIS server.

What is J2EE? - J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered, web-based applications.

What are the components of J2EE application?
- A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:

Application clients and applets are client components.

Java Servlet and JavaServer Pages technology components are web components.

Enterprise JavaBeans components (enterprise beans) are business components.

Resource adapter components provided by EIS and tool vendors.

What do Enterprise JavaBeans components contain? - Enterprise JavaBeans components contains Business code, which is logic
that solves or meets the needs of a particular business domain such as banking, retail, or finance, is handled by enterprise beans running in the business tier. All the business code is contained inside an Enterprise Bean which receives data from client programs, processes it (if necessary), and sends it to the enterprise information system tier for storage. An enterprise bean also retrieves data from storage, processes it (if necessary), and sends it back to the client program.

Is J2EE application only a web-based? - No, It depends on type of application that client wants. A J2EE application can be web-based or non-web-based. if an application client executes on the client machine, it is a non-web-based J2EE application. The J2EE application can provide a way for users to handle tasks such as J2EE system or application administration. It typically has a graphical user interface created from Swing or AWT APIs, or a command-line interface. When user request, it can open an HTTP connection to establish communication with a servlet running in the web tier.

Are JavaBeans J2EE components? - No. JavaBeans components are not considered J2EE components by the J2EE specification. They are written to manage the data flow between an application client or applet and components running on the J2EE server or between server components and a database. JavaBeans components written for the J2EE platform have instance variables and get and set methods for accessing the data in the instance variables. JavaBeans components used in this way are typically simple in design and implementation, but should conform to the naming and design conventions outlined in the JavaBeans component architecture.

Is HTML page a web component? - No. Static HTML pages and applets are bundled with web components during application assembly, but are not considered web components by the J2EE specification. Even the server-side utility classes are not considered web components, either.

What can be considered as a web component? - J2EE Web components can be either servlets or JSP pages. Servlets are Java programming language classes that dynamically process requests and construct responses. JSP pages are text-based documents that execute as servlets but allow a more natural approach to creating static content.

What is the container? - Containers are the interface between a component and the low-level platform specific functionality that supports the component. Before a Web, enterprise bean, or application client component can be executed, it must be assembled into a J2EE application and deployed into its container.

What are container services? - A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.

What is the web container? - Servlet and JSP containers are collectively referred to as Web containers. It manages the execution of JSP page and servlet components for J2EE applications. Web components and their container run on the J2EE server.

What is Enterprise JavaBeans (EJB) container? - It manages the execution of enterprise beans for J2EE applications.
Enterprise beans and their container run on the J2EE server.

What is Applet container? - IManages the execution of applets. Consists of a Web browser and Java Plugin running on the client together.

How do we package J2EE components? - J2EE components are packaged separately and bundled into a J2EE application for deployment. Each component, its related files such as GIF and HTML files or server-side utility classes, and a deployment descriptor are assembled into a module and added to the J2EE application. A J2EE application is composed of one or more enterprise bean,Web, or application client component modules. The final enterprise solution can use one J2EE application or be made up of two or more J2EE applications, depending on design requirements. A J2EE application and each of its modules has its own deployment descriptor. A deployment descriptor is an XML document with an .xml extension that describes a component’s deployment settings.

What is a thin client? - A thin client is a lightweight interface to the application that does not have such operations like query databases, execute complex business rules, or connect to legacy applications.

What are types of J2EE clients? - Following are the types of J2EE clients:

  • Applets

· Application clients

· Java Web Start-enabled rich clients, powered by Java Web Start technology.

· Wireless clients, based on Mobile Information Device Profile (MIDP) technology.

What is deployment descriptor? - A deployment descriptor is an Extensible Markup Language (XML) text-based file with an .xml extension that describes a component’s deployment settings. A J2EE application and each of its modules has its own deployment descriptor. For example, an enterprise bean module deployment descriptor declares transaction attributes and security authorizations
for an enterprise bean. Because deployment descriptor information is declarative, it can be changed without modifying the bean source code. At run time, the J2EE server reads the deployment descriptor and acts upon the component accordingly.

What is the EAR file? - An EAR file is a standard JAR file with an .ear extension, named from Enterprise ARchive file. A J2EE application with all of its modules is delivered in EAR file.

What is JTA and JTS? - JTA is the abbreviation for the Java Transaction API. JTS is the abbreviation for the Jave Transaction Service. JTA provides a standard interface and allows you to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK implements the transaction manager with JTS. But your code doesn’t call the JTS methods directly. Instead, it invokes the JTA methods, which then call the lower-level JTS routines. Therefore, JTA is a high level transaction interface that your application uses to control transaction. and JTS is a low level transaction interface and ejb uses behind the scenes (client code doesn’t directly interact with JTS. It is based on object transaction service(OTS) which is part of CORBA.

What is JAXP? - JAXP stands for Java API for XML. XML is a language for representing and describing text-based data which can be read and handled by any program or tool that uses XML APIs. It provides standard services to determine the type of an arbitrary piece of data, encapsulate access to it, discover the operations available on it, and create the appropriate JavaBeans component to perform those operations.

What is J2EE Connector? - The J2EE Connector API is used by J2EE tools vendors and system integrators to create resource adapters that support access to enterprise information systems that can be plugged into any J2EE product. Each type of database or EIS has a different resource adapter. Note: A resource adapter is a software component that allows J2EE application components to access and interact with the underlying resource manager. Because a resource adapter is specific to its resource manager, there is typically a different resource adapter for each type of database or enterprise information system.

What is JAAP? - The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. It is a standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.

What is Java Naming and Directory Service? - The JNDI provides naming and directory functionality. It provides applications with methods for performing standard directory operations, such as associating attributes with objects and searching for objects using their attributes. Using JNDI, a J2EE application can store and retrieve any type of named Java object. Because JNDI is independent of any specific implementations, applications can use JNDI to access multiple naming and directory services, including existing naming and
directory services such as LDAP, NDS, DNS, and NIS.

What is Struts? - A Web page development framework. Struts combines Java Servlets, Java Server Pages, custom tags, and message resources into a unified framework. It is a cooperative, synergistic platform, suitable for development teams, independent developers, and everyone between.

How is the MVC design pattern used in Struts framework? - In the MVC design pattern, application flow is mediated by a central Controller. The Controller delegates requests to an appropriate handler. The handlers are tied to a Model, and each handler acts as an adapter between the request and the Model. The Model represents, or encapsulates, an application’s business logic or state. Control is usually then forwarded back through the Controller to the appropriate View. The forwarding can be determined by consulting a set of mappings, usually loaded from a database or configuration file. This provides a loose coupling between the View and Model, which can make an application significantly easier to create and maintain. Controller: Servlet controller which supplied by Struts itself; View: what you can see on the screen, a JSP page and presentation components; Model: System state and a business logic JavaBeans.

*Q1. How could Java classes direct program messages to the system console, but error messages, say to a file?


A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);


*Q2. What's the difference between an interface and an abstract class?


A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.


*Q3. Why would you use a synchronized block vs. synchronized method?


A. Synchronized blocks place locks for shorter periods than synchronized methods.


*Q4. Explain the usage of the keyword transient?


A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).



*Q5. How can you force garbage collection?


A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.


*Q6. How do you know if an explicit object casting is needed?


A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.


*Q7. What's the difference between the methods sleep() and wait()


A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.


*Q8. Can you write a Java class that could be used both as an applet as well as an application?


A. Yes. Add a main() method to the applet.


*Q9. What's the difference between constructors and other methods?


A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.


*Q10. Can you call one constructor from another if a class has multiple constructors


A. Yes. Use this() syntax.


*Q11. Explain the usage of Java packages.


A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.


*Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?


A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee


*Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?


A.There's no difference, Sun Microsystems just re-branded this version.


*Q14. What would you use to compare two String variables - the operator == or the method equals()?


A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.


*Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?


A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.


*Q16. Can an inner class declared inside of a method access local variables of this method?


A. It's possible if these variables are final.


*Q17. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}

A. A single ampersand here would lead to a NullPointerException.


*Q18. What's the main difference between a Vector and an ArrayList


A. Java Vector class is internally synchronized and ArrayList is not.



*Q19. When should the method invokeLater()be used?


A. This method is used to ensure that Swing components are updated through the event-dispatching thread.


*Q20. How can a subclass call a method or a constructor defined in a superclass?


A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor

For senior-level developers:


**Q21. What's the difference between a queue and a stack?


A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule


**Q22. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?


A. Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.


**Q23. What comes to mind when you hear about a young generation in Java?


A. Garbage collection.


**Q24. What comes to mind when someone mentions a shallow copy in Java?


A. Object cloning.


**Q25. If you're overriding the method equals() of an object, which other method you might also consider?


A. hashCode()


**Q26. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:
ArrayList or LinkedList?


A. ArrayList


**Q27. How would you make a copy of an entire Java object with its state?


A. Have this class implement Cloneable interface and call its method clone().

**Q28. How can you minimize the need of garbage collection and make the memory use more effective?


A. Use object pooling and weak object references.

**Q29. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?


A. If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.


*Q30. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?


A. You do not need to specify any access level, and Java will use a default package access level.

Java Basics

1.The Java interpreter is used for the execution of the source code.

True

False

Ans: a.

2) On successful compilation a file with the class extension is created.

a) True

b) False

Ans: a.

3) The Java source code can be created in a Notepad editor.

a) True

b) False

Ans: a.

4) The Java Program is enclosed in a class definition.

a) True

b) False

Ans: a.

5) What declarations are required for every Java application?

Ans: A class and the main( ) method declarations.

6) What are the two parts in executing a Java program and their purposes?

Ans: Two parts in executing a Java program are:

Java Compiler and Java Interpreter.

The Java Compiler is used for compilation and the Java Interpreter is used for execution of the application.

7) What are the three OOPs principles and define them?

Ans : Encapsulation, Inheritance and Polymorphism are the three OOPs

Principles.

Encapsulation:

Is the Mechanism that binds together code and the data it manipulates, and keeps both safe from outside interference and misuse.

Inheritance:

Is the process by which one object acquires the properties of another object.

Polymorphism:

Is a feature that allows one interface to be used for a general class of actions.

8) What is a compilation unit?

Ans : Java source code file.

9) What output is displayed as the result of executing the following statement?

System.out.println("// Looks like a comment.");

// Looks like a comment

The statement results in a compilation error

Looks like a comment

No output is displayed

Ans : a.

10) In order for a source code file, containing the public class Test, to successfully compile, which of the following must be true?

It must have a package statement

It must be named Test.java

It must import java.lang

It must declare a public class named Test

Ans : b

11) What are identifiers and what is naming convention?

Ans : Identifiers are used for class names, method names and variable names. An identifier may be any descriptive sequence of upper case & lower case letters,numbers or underscore or dollar sign and must not begin with numbers.

12) What is the return type of program’s main( ) method?

Ans : void

13) What is the argument type of program’s main( ) method?

Ans : string array.

14) Which characters are as first characters of an identifier?

Ans : A – Z, a – z, _ ,$

15) What are different comments?

Ans : 1) // -- single line comment

2) /* --

*/ multiple line comment

3) /** --

*/ documentation

16) What is the difference between constructor method and method?

Ans : Constructor will be automatically invoked when an object is created. Whereas method has to be call explicitly.

17) What is the use of bin and lib in JDK?

Ans : Bin contains all tools such as javac, applet viewer, awt tool etc., whereas Lib

contains all packages and variables.

Data types,variables and Arrays

1) What is meant by variable?

Ans: Variables are locations in memory that can hold values. Before assigning any value to a variable, it must be declared.

2) What are the kinds of variables in Java? What are their uses?

Ans: Java has three kinds of variables namely, the instance variable, the local variable and the class variable.

Local variables are used inside blocks as counters or in methods as temporary variables and are used to store information needed by a single method.

Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.

Class variables are global to a class and to all the instances of the class and are useful for communicating between different objects of all the same class or keeping track of global states.

3) How are the variables declared?

Ans: Variables can be declared anywhere in the method definition and can be initialized during their declaration.They are commonly declared before usage at the beginning of the definition.

Variables with the same data type can be declared together. Local variables must be given a value before usage.

4) What are variable types?

Ans: Variable types can be any data type that java supports, which includes the eight primitive data types, the name of a class or interface and an array.

5) How do you assign values to variables?

Ans: Values are assigned to variables using the assignment operator =.

6) What is a literal? How many types of literals are there?

Ans: A literal represents a value of a certain type where the type describes how that value behaves.

There are different types of literals namely number literals, character literals,

boolean literals, string literals,etc.

7) What is an array?

Ans: An array is an object that stores a list of items.

8) How do you declare an array?

Ans: Array variable indicates the type of object that the array holds.

Ex: int arr[];

9) Java supports multidimensional arrays.

a)True

b)False

Ans: a.

10) An array of arrays can be created.

a)True

b)False

Ans: a.

11) What is a string?

Ans: A combination of characters is called as string.

12) Strings are instances of the class String.

a)True

b)False

Ans: a.

13) When a string literal is used in the program, Java automatically creates instances of the string class.

a)True

b)False

Ans: a.

14) Which operator is to create and concatenate string?

Ans: Addition operator(+).

15) Which of the following declare an array of string objects?

String[ ] s;

String [ ]s:

String[ s]:

String s[ ]:

Ans : a, b and d

16) What is the value of a[3] as the result of the following array declaration?

1

2

3

4

Ans : d

17) Which of the following are primitive types?

byte

String

integer

Float

Ans : a.

18) What is the range of the char type?

0 to 216

0 to 215

0 to 216-1

0 to 215-1

Ans. d

19) What are primitive data types?

Ans : byte, short, int, long

float, double

boolean

char

20) What are default values of different primitive types?

Ans : int - 0

short - 0

byte - 0

long - 0 l

float - 0.0 f

double - 0.0 d

boolean - false

char - null

21) Converting of primitive types to objects can be explicitly.

a)True

b)False

Ans: b.

22) How do we change the values of the elements of the array?

Ans : The array subscript expression can be used to change the values of the elements of the array.

23) What is final varaible?

Ans : If a variable is declared as final variable, then you can not change its value. It becomes constant.

24) What is static variable?

Ans : Static variables are shared by all instances of a class.

Operators

1) What are operators and what are the various types of operators available in Java?

Ans: Operators are special symbols used in expressions.

The following are the types of operators:

Arithmetic operators,

Assignment operators,

Increment & Decrement operators,

Logical operators,

Biwise operators,

Comparison/Relational operators and

Conditional operators

2) The ++ operator is used for incrementing and the -- operator is used for

decrementing.

a)True

b)False

Ans: a.

3) Comparison/Logical operators are used for testing and magnitude.

a)True

b)False

Ans: a.

4) Character literals are stored as unicode characters.

a)True

b)False

Ans: a.

5) What are the Logical operators?

Ans: OR(|), AND(&), XOR(^) AND NOT(~).

6) What is the % operator?

Ans : % operator is the modulo operator or reminder operator. It returns the reminder of dividing the first operand by second operand.

7) What is the value of 111 % 13?

3

5

7

9

Ans : c.

8) Is &&= a valid operator?

Ans : No.

9) Can a double value be cast to a byte?

Ans : Yes

10) Can a byte object be cast to a double value ?

Ans : No. An object cannot be cast to a primitive value.

11) What are order of precedence and associativity?

Ans : Order of precedence the order in which operators are evaluated in expressions.

Associativity determines whether an expression is evaluated left-right or right-left.

12) Which Java operator is right associativity?

Ans : = operator.

13) What is the difference between prefix and postfix of -- and ++ operators?

Ans : The prefix form returns the increment or decrement operation and returns the value of the increment or decrement operation.

The postfix form returns the current value of all of the expression and then

performs the increment or decrement operation on that value.

14) What is the result of expression 5.45 + "3,2"?

The double value 8.6

The string ""8.6"

The long value 8.

The String "5.453.2"

Ans : d

15) What are the values of x and y ?

x = 5; y = ++x;

Ans : x = 6; y = 6

16) What are the values of x and z?

x = 5; z = x++;

Ans : x = 6; z = 5

Control Statements

1) What are the programming constructs?

Ans: a) Sequential

b) Selection -- if and switch statements

c) Iteration -- for loop, while loop and do-while loop

2) class conditional {

public static void main(String args[]) {

int i = 20;

int j = 55;

int z = 0;

z = i < j ? i : j; // ternary operator

System.out.println("The value assigned is " + z);

}

}

What is output of the above program?

Ans: The value assigned is 20

3) The switch statement does not require a break.

a)True

b)False

Ans: b.

4) The conditional operator is otherwise known as the ternary operator.

a)True

b)False

Ans: a.

5) The while loop repeats a set of code while the condition is false.

a)True

b)False

Ans: b.

6) The do-while loop repeats a set of code atleast once before the condition is tested.

a)True

b)False

Ans: a.

7) What are difference between break and continue?

Ans: The break keyword halts the execution of the current loop and forces control out of the loop.

The continue is similar to break, except that instead of halting the execution of the loop, it starts the next iteration.

8) The for loop repeats a set of statements a certain number of times until a condition is matched.

a)True

b)False

Ans: a.

9) Can a for statement loop indefintely?

Ans : Yes.

10) What is the difference between while statement and a do statement/

Ans : A while statement checks at the beginning of a loop to see whether the next loop iteration should occur.

A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Introduction to Classes and Methods

1) Which is used to get the value of the instance variables?

Ans: Dot notation.

2) The new operator creates a single instance named class and returns a

reference to that object.

a)True

b)False

Ans: a.

3) A class is a template for multiple objects with similar features.

a)True

b)False

Ans: a.

4) What is mean by garbage collection?

Ans: When an object is no longer referred to by any variable, Java automatically

reclaims memory used by that object. This is known as garbage collection.

5) What are methods and how are they defined?

Ans: Methods are functions that operate on instances of classes in which they are defined.Objects can communicate with each other using methods and can call methods in other classes.

Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method.

A method's signature is a combination of the first three parts mentioned above.

6) What is calling method?

Ans: Calling methods are similar to calling or referring to an instance variable. These methods are accessed using dot notation.

Ex: obj.methodname(param1,param2)

7) Which method is used to determine the class of an object?

Ans: getClass( ) method can be used to find out what class the belongs to. This class is defined in the object class and is available to all objects.

8) All the classes in java.lang package are automatically imported when

a program is compiled.

a)True

b)False

Ans: a.

9) How can class be imported to a program?

Ans: To import a class, the import keyword should be used as shown.;

import classname;

10) How can class be imported from a package to a program?

Ans: import java . packagename . classname (or) import java.package name.*;

11) What is a constructor?

Ans: A constructor is a special kind of method that determines how an object is

initialized when created.

12) Which keyword is used to create an instance of a class?

Ans: new.

13) Which method is used to garbage collect an object?

Ans: finalize ().

14) Constructors can be overloaded like regular methods.

a)True

b)False

Ans: a.

15) What is casting?

Ans: Casting is bused to convert the value of one type to another.

16) Casting between primitive types allows conversion of one primitive type to another.

a)True

b)False

Ans: a.

17) Casting occurs commonly between numeric types.

a)True

b)False

Ans: a.

18) Boolean values can be cast into any other primitive type.

a)True

b)False

Ans: b.

19) Casting does not affect the original object or value.

a)True

b)False

Ans: a.

20) Which cast must be used to convert a larger value into a smaller one?

Ans: Explicit cast.

21) Which cast must be used to cast an object to another class?

Ans: Specific cast.

22) Which of the following features are common to both Java & C++?

A.The class declaration

b.The access modifiers

c.The encapsulation of data & methods with in objects

d.The use of pointers

Ans: a,b,c.

23) Which of the following statements accurately describe the use of access modifiers within a class definition?

a.They can be applied to both data & methods

b.They must precede a class's data variables or methods

c.They can follow a class's data variables or methods

d.They can appear in any order

e.They must be applied to data variables first and then to methods

Ans: a,b,d.

24) Suppose a given instance variable has been declared private.

Can this instance variable be manipulated by methods out side its class?

a.yes

b.no

Ans: b.

25) Which of the following statements can be used to describe a public method?

a.It is accessible to all other classes in the hierarchy

b.It is accessablde only to subclasses of its parent class

c.It represents the public interface of its class

d.The only way to gain access to this method is by calling one of the public class

methods

Ans: a,c.

26) Which of the following types of class members can be part of the internal part of a class?

a.Public instance variables

b.Private instance variables

c.Public methods

d.Private methods

Ans: b,d.

27) You would use the ____ operator to create a single instance of a named class.

a.new

b.dot

Ans: a.

28) Which of the following statements correctly describes the relation between an object and the instance variable it stores?

a.Each new object has its own distinctive set of instance variables

b.Each object has a copy of the instance variables of its class

c.the instance variable of each object are seperate from the variables of other objects

d.The instance variables of each object are stored together with the variables of other objects

Ans: a,b,c.

29) If no input parameters are specified in a method declaration then the declaration will include __.

a.an empty set of parantheses

b.the term void

Ans: a.

30) What are the functions of the dot(.) operator?

a.It enables you to access instance variables of any objects within a class

b.It enables you to store values in instance variables of an object

c.It is used to call object methods

d.It is to create a new object

Ans: a,b,c.

31) Which of the following can be referenced by this variable?

a.The instance variables of a class only

b.The methods of a class only

c.The instance variables and methods of a class

Ans: c.

32) The this reference is used in conjunction with ___methods.

a.static

b.non-static

Ans: b.

33) Which of the following operators are used in conjunction with the this and super references?

a.The new operator

b.The instanceof operator

c.The dot operator

Ans: c.

34) A constructor is automatically called when an object is instantiated

a. true

b. false

Ans: a.

35) When may a constructor be called without specifying arguments?

a. When the default constructor is not called

b. When the name of the constructor differs from that of the class

c. When there are no constructors for the class

Ans: c.

36) Each class in java can have a finalizer method

a. true

b.false

Ans: a.

37) When an object is referenced, does this mean that it has been identified by the finalizer method for garbage collection?

a.yes

b.no

Ans: b.

38) Because finalize () belongs to the java.lang.Object class, it is present in all ___.

a.objects

b.classes

c.methods

Ans: b.

39) Identify the true statements about finalization.

a.A class may have only one finalize method

b.Finalizers are mostly used with simple classes

c.Finalizer overloading is not allowed

Ans: a,c.

40) When you write finalize() method for your class, you are overriding a finalizer

inherited from a super class.

a.true

b.false

Ans: a.

41) Java memory management mechanism garbage collects objects which are no longer referenced

a true

b.false

Ans: a.

42) are objects referenced by a variable candidates for garbage collection when the variable goes out of scope?

a yes

b. no

Ans: a.

43) Java's garbage collector runs as a ___ priority thread waiting for __priority threads to relinquish the processor.

a.high

b.low

Ans: a,b.

44) The garbage collector will run immediately when the system is out of memory

a.true

b.false

Ans: a.

45) You can explicitly drop a object reference by setting the value of a variable whose data type is a reference type to ___

Ans: null

46) When might your program wish to run the garbage collecter?

a. before it enters a compute-intense section of code

b. before it enters a memory-intense section of code

c. before objects are finalized

d. when it knows there will be some idle time

Ans: a,b,d

47) For externalizable objects the class is solely responsible for the external format of its contents

a.true

b.false

Ans: a

48) When an object is stored, are all of the objects that are reachable from that object stored as well?

a.true

b.false

Ans: a

49) The default__ of objects protects private and trancient data, and supports the __ of the classes

a.evolution

b.encoding

Ans: b,a.

50) Which are keywords in Java?

a) NULL

b) sizeof

c) friend

d) extends

e) synchronized

Ans : d and e

51) When must the main class and the file name coincide?

Ans :When class is declared public.

52) What are different modifiers?

Ans : public, private, protected, default, static, trancient, volatile, final, abstract.

53) What are access modifiers?

Ans : public, private, protected, default.

54) What is meant by "Passing by value" and " Passing by reference"?

Ans : objects – pass by referrence

Methods - pass by value

55) Is a class a subclass of itself?

Ans : A class is a subclass itself.

56) What modifiers may be used with top-level class?

Ans : public, abstract, final.

57) What is an example of polymorphism?

Inner class

Anonymous classes

Method overloading

Method overriding

Ans : c

Packages and interface

1) What are packages ? what is use of packages ?

Ans :The package statement defines a name space in which classes are stored.If you omit the package, the classes are put into the default package.

Signature... package pkg;

Use: * It specifies to which package the classes defined in a file belongs to. * Package is both naming and a visibility control mechanism.

2) What is difference between importing "java.applet.Applet" and "java.applet.*;" ?

Ans :"java.applet.Applet" will import only the class Applet from the package java.applet

Where as "java.applet.*" will import all the classes from java.applet package.

3) What do you understand by package access specifier?

Ans : public: Anything declared as public can be accessed from anywhere

private: Anything declared in the private can’t be seen outside of its class.

default: It is visible to subclasses as well as to other classes in the same package.

4) What is interface? What is use of interface?

Ans : It is similar to class which may contain method’s signature only but not bodies.

Methods declared in interface are abstract methods. We can implement many interfaces on a class which support the multiple inheritance.

5) Is it is necessary to implement all methods in an interface?

Ans : Yes. All the methods have to be implemented.

6) Which is the default access modifier for an interface method?

Ans : public.

7) Can we define a variable in an interface ?and what type it should be ?

Ans : Yes we can define a variable in an interface. They are implicitly final and static.

8) What is difference between interface and an abstract class?

Ans : All the methods declared inside an Interface are abstract. Where as abstract class must have at least one abstract method and others may be concrete or abstract.

In Interface we need not use the keyword abstract for the methods.

9) By default, all program import the java.lang package.

True/False

Ans : True

10) Java compiler stores the .class files in the path specified in CLASSPATH

environmental variable.

True/False

Ans : False

11) User-defined package can also be imported just like the standard packages.

True/False

Ans : True

12) When a program does not want to handle exception, the ______class is used.

Ans : Throws

13) The main subclass of the Exception class is _______ class.

Ans : RuntimeException

14) Only subclasses of ______class may be caught or thrown.

Ans : Throwable

15) Any user-defined exception class is a subclass of the _____ class.

Ans : Exception

16) The catch clause of the user-defined exception class should ______ its

Base class catch clause.

Ans : Exception

17) A _______ is used to separate the hierarchy of the class while declaring an

Import statement.

Ans : Package

18) All standard classes of Java are included within a package called _____.

Ans : java.lang

19) All the classes in a package can be simultaneously imported using ____.

Ans : *

20) Can you define a variable inside an Interface. If no, why? If yes, how?

Ans.: YES. final and static

21) How many concrete classes can you have inside an interface?

Ans.: None

22) Can you extend an interface?

Ans.: Yes

23) Is it necessary to implement all the methods of an interface while implementing the interface?

Ans.: No

24) If you do not implement all the methods of an interface while implementing , what specifier should you use for the class ?

Ans.: abstract

25) How do you achieve multiple inheritance in Java?

Ans: Using interfaces.

26) How to declare an interface example?

Ans : access class classname implements interface.

27) Can you achieve multiple interface through interface?

a)True

b) false

Ans : a.

28) Can variables be declared in an interface ? If so, what are the modifiers?

Ans : Yes. final and static are the modifiers can be declared in an interface.

29) What are the possible access modifiers when implementing interface methods?

Ans : public.

30) Can anonymous classes be implemented an interface?

Ans : Yes.

31) Interfaces can’t be extended.

a)True

b)False

Ans : b.

32) Name interfaces without a method?

Ans : Serializable, Cloneble & Remote.

33) Is it possible to use few methods of an interface in a class ? If so, how?

Ans : Yes. Declare the class as abstract.

Exception Handling

1) What is the difference between ‘throw’ and ‘throws’ ?And it’s application?

Ans : Exceptions that are thrown by java runtime systems can be handled by Try and catch blocks. With throw exception we can handle the exceptions thrown by the program itself. If a method is capable of causing an exception that it does not

handle, it must specify this behavior so the callers of the method can guard

against that exception.

2) What is the difference between ‘Exception’ and ‘error’ in java?

Ans : Exception and Error are the subclasses of the Throwable class. Exception class is used for exceptional conditions that user program should catch. With exception class we can subclass to create our own custom exception.

Error defines exceptions that are not excepted to be caught by you program. Example is Stack Overflow.

3) What is ‘Resource leak’?

Ans : Freeing up other resources that might have been allocated at the beginning of a method.

4)What is the ‘finally’ block?

Ans : Finally block will execute whether or not an exception is thrown. If an exception is thrown, the finally block will execute even if no catch statement match the exception. Any time a method is about to return to the caller from inside try/catch block, via an uncaught exception or an explicit return statement, the finally clause is also execute.

5) Can we have catch block with out try block? If so when?

Ans : No. Try/Catch or Try/finally form a unit.

6) What is the difference between the following statements?

Catch (Exception e),

Catch (Error err),

Catch (Throwable t)

Ans :

7) What will happen to the Exception object after exception handling?

Ans : It will go for Garbage Collector. And frees the memory.

8) How many Exceptions we can define in ‘throws’ clause?

Ans : We can define multiple exceptions in throws clause.

Signature is..

type method-name (parameter-list) throws exception-list

9) The finally block is executed when an exception is thrown, even if no catch matches it.

True/False

Ans : True

10) The subclass exception should precede the base class exception when used within the catch clause.

True/False

Ans : True

11) Exceptions can be caught or rethrown to a calling method.

True/False

Ans : True

12) The statements following the throw keyword in a program are not executed.

True/False

Ans : True

13) The toString ( ) method in the user-defined exception class is overridden.

True/False

Ans : True

MULTI THREADING

1) What are the two types of multitasking?

Ans : 1.process-based

2.Thread-based

2) What are the two ways to create the thread?

Ans : 1.by implementing Runnable

2.by extending Thread

3) What is the signature of the constructor of a thread class?

Ans : Thread(Runnable threadob,String threadName)

4) What are all the methods available in the Runnable Interface?

Ans : run()

5) What is the data type for the method isAlive() and this method is

available in which class?

Ans : boolean, Thread

6) What are all the methods available in the Thread class?

Ans : 1.isAlive()

2.join()

3.resume()

4.suspend()

5.stop()

6.start()

7.sleep()

8.destroy()

7) What are all the methods used for Inter Thread communication and what is the class in which these methods are defined?

Ans :1. wait(),notify() & notifyall()

2. Object class

8) What is the mechanisam defind by java for the Resources to be used by only one Thread at a time?

Ans : Synchronisation

9) What is the procedure to own the moniter by many threads?

Ans : not possible

10) What is the unit for 1000 in the below statement?

ob.sleep(1000)

Ans : long milliseconds

11) What is the data type for the parameter of the sleep() method?

Ans : long

12) What are all the values for the following level?

max-priority

min-priority

normal-priority

Ans : 10,1,5

13) What is the method available for setting the priority?

Ans : setPriority()

14) What is the default thread at the time of starting the program?

Ans : main thread

15) The word synchronized can be used with only a method.

True/ False

Ans : False

16) Which priority Thread can prompt the lower primary Thread?

Ans : Higher Priority

17) How many threads at a time can access a monitor?

Ans : one

18) What are all the four states associated in the thread?

Ans : 1. new 2. runnable 3. blocked 4. dead

19) The suspend()method is used to teriminate a thread?

True /False

Ans : False

20) The run() method should necessary exists in clases created as subclass of thread?

True /False

Ans : True

21) When two threads are waiting on each other and can't proceed the programe is said to be in a deadlock?

True/False

Ans : True

22) Which method waits for the thread to die ?

Ans : join() method

23) Which of the following is true?

1) wait(),notify(),notifyall() are defined as final & can be called only from with in a synchronized method

2) Among wait(),notify(),notifyall() the wait() method only throws IOException

3) wait(),notify(),notifyall() & sleep() are methods of object class

1

2

3

1 & 2

1,2 & 3

Ans : D

24) Garbage collector thread belongs to which priority?

Ans : low-priority

25) What is meant by timeslicing or time sharing?

Ans : Timeslicing is the method of allocating CPU time to individual threads in a priority schedule.

26) What is meant by daemon thread? In java runtime, what is it's role?

Ans : Daemon thread is a low priority thread which runs intermittently in the background doing the garbage collection operation for the java runtime system.

Inheritance

1) What is the difference between superclass & subclass?

Ans : A super class is a class that is inherited whereas subclass is a class that does the inheriting.

2) Which keyword is used to inherit a class?

Ans : extends

3) Subclasses methods can access superclass members/ attributes at all times?

True/False

Ans : False

4) When can subclasses not access superclass members?

Ans : When superclass is declared as private.

5) Which class does begin Java class hierarchy?

Ans : Object class

6) Object class is a superclass of all other classes?

True/False

Ans : True

7) Java supports multiple inheritance?

True/False

Ans : False

8) What is inheritance?

Ans : Deriving an object from an existing class. In the other words, Inheritance is the process of inheriting all the features from a class

9) What are the advantages of inheritance?

Ans : Reusability of code and accessibility of variables and methods of the superclass by subclasses.

10) Which method is used to call the constructors of the superclass from the subclass?

Ans : super(argument)

11) Which is used to execute any method of the superclass from the subclass?

Ans : super.method-name(arguments)

12) Which methods are used to destroy the objects created by the constructor methods?

Ans : finalize()

13) What are abstract classes?

Ans : Abstract classes are those for which instances can’t be created.

14) What must a class do to implement an interface?

Ans: It must provide all of the methods in the interface and identify the interface in its implements clause.

15) Which methods in the Object class are declared as final?

Ans : getClass(), notify(), notifyAll(), and wait()

16) Final methods can be overridden.

True/False

Ans : False

17) Declaration of methods as final results in faster execution of the program?

True/False

Ans: True

18) Final variables should be declared in the beginning?

True/False

Ans : True

19) Can we declare variable inside a method as final variables? Why?

Ans : Cannot because, local variable cannot be declared as final variables.

20) Can an abstract class may be final?

Ans : An abstract class may not be declared as final.

21) Does a class inherit the constructors of it's super class?

Ans: A class does not inherit constructors from any of it's super classes.

22) What restrictions are placed on method overloading?

Ans: Two methods may not have the same name and argument list but different return types.

23) What restrictions are placed on method overriding?

Ans : Overridden methods must have the same name , argument list , and return type. The overriding method may not limit the access of the method it overridees.The overriding method may not throw any exceptions that may not be thrown by the overridden method.

24) What modifiers may be used with an inner class that is a member of an outer class?

Ans : a (non-local) inner class may be declared as public, protected, private, static, final or abstract.

25) How this() is used with constructors?

Ans: this() is used to invoke a constructor of the same class

26) How super() used with constructors?

Ans : super() is used to invoke a super class constructor

27) Which of the following statements correctly describes an interface?

a)It's a concrete class

b)It's a superclass

c)It's a type of abstract class

Ans: c

28) An interface contains __ methods

a)Non-abstract

b)Implemented

c)unimplemented

Ans:c

STRING HANDLING

Which package does define String and StringBuffer classes?

Ans : java.lang package.

Which method can be used to obtain the length of the String?

Ans : length( ) method.

How do you concatenate Strings?

Ans : By using " + " operator.

Which method can be used to compare two strings for equality?

Ans : equals( ) method.

Which method can be used to perform a comparison between strings that ignores case differences?

Ans : equalsIgnoreCase( ) method.

What is the use of valueOf( ) method?

Ans : valueOf( ) method converts data from its internal format into a human-readable form.

What are the uses of toLowerCase( ) and toUpperCase( ) methods?

Ans : The method toLowerCase( ) converts all the characters in a string from uppercase to

lowercase.

The method toUpperCase( ) converts all the characters in a string from lowercase to

uppercase.

Which method can be used to find out the total allocated capacity of a StrinBuffer?

Ans : capacity( ) method.

Which method can be used to set the length of the buffer within a StringBuffer object?

Ans : setLength( ).

What is the difference between String and StringBuffer?

Ans : String objects are constants, whereas StringBuffer objects are not.

String class supports constant strings, whereas StringBuffer class supports growable, modifiable strings.

What are wrapper classes?

Ans : Wrapper classes are classes that allow primitive types to be accessed as objects.

Which of the following is not a wrapper class?

String

Integer

Boolean

Character

Ans : a.

What is the output of the following program?

public class Question {

public static void main(String args[]) {

String s1 = "abc";

String s2 = "def";

String s3 = s1.concat(s2.toUpperCase( ) );

System.out.println(s1+s2+s3);

}

}

abcdefabcdef

abcabcDEFDEF

abcdefabcDEF

None of the above

ANS : c.

Which of the following methods are methods of the String class?

delete( )

append( )

reverse( )

replace( )

Ans : d.

Which of the following methods cause the String object referenced by s to be changed?

s.concat( )

s.toUpperCase( )

s.replace( )

s.valueOf( )

Ans : a and b.

String is a wrapper class?

True

False

Ans : b.

17) If you run the code below, what gets printed out?

String s=new String("Bicycle");

int iBegin=1;

char iEnd=3;

System.out.println(s.substring(iBegin,iEnd));

Bic

ic

c) icy
d) error: no method matching substring(int,char)

Ans : b.

18) Given the following declarations

String s1=new String("Hello")

String s2=new String("there");

String s3=new String();

Which of the following are legal operations?

s3=s1 + s2;

s3=s1 - s2;

c) s3=s1 & s2
d) s3=s1 && s2

Ans : a.

19) Which of the following statements are true?

The String class is implemented as a char array, elements are addressed using the stringname[] convention

b) Strings are a primitive type in Java that overloads the + operator for concatenation
c) Strings are a primitive type in Java and the StringBuffer is used as the matching wrapper type
d) The size of a string can be retrieved using the length property.

Ans : b.

EXPLORING JAVA.LANG

java.lang package is automatically imported into all programs.

True

False

Ans : a

What are the interfaces defined by java.lang?

Ans : Cloneable, Comparable and Runnable.

What are the constants defined by both Flaot and Double classes?

Ans : MAX_VALUE,

MIN_VALUE,

NaN,

POSITIVE_INFINITY,

NEGATIVE_INFINITY and

TYPE.

What are the constants defined by Byte, Short, Integer and Long?

Ans : MAX_VALUE,

MIN_VALUE and

TYPE.

What are the constants defined by both Float and Double classes?

Ans : MAX_RADIX,

MIN_RADIX,

MAX_VALUE,

MIN_VALUE and

TYPE.

What is the purpose of the Runtime class?

Ans : The purpose of the Runtime class is to provide access to the Java runtime system.

What is the purpose of the System class?

Ans : The purpose of the System class is to provide access to system resources.

Which class is extended by all other classes?

Ans : Object class is extended by all other classes.

Which class can be used to obtain design information about an object?

Ans : The Class class can be used to obtain information about an object’s design.

Which method is used to calculate the absolute value of a number?

Ans : abs( ) method.

What are E and PI?

Ans : E is the base of the natural logarithm and PI is the mathematical value pi.

Which of the following classes is used to perform basic console I/O?

System

SecurityManager

Math

Runtime

Ans : a.

Which of the following are true?

The Class class is the superclass of the Object class.

The Object class is final.

The Class class can be used to load other classes.

The ClassLoader class can be used to load other classes.

Ans : c and d.

Which of the following methods are methods of the Math class?

absolute( )

log( )

cosine( )

sine( )

Ans : b.

Which of the following are true about the Error and Exception classes?

Both classes extend Throwable.

The Error class is final and the Exception class is not.

The Exception class is final and the Error is not.

Both classes implement Throwable.

Ans : a.

Which of the following are true?

The Void class extends the Class class.

The Float class extends the Double class.

The System class extends the Runtime class.

The Integer class extends the Number class.

Ans : d.

17) Which of the following will output -4.0

System.out.println(Math.floor(-4.7));

System.out.println(Math.round(-4.7));

System.out.println(Math.ceil(-4.7));

d) System.out.println(Math.Min(-4.7));

Ans : c.

18) Which of the following are valid statements

a) public class MyCalc extends Math
b) Math.max(s);
c) Math.round(9.99,1);
d) Math.mod(4,10);

e) None of the above.

Ans : e.

19) What will happen if you attempt to compile and run the following code?

Integer ten=new Integer(10);

Long nine=new Long (9);

System.out.println(ten + nine);

int i=1;

System.out.println(i + ten);

19 followed by 20

19 followed by 11

Error: Can't convert java lang Integer

d) 10 followed by 1

Ans : c.

INPUT / OUTPUT : EXPLORING JAVA.IO

What is meant by Stream and what are the types of Streams and classes of the Streams?

Ans : A Stream is an abstraction that either produces or consumes information.

There are two types of Streams. They are:

Byte Streams : Byte Streams provide a convenient means for handling input and output of bytes.

Character Streams : Character Streams provide a convenient means for handling input and output of characters.

Byte Stream classes : Byte Streams are defined by using two abstract classes. They are:InputStream and OutputStream.

Character Stream classes : Character Streams are defined by using two abstract classes. They are : Reader and Writer.

Which of the following statements are true?

UTF characters are all 8-bits.

UTF characters are all 16-bits.

UTF characters are all 24-bits.

Unicode characters are all 16-bits.

Bytecode characters are all 16-bits.

Ans : d.

Which of the following statements are true?

When you construct an instance of File, if you do not use the filenaming semantics of the local machine, the constructor will throw an IOException.

When you construct an instance of File, if the corresponding file does not exist on the local file system, one will be created.

When an instance of File is garbage collected, the corresponding file on the local file system is deleted.

None of the above.

Ans : a,b and c.

The File class contains a method that changes the current working directory.

True

False

Ans : b.

It is possible to use the File class to list the contents of the current working directory.

True

False

Ans : a.

Readers have methods that can read and return floats and doubles.

True

False

Ans : b.

You execute the code below in an empty directory. What is the result?

File f1 = new File("dirname");

File f2 = new File(f1, "filename");

A new directory called dirname is created in the current working directory.

A new directory called dirname is created in the current working directory. A new file called filename is created in directory dirname.

A new directory called dirname and a new file called filename are created, both in the current working directory.

A new file called filename is created in the current working directory.

No directory is created, and no file is created.

Ans : e.

What is the difference between the Reader/Writer class hierarchy and the

InputStream/OutputStream class hierarchy?

Ans : The Reader/Writer class hierarchy is character-oriented and the InputStream/OutputStream class hierarchy is byte-oriented.

What is an I/O filter?

Ans : An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

What is the purpose of the File class?

Ans : The File class is used to create objects that provide access to the files and directories of a local file system.

What interface must an object implement before it can be written to a stream as an object?

Ans : An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

What is the difference between the File and RandomAccessFile classes?

Ans : The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

What class allows you to read objects directly from a stream?

Ans : The ObjectInputStream class supports the reading of objects from input streams.

What value does read( ) return when it has reached the end of a file?

Ans : The read( ) method returns – 1 when it has reached the end of a file.

What value does readLine( ) return when it has reached the end of a file?

Ans : The readLine( ) method returns null when it has reached the end of a file.

How many bits are used to represent Unicode, ASCII, UTF-16 and UTF-8 characters?

Ans : Unicode requires 16-bits and ASCII requires 8-bits. Although the ASCII character set uses only 1-bits, it is usually represented as 8-bits. UTF-8 represents characters using 8, 16 and 18-bit patterns. UTF-16 uses 16-bit and larger bit patterns.

Which of the following are true?

The InputStream and OutputStream classes are byte-oriented.

The ObjectInputStream and ObjectOutputStream do not support serialized object input and output.

The Reader and Writer classes are character-oriented.

The Reader and Writer classes are the preferred solution to serialized object output.

Ans : a and c.

Which of the following are true about I/O filters?

Filters are supported on input, but not on output.

Filters are supported by the InputStream/OutputStream class hierarchy, but not by the Reader/Writer class hierarchy.

Filters read from one stream and write to another.

A filter may alter data that is read from one stream and written to another.

Ans : c and d.

Which of the following are true?

Any Unicode character is represented using 16-bits.

7-bits are needed to represent any ASCII character.

UTF-8 characters are represented using only 8-bits.

UTF-16 characters are represented using only 16-bits.

Ans : a and b.

Which of the following are true?

The Serializable interface is used to identify objects that may be written to an output stream.

The Externalizable interface is implemented by classes that control the way in which their objects are serialized.

The Serializable interface extends the Externalizable interface.

The Externalizable interface extends the Serializable interface.

Ans : a, b and d.

Which of the following are true about the File class?

A File object can be used to change the current working directory.

A File object can be used to access the files in the current directory.

When a File object is created, a corresponding directory or file is created in the local file system.

File objects are used to access files and directories on the local file system.

File objects can be garbage collected.

When a File object is garbage collected, the corresponding file or directory is deleted.

Ans : b, d and e.

How do you create a Reader object from an InputStream object?

Use the static createReader( ) method of InputStream class.

Use the static createReader( ) method of Reader class.

Create an InputStreamReader object, passing the InputStream object as an argument to the InputStreamReader constructor.

Create an OutputStreamReader object, passing the InputStream object as an argument to the OutputStreamReader constructor.

Ans : c.

Which of the following are true?

Writer classes can be used to write characters to output streams using different character encodings.

Writer classes can be used to write Unicode characters to output streams.

Writer classes have methods that support the writing of the values of any Java primitive type to output streams.

Writer classes have methods that support the writing of objects to output streams.

Ans : a and b.

The isFile( ) method returns a boolean value depending on whether the file object is a file or a directory.

True.

False.

Ans : a.

Reading or writing can be done even after closing the input/output source.

True.

False.

Ans : b.

The ________ method helps in clearing the buffer.

Ans : flush( ).

The System.err method is used to print error message.

True.

False.

Ans : a.

What is meant by StreamTokenizer?

Ans : StreamTokenizer breaks up InputStream into tokens that are delimited by sets of characters.

It has the constructor : StreamTokenizer(Reader inStream).

Here inStream must be some form of Reader.

What is Serialization and deserialization?

Ans : Serialization is the process of writing the state of an object to a byte stream.

Deserialization is the process of restoring these objects.

30) Which of the following can you perform using the File class?

a) Change the current directory
b) Return the name of the parent directory
c) Delete a file
d) Find if a file contains text or binary information

Ans : b and c.

31)How can you change the current working directory using an instance of the File class called FileName?

FileName.chdir("DirName").

FileName.cd("DirName").

FileName.cwd("DirName").

The File class does not support directly changing the current directory.

Ans : d.

EVENT HANDLING

The event delegation model, introduced in release 1.1 of the JDK, is fully compatible with the

event model.

True

False

Ans : b.

A component subclass that has executed enableEvents( ) to enable processing of a certain kind of event cannot also use an adapter as a listener for the same kind of event.

True

False

Ans : b.

What is the highest-level event class of the event-delegation model?

Ans : The java.util.eventObject class is the highest-level class in the event-delegation hierarchy.

What interface is extended by AWT event listeners?

Ans : All AWT event listeners extend the java.util.EventListener interface.

What class is the top of the AWT event hierarchy?

Ans : The java.awt.AWTEvent class is the highest-level class in the AWT event class hierarchy.

What event results from the clicking of a button?

Ans : The ActionEvent event is generated as the result of the clicking of a button.

What is the relationship between an event-listener interface and an event-adapter class?

Ans : An event-listener interface defines the methods that must be implemented by an event

handler for a particular kind of event.

An event adapter provides a default implementation of an event-listener interface.

In which package are most of the AWT events that support the event-delegation model defined?

Ans : Most of the AWT–related events of the event-delegation model are defined in the

java.awt.event package. The AWTEvent class is defined in the java.awt package.

What is the advantage of the event-delegation model over the earlier event-inheritance model?

Ans : The event-delegation has two advantages over the event-inheritance model. They are :

It enables event handling by objects other than the ones that generate the events. This

allows a clean separation between a component’s design and its use.

It performs much better in applications where many events are generated. This

performance improvement is due to the fact that the event-delegation model does not

have to repeatedly process unhandled events, as is the case of the event-inheritance

model.

What is the purpose of the enableEvents( ) method?

Ans :The enableEvents( ) method is used to enable an event for a particular object.

Which of the following are true?

The event-inheritance model has replaced the event-delegation model.

The event-inheritance model is more efficient than the event-delegation model.

The event-delegation model uses event listeners to define the methods of event-handling classes.

The event-delegation model uses the handleEvent( ) method to support event handling.

Ans : c.

Which of the following is the highest class in the event-delegation model?

java.util.EventListener

java.util.EventObject

java.awt.AWTEvent

java.awt.event.AWTEvent

Ans : b.

When two or more objects are added as listeners for the same event, which listener is first invoked to handle the event?

The first object that was added as listener.

The last object that was added as listener.

There is no way to determine which listener will be invoked first.

It is impossible to have more than one listener for a given event.

Ans : c.

Which of the following components generate action events?

Buttons

Labels

Check boxes

Windows

Ans : a.

Which of the following are true?

A TextField object may generate an ActionEvent.

A TextArea object may generate an ActionEvent.

A Button object may generate an ActionEvent.

A MenuItem object may generate an ActionEvent.

Ans : a,c and d.

Which of the following are true?

The MouseListener interface defines methods for handling mouse clicks.

The MouseMotionListener interface defines methods for handling mouse clicks.

The MouseClickListener interface defines methods for handling mouse clicks.

The ActionListener interface defines methods for handling the clicking of a button.

Ans : a and d.

Suppose that you want to have an object eh handle the TextEvent of a TextArea object t. How should you add eh as the event handler for t?

t.addTextListener(eh);

eh.addTextListener(t);

addTextListener(eh.t);

addTextListener(t,eh);

Ans : a.

What is the preferred way to handle an object’s events in Java 2?

Override the object’s handleEvent( ) method.

Add one or more event listeners to handle the events.

Have the object override its processEvent( ) methods.

Have the object override its dispatchEvent( ) methods.

Ans : b.

Which of the following are true?

A component may handle its own events by adding itself as an event listener.

A component may handle its own events by overriding its event-dispatching method.

A component may not handle oits own events.

A component may handle its own events only if it implements the handleEvent( ) method.

Ans : a and b.

APPLETS

What is an Applet? Should applets have constructors?

Ans : Applet is a dynamic and interactive program that runs inside a Web page

displayed by a Java capable browser. We don’t have the concept of Constructors in Applets.

How do we read number information from my applet’s parameters, given that Applet’s getParameter() method returns a string?

Ans : Use the parseInt() method in the Integer Class, the Float(String) constructor in the

Class Float, or the Double(String) constructor in the class Double.

How can I arrange for different applets on a web page to communicate with each other?

Ans : Name your applets inside the Applet tag and invoke AppletContext’s getApplet()

method in your applet code to obtain references to the other applets on the page.

How do I select a URL from my Applet and send the browser to that page?

Ans : Ask the applet for its applet context and invoke showDocument() on that context object.

Eg. URL targetURL;

String URLString

AppletContext context = getAppletContext();

try{

targetUR L = new URL(URLString);

} catch (Malformed URLException e){

// Code for recover from the exception

}

context. showDocument (targetURL);

Can applets on different pages communicate with each other?

Ans : No. Not Directly. The applets will exchange the information at one meeting place

either on the local file system or at remote system.

How do Applets differ from Applications?

Ans : Appln: Stand Alone

Applet: Needs no explicit installation on local m/c.

Appln: Execution starts with main() method.

Applet: Execution starts with init() method.

Appln: May or may not be a GUI

Applet: Must run within a GUI (Using AWT)

How do I determine the width and height of my application?

Ans : Use the getSize() method, which the Applet class inherits from the Component

class in the Java.awt package. The getSize() method returns the size of the applet as

a Dimension object, from which you extract separate width, height fields.

Eg. Dimension dim = getSize ();

int appletwidth = dim.width ();

8) What is AppletStub Interface?

Ans : The applet stub interface provides the means by which an applet and the browser communicate. Your code will not typically implement this interface.

It is essential to have both the .java file and the .html file of an applet in the same

directory.

True.

False.

Ans : b.

The <PARAM> tag contains two attributes namely _________ and _______.

Ans : Name , value.

Passing values to parameters is done in the _________ file of an applet.

Ans : .html.

12) What tags are mandatory when creating HTML to display an applet

name, height, width

code, name

codebase, height, width

d) code, height, width

Ans : d.

Applet’s getParameter( ) method can be used to get parameter values.

True.

False.

Ans : a.

What are the Applet’s Life Cycle methods? Explain them?

Ans : init( ) method - Can be called when an applet is first loaded.

start( ) method - Can be called each time an applet is started.

paint( ) method - Can be called when the applet is minimized or refreshed.

stop( ) method - Can be called when the browser moves off the applet’s page.

destroy( ) method - Can be called when the browser is finished with the applet.

What are the Applet’s information methods?

Ans : getAppletInfo( ) method : Returns a string describing the applet, its author ,copy

right information, etc.

getParameterInfo( ) method : Returns an array of string describing the applet’s parameters.

All Applets are subclasses of Applet.

True.

False.

Ans : a.

All Applets must import java.applet and java.awt.

True.

False.

Ans : a.

What are the steps involved in Applet development?

Ans : a) Edit a Java source file,

b) Compile your program and

c) Execute the appletviewer, specifying the name of your applet’s source file.

Applets are executed by the console based Java run-time interpreter.

True.

False.

Ans : b.

Which classes and interfaces does Applet class consist?

Ans : Applet class consists of a single class, the Applet class and three interfaces: AppletContext,

AppletStub and AudioClip.

What is the sequence for calling the methods by AWT for applets?

Ans : When an applet begins, the AWT calls the following methods, in this sequence.

init( )

start( )

paint( )

When an applet is terminated, the following sequence of method cals takes place :

stop( )

destroy( )

Which method is used to output a string to an applet?

Ans : drawString ( ) method.

Every color is created from an RGB value.

True.

False

Ans : a.

AWT : WINDOWS, GRAPHICS AND FONTS

How would you set the color of a graphics context called g to cyan?

g.setColor(Color.cyan);

g.setCurrentColor(cyan);

g.setColor("Color.cyan");

g.setColor("cyan’);

g.setColor(new Color(cyan));

Ans : a.

The code below draws a line. What color is the line?

g.setColor(Color.red.green.yellow.red.cyan);

g.drawLine(0, 0, 100,100);

Red

Green

Yellow

Cyan

Black

Ans : d.

What does the following code draw?

g.setColor(Color.black);

g.drawLine(10, 10, 10, 50);

g.setColor(Color.RED);

g.drawRect(100, 100, 150, 150);

A red vertical line that is 40 pixels long and a red square with sides of 150 pixels

A black vertical line that is 40 pixels long and a red square with sides of 150 pixels

A black vertical line that is 50 pixels long and a red square with sides of 150 pixels

A red vertical line that is 50 pixels long and a red square with sides of 150 pixels

A black vertical line that is 40 pixels long and a red square with sides of 100 pixel

Ans : b.

Which of the statements below are true?

A polyline is always filled.

b) A polyline can not be filled.

c) A polygon is always filled.

d) A polygon is always closed

e) A polygon may be filled or not filled

Ans : b, d and e.

What code would you use to construct a 24-point bold serif font?

new Font(Font.SERIF, 24,Font.BOLD);

new Font("SERIF", 24, BOLD");

new Font("BOLD ", 24,Font.SERIF);

new Font("SERIF", Font.BOLD,24);

new Font(Font.SERIF, "BOLD", 24);

Ans : d.

What does the following paint( ) method draw?

Public void paint(Graphics g) {

g.drawString("question #6",10,0);

}

The string "question #6", with its top-left corner at 10,0

A little squiggle coming down from the top of the component, a little way in from the left edge

Ans : b.

What does the following paint( ) method draw?

Public void paint(Graphics g) {

g.drawString("question #6",10,0);

}

A circle at (100, 100) with radius of 44

A circle at (100, 44) with radius of 100

A circle at (100, 44) with radius of 44

The code does not compile

Ans : d.

8)What is relationship between the Canvas class and the Graphics class?

Ans : A Canvas object provides access to a Graphics object via its paint( ) method.

What are the Component subclasses that support painting.

Ans : The Canvas, Frame, Panel and Applet classes support painting.

What is the difference between the paint( ) and repaint( ) method?

Ans : The paint( ) method supports painting via a Graphics object. The repaint( ) method is used

to cause paint( ) to be invoked by the AWT painting method.

What is the difference between the Font and FontMetrics classes?

Ans : The FontMetrics class is used to define implementation-specific properties, such as ascent

and descent, of a Font object.

Which of the following are passed as an argument to the paint( ) method?

A Canvas object

A Graphics object

An Image object

A paint object

Ans : b.

Which of the following methods are invoked by the AWT to support paint and repaint operations?

paint( )

repaint( )

draw( )

redraw( )

Ans : a.

Which of the following classes have a paint( ) method?

Canvas

Image

Frame

Graphics

Ans : a and c.

Which of the following are methods of the Graphics class?

drawRect( )

drawImage( )

drawPoint( )

drawString( )

Ans : a, b and d.

Which Font attributes are available through the FontMetrics class?

ascent

leading

case

height

Ans : a, b and d.

Which of the following are true?

The AWT automatically causes a window to be repainted when a portion of a window has been minimized and then maximized.

The AWT automatically causes a window to be repainted when a portion of a window has been covered and then uncovered.

The AWT automatically causes a window to be repainted when application data is changed.

The AWT does not support repainting operations.

Ans : a and b.

Which method is used to size a graphics object to fit the current size of the window?

Ans : getSize( ) method.

What are the methods to be used to set foreground and background colors?

Ans : setForeground( ) and setBackground( ) methods.

19) You have created a simple Frame and overridden the paint method as follows

public void paint(Graphics g){

g.drawString("Dolly",50,10);

}

What will be the result when you attempt to compile and run the program?

The string "Dolly" will be displayed at the centre of the frame

b) An error at compilation complaining at the signature of the paint method
c) The lower part of the word Dolly will be seen at the top of the form, with the top hidden.
d) The string "Dolly" will be shown at the bottom of the form

Ans : c.

20) Where g is a graphics instance what will the following code draw on the screen.

g.fillArc(45,90,50,50,90,180);

a) An arc bounded by a box of height 45, width 90 with a centre point of 50,50, starting
at an angle of 90 degrees traversing through 180 degrees counter clockwise.

b) An arc bounded by a box of height 50, width 50, with a centre point of 45,90 starting
at an angle of 90 degrees traversing through 180 degrees clockwise.

c) An arc bounded by a box of height 50, width 50, with a top left at coordinates of 45,
90, starting at 90 degrees and traversing through 180 degrees counter clockwise.

d) An arc starting at 45 degrees, traversing through 90 degrees clockwise bounded by a
box of height 50, width 50 with a centre point of 90, 180.

Ans : c.

21) Given the following code
import java.awt.*;
public class SetF extends Frame{
public static void main(String argv[]){
SetF s = new SetF();
s.setSize(300,200);
s.setVisible(true);
}
}
How could you set the frame surface color to pink

a)s.setBackground(Color.pink);
b)s.setColor(PINK);
c)s.Background(pink);
d)s.color=Color.pink

Ans : a.

AWT: CONTROLS, LAYOUT MANAGERS AND MENUS

What is meant by Controls and what are different types of controls?

Ans : Controls are componenets that allow a user to interact with your application.

The AWT supports the following types of controls:

Labels

Push buttons

Check boxes

Choice lists

Lists

Scroll bars

Text components

These controls are subclasses of Component.

You want to construct a text area that is 80 character-widths wide and 10 character-heights tall. What code do you use?

new TextArea(80, 10)

new TextArea(10, 80)

Ans: b.

A text field has a variable-width font. It is constructed by calling new

TextField("iiiii"). What happens if you change the contents of the text field to

"wwwww"? (Bear in mind that is one of the narrowest characters, and w is one of the widest.)

The text field becomes wider.

The text field becomes narrower.

The text field stays the same width; to see the entire contents you will have to scroll by using the ß and à keys.

The text field stays the same width; to see the entire contents you will have to scroll by using the text field’s horizontal scroll bar.

Ans : c.

The CheckboxGroup class is a subclass of the Component class.

True

False

Ans : b.

5) What are the immediate super classes of the following classes?

a) Container class

b) MenuComponent class

c) Dialog class

d) Applet class

e) Menu class

Ans : a) Container - Component

b) MenuComponent - Object

c) Dialog - Window

d) Applet - Panel

e) Menu - MenuItem

6) What are the SubClass of Textcomponent Class?

Ans : TextField and TextArea

7) Which method of the component class is used to set the position and the size of a component?

Ans : setBounds()

8) Which TextComponent method is used to set a TextComponent to the read-only state?

Ans : setEditable()

9) How can the Checkbox class be used to create a radio button?

Ans : By associating Checkbox objects with a CheckboxGroup.

10) What Checkbox method allows you to tell if a Checkbox is checked?

Ans : getState()

11) Which Component method is used to access a component's immediate Container?

getVisible()

getImmediate

getParent()

getContainer

Ans : c.

12) What methods are used to get and set the text label displayed by a Button object?

Ans : getLabel( ) and setLabel( )

13) What is the difference between a Choice and a List?

Ans : A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice.

A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

14) Which Container method is used to cause a container to be laid out and redisplayed?

Ans : validate( )

15) What is the difference between a Scollbar and a Scrollpane?

Ans : A Scrollbar is a Component, but not a Container.

A Scrollpane is a Container and handles its own events and performs its own

scrolling.

16) Which Component subclass is used for drawing and painting?

Ans : Canvas.

17) Which of the following are direct or indirect subclasses of Component?

Button

Label

CheckboxMenuItem

Toolbar

Frame

Ans : a, b and e.

18) Which of the following are direct or indirect subclasses of Container?

Frame

TextArea

MenuBar

FileDialog

Applet

Ans : a,d and e.

19) Which method is used to set the text of a Label object?

setText( )

setLabel( )

setTextLabel( )

setLabelText( )

Ans : a.

20) Which constructor creates a TextArea with 10 rows and 20 columns?

new TextArea(10, 20)

new TextArea(20, 10)

new TextArea(new Rows(10), new columns(20))

new TextArea(200)

Ans : a.

(Usage is TextArea(rows, columns)

21) Which of the following creates a List with 5 visible items and multiple selection enabled?

new List(5, true)

new List(true, 5)

new List(5, false)

new List(false,5)

Ans : a.

[Usage is List(rows, multipleMode)]

22) Which are true about the Container class?

The validate( ) method is used to cause a Container to be laid out and redisplayed.

The add( ) method is used to add a Component to a Container.

The getBorder( ) method returns information about a Container’s insets.

The getComponent( ) method is used to access a Component that is contained in a Container.

Ans : a, b and d.

23) Suppose a Panel is added to a Frame and a Button is added to the Panel. If the Frame’s font is set to 12-point TimesRoman, the Panel’s font is set to 10-point TimesRoman, and the Button’s font is not set, what font will be used to dispaly the Button’s label?

12-point TimesRoman

11-point TimesRoman

10-point TimesRoman

9-point TimesRoman

Ans : c.

A Frame’s background color is set to Color.Yellow, and a Button’s background color is to Color.Blue. Suppose the Button is added to a Panel, which is added to the Frame. What background color will be used with the Panel?

Colr.Yellow

Color.Blue

Color.Green

Color.White

Ans : a.

25) Which method will cause a Frame to be displayed?

show( )

setVisible( )

display( )

displayFrame( )

Ans : a and b.

26) All the componenet classes and container classes are derived from _________ class.

Ans : Object.

27) Which method of the container class can be used to add components to a Panel.

Ans : add ( ) method.

28) What are the subclasses of the Container class?

Ans : The Container class has three major subclasses. They are :

Window

Panel

ScrollPane

29) The Choice component allows multiple selection.

True.

False.

Ans : b.

30) The List component does not generate any events.

True.

False.

Ans : b.

31) Which components are used to get text input from the user.

Ans : TextField and TextArea.

32) Which object is needed to group Checkboxes to make them exclusive?

Ans : CheckboxGroup.

33) Which of the following components allow multiple selections?

Non-exclusive Checkboxes.

Radio buttons.

Choice.

List.

Ans : a and d.

34) What are the types of Checkboxes and what is the difference between them?

Ans : Java supports two types of Checkboxes. They are : Exclusive and Non-exclusive.

In case of exclusive Checkboxes, only one among a group of items can be selected at a time. I f an item from the group is selected, the checkbox currently checked is deselected and the new selection is highlighted. The exclusive Checkboxes are also called as Radio buttons.

The non-exclusive checkboxes are not grouped together and each one can be selected independent of the other.

35) What is a Layout Manager and what are the different Layout Managers available in java.awt and what is the default Layout manager for the panal and the panal subclasses?

Ans: A layout Manager is an object that is used to organize components in a container.

The different layouts available in java.awt are :

FlowLayout, BorderLayout, CardLayout, GridLayout and GridBag Layout.

The default Layout Manager of Panal and Panal sub classes is FlowLayout".

36) Can I exert control over the size and placement of components in my interface?

Ans : Yes.

myPanal.setLayout(null);

myPanal.setbounds(20,20,200,200);

37) Can I add the same component to more than one container?

Ans : No. Adding a component to a container automatically removes it from any previous parent(container).

38) How do I specify where a window is to be placed?

Ans : Use setBounds, setSize, or setLocation methods to implement this.

setBounds(int x, int y, int width, int height)

setBounds(Rectangle r)

setSize(int width, int height)

setSize(Dimension d)

setLocation(int x, int y)

setLocation(Point p)

39) How can we create a borderless window?

Ans : Create an instance of the Window class, give it a size, and show it on the screen.

eg. Frame aFrame = ......

Window aWindow = new Window(aFrame);

aWindow.setLayout(new FlowLayout());

aWindow.add(new Button("Press Me"));

aWindow.getBounds(50,50,200,200);

aWindow.show();

40) Can I create a non-resizable windows? If so, how?

Ans: Yes. By using setResizable() method in class Frame.

41) What is the default Layout Manager for the Window and Window subclasses (Frame,Dialog)?

Ans : BorderLayout().

42) How are the elements of different layouts organized?

Ans : FlowLayout : The elements of a FlowLayout are organized in a top to bottom, left to right fashion.

BorderLayout : The elements of a BorderLayout are organized at the

borders (North, South, East and West) and the center of a

container.

CardLayout : The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

GridLayout : The elements of a GridLayout are of equal size and are laid out using the square of a grid.

GridBagLayout : The elements of a GridBagLayout are organized according to a grid.However, the elements are of different sizes and may occupy

more than one row or column of the grid. In addition, the rows and columns may have different sizes.

43) Which containers use a BorderLayout as their default layout?

Ans : The Window, Frame and Dialog classes use a BorderLayout as their default layout.

44) Which containers use a FlowLayout as their default layout?

Ans : The Panel and the Applet classes use the FlowLayout as their default layout.

45) What is the preferred size of a component?

Ans : The preferred size of a component size that will allow the component to display normally.

46) Which method is method to set the layout of a container?

startLayout( )

initLayout( )

layoutContainer( )

setLayout( )

Ans : d.

47) Which method returns the preferred size of a component?

getPreferredSize( )

getPreferred( )

getRequiredSize( )

getLayout( )

Ans : a.

48) Which layout should you use to organize the components of a container in a

tabular form?

CardLayout

BorederLayout

FlowLayout

GridLayout

Ans : d.

An application has a frame that uses a Border layout manager. Why is it probably not a good idea to put a vertical scroll bar at North in the frame?

The scroll bar’s height would be its preferred height, which is not likely to be enough.

The scroll bar’s width would be the entire width of the frame, which would be much wider than necessary.

Both a and b.

Neither a nor b. There is no problem with the layout as described.

Ans : c.

What is the default layouts for a applet, a frame and a panel?

Ans : For an applet and a panel, Flow layout is the default layout, whereas Border layout is default layout for a frame.

If a frame uses a Grid layout manager and does not contain any panels, then all the components within the frame are the same width and height.

True

False.

Ans : a.

If a frame uses its default layout manager and does not contain any panels, then all the components within the frame are the same width and height.

True

False.

Ans : b.

With a Border layout manager, the component at Center gets all the space that is left over, after the components at North and South have been considered.

True

False

Ans : b.

An Applet has its Layout Manager set to the default of FlowLayout. What code would be the correct to change to another Layout Manager?

setLayoutManager(new GridLayout());

setLayout(new GridLayout(2,2));

c) setGridLayout(2,2,))
d setBorderLayout();

Ans : b.

55) How do you indicate where a component will be positioned using Flowlayout?

a) North, South,East,West
b) Assign a row/column grid reference
c) Pass a X/Y percentage parameter to the add method
d) Do nothing, the FlowLayout will position the component

Ans :d.

56) How do you change the current layout manager for a container?

a) Use the setLayout method
b) Once created you cannot change the current layout manager of a component
c) Use the setLayoutManager method
d) Use the updateLayout method

Ans :a.

57)When using the GridBagLayout manager, each new component requires a new instance of the GridBagConstraints class. Is this statement true or false?

a) true
b) false

Ans : b.

58) Which of the following statements are true?

a)The default layout manager for an Applet is FlowLayout
b) The default layout manager for an application is FlowLayout
c) A layout manager must be assigned to an Applet before the setSize method is called
d) The FlowLayout manager attempts to honor the preferred size of any components

Ans : a and d.

59) Which method does display the messages whenever there is an item selection or deselection of the CheckboxMenuItem menu?

Ans : itemStateChanged method.

60) Which is a dual state menu item?

Ans : CheckboxMenuItem.

61) Which method can be used to enable/diable a checkbox menu item?

Ans : setState(boolean).

Which of the following may a menu contain?

A separator

A check box

A menu

A button

A panel

Ans : a and c.

Which of the following may contain a menu bar?

A panel

A frame

An applet

A menu bar

A menu

Ans : b

64) What is the difference between a MenuItem and a CheckboxMenuItem?

Ans : The CheckboxMenuItem class extends the MenuItem class to support a menu item

that may be checked or unchecked.


65) Which of the following are true?

A Dialog can have a MenuBar.

MenuItem extends Menu.

A MenuItem can be added to a Menu.

A Menu can be added to a Menu.

Ans : c and d.

Description: one

Which colour is used to indicate instance methods in the standard "javadoc" format documentation:

1) blue

2) red

3) purple

4) orange

Answer : 2

explain

In JDK 1.1 the variabels, methods and constructors are colour coded to simplifytheir identification.

endExplain

What is the correct ordering for the import, class and package declarations when found in a single file?

1) package, import, class

2) class, import, package

3) import, package, class

4) package, class, import

Answer : 1

explain

This is my explanation for question 2

endExplain

Which methods can be legally applied to a string object?

(Multiple)

1) equals(String)

2) equals(Object)

3) trim()

4) round()

5) toString()

Answer : 1,2,3,5

What is the parameter specification for the public static void main method?

(multiple)

1) String args []

2) String [] args

3) Strings args []

4) String args

Answer : 1,2

What does the zeroth element of the string array passed to the public static void main method contain?

(multiple)

1) The name of the program

2) The number of arguments

3) The first argument if one is present

Answer : 3

Which of the following are Java keywords?

(multiple)

1) goto

2) malloc

3) extends

4) FALSE

Answer : 3

What will be the result of compiling the following code:

public class Test {

public static void main (String args []) {

int age;

age = age + 1;

System.out.println("The age is " + age);

}

}

1) Compiles and runs with no output

2) Compiles and runs printing out The age is 1

3) Compiles but generates a runtime error

4) Does not compile

5) Compiles but generates a compile time error

Answer : 4

Which of these is the correct format to use to create the literal char value a?

(multiple)

1) 'a'

2) "a"

3) new Character(a)

4) \000a

Answer : 1

What is the legal range of a byte integral type?

1) 0 - 65, 535

2) (-128) - 127

3) (-32,768) - 32,767

4) (-256) - 255

Answer : 2

Which of the following is illegal:

1) int i = 32;

2) float f = 45.0;

3) double d = 45.0;

Answer 2

What will be the result of compiling the following code:

public class Test {

static int age;

public static void main (String args []) {

age = age + 1;

System.out.println("The age is " + age);

}

}

1) Compiles and runs with no output

2) Compiles and runs printing out The age is 1

3) Compiles but generates a runtime error

4) Does not compile

5) Compiles but generates a compile time error

Answer : 2

Which of the following are correct?

(multiple)

1) 128 >> 1 gives 64

2) 128 >>> 1 gives 64

3) 128 >> 1 gives -64

4) 128 >>> 1 gives -64

Answer : 1

Which of the following return true?

(multiple)

1) "john" == new String("john")

2) "john".equals("john")

3) "john" = "john"

4) "john".equals(new Button("john"))

Answer : 2

Which of the following do not lead to a runtime error?

(multiple)

1) "john" + " was " + " here"

2) "john" + 3

3) 3 + 5

4) 5 + 5.5

answer 1,2,3,4

Which of the following are so called "short circuit" logical operators?

(multiple)

1) &

2) ||

3) &&

4) |

Answer : 2,3

Which of the following are acceptable?

(multiple)

1) Object o = new Button("A");

2) Boolean flag = true;

3) Panel p = new Frame();

4) Frame f = new Panel();

5) Panel p = new Applet();

Answer : 1,5

What is the result of compiling and running the following code:

public class Test {

static int total = 10;

public static void main (String args []) {

new Test();

}

public Test () {

System.out.println("In test");

System.out.println(this);

int temp = this.total;

if (temp > 5) {

System.out.println(temp);

}

}

}

(multiple)

1) The class will not compile

2) The compiler reports and error at line 2

3) The compiler reports an error at line 9

4) The value 10 is one of the elements printed to the standard output

5) The class compiles but generates a runtime error

Answer : 4

Which of the following is correct:

1) String temp [] = new String {"j" "a" "z"};

2) String temp [] = { "j " " b" "c"};

3) String temp = {"a", "b", "c"};

4) String temp [] = {"a", "b", "c"};

Answer 4

What is the correct declaration of an abstract method that is intended to be public:

1) public abstract void add();

2) public abstract void add() {}

3) public abstract add();

4) public virtual add();

Answer : 1

Under what situations do you obtain a default constructor?

1) When you define any class

2) When the class has no other constructors

3) When you define at least one constructor

Answer : 2

Which of the following can be used to define a constructor for this class, given the following code:

public class Test {

...

}

1) public void Test() {...}

2) public Test() {...}

3) public static Test() {...}

4) public static void Test() {...}

Answer : 2

Which of the following are acceptable to the Java compiler:

(multiple)

1) if (2 == 3) System.out.println("Hi");

2) if (2 = 3) System.out.println("Hi");

3) if (true) System.out.println("Hi");

4) if (2 != 3) System.out.println("Hi");

5) if (aString.equals("hello")) System.out.println("Hi");

Answer : 1,3,4,5

Assuming a method contains code which may raise an Exception (but not a RuntimeException), what is the correct way for a method to indicate that it expects the caller to handle that exception:

1) throw Exception

2) throws Exception

3) new Exception

4) Don't need to specify anything

Answer : 2

What is the result of executing the following code, using the parameters 4 and 0:

public void divide(int a, int b) {

try {

int c = a / b;

} catch (Exception e) {

System.out.print("Exception ");

} finally {

System.out.println("Finally");

}

1) Prints out: Exception Finally

2) Prints out: Finally

3) Prints out: Exception

4) No output

Answer : 1

Which of the following is a legal return type of a method overloading the following method:

public void add(int a) {...}

1) void

2) int

3) Can be anything

Answer : 3

Which of the following statements is correct for a method which is overriding the following method:

public void add(int a) {...}

1) the overriding method must return void

2) the overriding method must return int

3) the overriding method can return whatever it likes

Answer : 1

Given the following classes defined in separate files, what will be the effect of compiling and running this class Test?

class Vehicle {

public void drive() {

System.out.println("Vehicle: drive");

}

}

class Car extends Vehicle {

public void drive() {

System.out.println("Car: drive");

}

}

public class Test {

public static void main (String args []) {

Vehicle v;

Car c;

v = new Vehicle();

c = new Car();

v.drive();

c.drive();

v = c;

v.drive();

}

}

1) Generates a Compiler error on the statement v= c;

2) Generates runtime error on the statement v= c;

3) Prints out:

Vehicle: drive

Car: drive

Car: drive

4) Prints out:

Vehicle: drive

Car: drive

Vehicle: drive

Answer : 3

Where in a constructor, can you place a call to a constructor defined in the super class?

1) Anywhere

2) The first statement in the constructor

3) The last statement in the constructor

4) You can't call super in a constructor

Answer : 2

Which variables can an inner class access from the class which encapsulates it?

(multiple)

1) All static variables

2) All final variables

3) All instance variables

4) Only final instance variables

5) Only final static variables

Answer : 1,2,3

What class must an inner class extend:

1) The top level class

2) The Object class

3) Any class or interface

4) It must extend an interface

Answer 3

In the following code, which is the earliest statement, where the object originally held in e, may be garbage collected:

1. public class Test {

2. public static void main (String args []) {

3. Employee e = new Employee("Bob", 48);

4. e.calculatePay();

5. System.out.println(e.printDetails());

6. e = null;

7. e = new Employee("Denise", 36);

8. e.calculatePay();

9. System.out.println(e.printDetails());

10. }

11. }

1) Line 10

2) Line 11

3) Line 7

4) Line 8

5) Never

Answer : 3

What is the name of the interface that can be used to define a class that can execute within its own thread?

1) Runnable

2) Run

3) Threadable

4) Thread

5) Executable

Answer : 1

What is the name of the method used to schedule a thread for execution?

1) init();

2) start();

3) run();

4) resume();

5) sleep();

Answer : 2

Which methods may cause a thread to stop executing?

(multiple)

1) sleep();

2) stop();

3) yield();

4) wait();

5) notify();

6) notifyAll()

7) synchronized()

Answer : 1,2,3,4

Which of the following would create a text field able to display 10 characters (assuming a fixed size font) displaying the initial string "hello":

1) new TextField("hello", 10);

2) new TextField("hello");

3) new textField(10);

4) new TextField();

Answer : 1

Which of the following methods are defined on the Graphics class:

(multiple)

1) drawLine(int, int, int, int)

2) drawImage(Image, int, int, ImageObserver)

3) drawString(String, int, int)

4) add(Component);

5) setVisible(boolean);

6) setLayout(Object);

Answer : 1,2,3

Which of the following layout managers honours the preferred size of a component:

(multiple)

1) CardLayout

2) FlowLayout

3) BorderLayout

4) GridLayout

Answer : 2

Given the following code what is the effect of a being 5:

public class Test {

public void add(int a) {

loop: for (int i = 1; i < 3; i++){

for (int j = 1; j < 3; j++) {

if (a == 5) {

break loop;

}

System.out.println(i * j);

}

}

}

}

1) Generate a runtime error

2) Throw an ArrayIndexOutOfBoundsException

3) Print the values: 1, 2, 2, 4

4) Produces no output

Answer : 4

What is the effect of issuing a wait() method on an object

1) If a notify() method has already been sent to that object then it has no effect

2) The object issuing the call to wait() will halt until another object sends a notify() or notifyAll() method

3) An exception will be raised

4) The object issuing the call to wait() will be automatically synchronized with any other objects using the receiving object.

Answer : 2

The layout of a container can be altered using which of the following methods:

(multiple)

1) setLayout(aLayoutManager);

2) addLayout(aLayoutManager);

3) layout(aLayoutManager);

4) setLayoutManager(aLayoutManager);

Answer : 1

Using a FlowLayout manager, which is the correct way to add elements to a container:

1) add(component);

2) add("Center", component);

3) add(x, y, component);

4) set(component);

Answer : 1

Given that a Button can generate an ActionEvent which listener would you expect to have to implement, in a class which would handle this event?

1) FocusListener

2) ComponentListener

3) WindowListener

4) ActionListener

5) ItemListener

Answer : 4

Which of the following, are valid return types, for listener methods:

1) boolean

2) the type of event handled

3) void

4) Component

Answer : 3

Assuming we have a class which implements the ActionListener interface, which method should be used to register this with a Button?

1) addListener(*);

2) addActionListener(*);

3) addButtonListener(*);

4) setListener(*);

Answer : 2

In order to cause the paint(Graphics) method to execute, which of the following is the most appropriate method to call:

1) paint()

2) repaint()

3) paint(Graphics)

4) update(Graphics)

5) None - you should never cause paint(Graphics) to execute

Answer : 2

Which of the following illustrates the correct way to pass a parameter into an applet:

1) <applet code=Test.class age=33 width=100 height=100>

2) <param name=age value=33>

3) <applet code=Test.class name=age value=33 width=100 height=100>

4) <applet Test 33>

Answer : 2

Which of the following correctly illustrate how an InputStreamReader can be created:

(multiple)

1) new InputStreamReader(new FileInputStream("data"));

2) new InputStreamReader(new FileReader("data"));

3) new InputStreamReader(new BufferedReader("data"));

4) new InputStreamReader("data");

5) new InputStreamReader(System.in);

Answer : 1,5

What is the permanent effect on the file system of writing data to a new FileWriter("report"), given the file report already exists?

1) The data is appended to the file

2) The file is replaced with a new file

3) An exception is raised as the file already exists

4) The data is written to random locations within the file

Answer : 2

What is the effect of adding the sixth element to a vector created in the following manner:

new Vector(5, 10);

1) An IndexOutOfBounds exception is raised.

2) The vector grows in size to a capacity of 10 elements

3) The vector grows in size to a capacity of 15 elements

4) Nothing, the vector will have grown when the fifth element was added

Answer : 3

What is the result of executing the following code when the value of x is 2:

switch (x) {

case 1:

System.out.println(1);

case 2:

case 3:

System.out.println(3);

case 4:

System.out.println(4);

}

1) Nothing is printed out

2) The value 3 is printed out

3) The values 3 and 4 are printed out

4) The values 1, 3 and 4 are printed out

Answer : 3

What is the result of compiling and running the Second class?

Consider the following example:

class First {

public First (String s) {

System.out.println(s);

}

}

public class Second extends First {

public static void main(String args []) {

new Second();

}

}

1) Nothing happens

2) A string is printed to the standard out

3) An instance of the class First is generated

4) An instance of the class Second is created

5) An exception is raised at runtime stating that there is no null parameter constructor in class First.

6) The class second will not compile as there is no null parameter constructor in the class First

Answer : 6

What is the result of executing the following fragment of code:

boolean flag = false;

if (flag = true) {

System.out.println("true");

} else {

System.out.println("false");

}

1) true is printed to standard out

2) false is printed to standard out

3) An exception is raised

4) Nothing happens

Answer : 1

Consider the following classes. What is the result of compiling and running this class?

public class Test {

public static void test() {

this.print();

}

public static void print() {

System.out.println("Test");

}

public static void main(String args []) {

test();

}

}

(multiple)

1) The string Test is printed to the standard out.

2) A runtime exception is raised stating that an object has not been created.

3) Nothing is printed to the standard output.

4) An exception is raised stating that the method test cannot be found.

5) An exception is raised stating that the variable this can only be used within an instance.

6) The class fails to compile stating that the variable this is undefined.

Answer : 6

Examine the following class definition:

public class Test {

public static void test() {

print();

}

public static void print() {

System.out.println("Test");

}

public void print() {

System.out.println("Another Test");

}

}

What is the result of compiling this class:

1) A successful compilation.

2) A warning stating that the class has no main method.

3) An error stating that there is a duplicated method.

4) An error stating that the method test() will call one or other of the print() methods.

Answer : 3

What is the result of compiling and executing the following Java class:

public class ThreadTest extends Thread {

public void run() {

System.out.println("In run");

suspend();

resume();

System.out.println("Leaving run");

}

public static void main(String args []) {

(new ThreadTest()).start();

}

}

1) Compilation will fail in the method main.

2) Compilation will fail in the method run.

3) A warning will be generated for method run.

4) The string "In run" will be printed to standard out.

5) Both strings will be printed to standard out.

6) Nothing will happen.

Answer : 4

Given the following sequence of Java statements, Which of the following options are true:

1. StringBuffer sb = new StringBuffer("abc");

2. String s = new String("abc");

3. sb.append("def");

4. s.append("def");

5. sb.insert(1, "zzz");

6. s.concat(sb);

7. s.trim();

(multiple)

1) The compiler would generate an error for line 1.

2) The compiler would generate an error for line 2.

3) The compiler would generate an error for line 3.

4) The compiler would generate an error for line 4.

5) The compiler would generate an error for line 5.

6) The compiler would generate an error for line 6.

7) The compiler would generate an error for line 7.

Answer : 4,6

What is the result of executing the following Java class:

import java.awt.*;

public class FrameTest extends Frame {

public FrameTest() {

add (new Button("First"));

add (new Button("Second"));

add (new Button("Third"));

pack();

setVisible(true);

}

public static void main(String args []) {

new FrameTest();

}

}

1) Nothing happens.

2) Three buttons are displayed across a window.

3) A runtime exception is generated (no layout manager specified).

4) Only the "first" button is displayed.

5) Only the "second" button is displayed.

6) Only the "third" button is displayed.

Answer : 6

Consider the following tags and attributes of tags, which can be used with the <AAPLET> and </APPLET> tags?

1. CODEBASE

2. ALT

3. NAME

4. CLASS

5. JAVAC

6. HORIZONTALSPACE

7. VERTICALSPACE

8. WIDTH

9. PARAM

10. JAR

(multiple)

1) line 1, 2, 3

2) line 2, 5, 6, 7

3) line 3, 4, 5

4) line 8, 9, 10

5) line 8, 9

Answer : 1,5

Which of the following is a legal way to construct a RandomAccessFile:

1) RandomAccessFile("data", "r");

2) RandomAccessFile("r", "data");

3) RandomAccessFile("data", "read");

4) RandomAccessFile("read", "data");

Answer : 1

Carefully examine the following code, When will the string "Hi there" be printed?

public class StaticTest {

static {

System.out.println("Hi there");

}

public void print() {

System.out.println("Hello");

}

public static void main(String args []) {

StaticTest st1 = new StaticTest();

st1.print();

StaticTest st2 = new StaticTest();

st2.print();

}

}

1) Never.

2) Each time a new instance is created.

3) Once when the class is first loaded into the Java virtual machine.

4) Only when the static method is called explicitly.

Answer : 3

What is the result of the following program:

public class Test {

public static void main (String args []) {

boolean a = false;

if (a = true)

System.out.println("Hello");

else

System.out.println("Goodbye");

}

}

1) Program produces no output but terminates correctly.

2) Program does not terminate.

3) Prints out "Hello"

4) Prints out "Goodbye"

Answer : 3

Examine the following code, it includes an inner class, what is the result:

public final class Test4 {

class Inner {

void test() {

if (Test4.this.flag); {

sample();

}

}

}

private boolean flag = true;

public void sample() {

System.out.println("Sample");

}

public Test4() {

(new Inner()).test();

}

public static void main(String args []) {

new Test4();

}

}

1) Prints out "Sample"

2) Program produces no output but terminates correctly.

3) Program does not terminate.

4) The program will not compile

Answer : 1

Carefully examine the following class:

public class Test5 {

public static void main (String args []) {

/* This is the start of a comment

if (true) {

Test5 = new test5();

System.out.println("Done the test");

}

/* This is another comment */

System.out.println ("The end");

}

}

1) Prints out "Done the test" and nothing else.

2) Program produces no output but terminates correctly.

3) Program does not terminate.

4) The program will not compile.

5) The program generates a runtime exception.

6) The program prints out "The end" and nothing else.

7) The program prints out "Done the test" and "The end"

Answer : 6

What is the result of compiling and running the following applet:

import java.applet.Applet;

import java.awt.*;

public class Sample extends Applet {

private String text = "Hello World";

public void init() {

add(new Label(text));

}

public Sample (String string) {

text = string;

}

}

It is accessed form the following HTML page:

<html>

<title>Sample Applet</title>

<body>

<applet code="Sample.class" width=200 height=200></applet>

</body>

</html>

1) Prints "Hello World".

2) Generates a runtime error.

3) Does nothing.

4) Generates a compile time error.

Answer : 2

What is the effect of compiling and (if possible) running this class:

public class Calc {

public static void main (String args []) {

int total = 0;

for (int i = 0, j = 10; total > 30; ++i, --j) {

System.out.println(" i = " + i + " : j = " + j);

total += (i + j);

}

System.out.println("Total " + total);

}

}

1) Produce a runtime error

2) Produce a compile time error

3) Print out "Total 0"

4) Generate the following as output:

i = 0 : j = 10

i = 1 : j = 9

i = 2 : j = 8

Total 30

Answer : 3

Utility Package

1) What is the Vector class?

ANSWER : The Vector class provides the capability to implement a growable array of objects.

2) What is the Set interface?

ANSWER : The Set interface provides methods for accessing the elements of a finite mathematical set.Sets do not allow duplicate elements.

3) What is Dictionary class?

ANSWER : The Dictionary class is the abstarct super class of Hashtable and Properties class.Dictionary provides the abstarct functions used to store and retrieve objects by key-value.This class allows any object to be used as a key or value.

4) What is the Hashtable class?

ANSWER : The Hashtable class implements a hash table data structure. A hash table indexes and stores objects in a dictionary using hash codes as the objects' keys. Hash codes are integer values that identify objects.

5) What is the Properties class?

Answer : The properties class is a subclass of Hashtable that can be read from or written to a stream.It also provides the capability to specify a set of default values to be used if a specified key is not found in the table. We have two methods load() and save().

6) What changes are needed to make the following prg to compile?

import java.util.*;

class Ques{

public static void main (String args[]) {

String s1 = "abc";

String s2 = "def";

Vector v = new Vector();

v.add(s1);

v.add(s2);

String s3 = v.elementAt(0) + v.elementAt(1);

System.out.println(s3);

}

}

ANSWER : Declare Ques as public B) Cast v.elementAt(0) to a String

C) Cast v.elementAt(1) to an Object. D) Import java.lang

ANSWER : B) Cast v.elementAt(0) to a String

8) What is the output of the prg.

import java.util.*;

class Ques{

public static void main (String args[]) {

String s1 = "abc";

String s2 = "def";

Stack stack = new Stack();

stack.push(s1);

stack.push(s2);

try{

String s3 = (String) stack.pop() + (String) stack.pop() ;

System.out.println(s3);

}catch (EmptyStackException ex){}

}

}

ANSWER : abcdef B) defabc C) abcabc D) defdef

ANSWER : B) defabc

9) Which of the following may have duplicate elements?

ANSWER : Collection B) List C) Map D) Set

ANSWER : A and B Neither a Map nor a Set may have duplicate elements.

10) Can null value be added to a List?

ANSWER : Yes.A Null value may be added to any List.

11) What is the output of the following prg.

import java.util.*;

class Ques{

public static void main (String args[]) {

HashSet set = new HashSet();

String s1 = "abc";

String s2 = "def";

String s3 = "";

set.add(s1);

set.add(s2);

set.add(s1);

set.add(s2);

Iterator i = set.iterator();

while(i.hasNext())

{

s3 += (String) i.next();

}

System.out.println(s3);

}

}

A) abcdefabcdef B) defabcdefabc C) fedcbafedcba D) defabc

ANSWER : D) defabc. Sets may not have duplicate elements.

12) Which of the following java.util classes support internationalization?

A) Locale B) ResourceBundle C) Country D) Language

ANSWER : A and B . Country and Language are not java.util classes.

13) What is the ResourceBundle?

The ResourceBundle class also supports internationalization.

ResourceBundle subclasses are used to store locale-specific resources that can be loaded by a program to tailor the program's appearence to the paticular locale in which it is being run. Resource Bundles provide the capability to isolate a program's locale-specific resources in a standard and modular manner.

14) How are Observer Interface and Observable class, in java.util package, used?

ANSWER : Objects that subclass the Observable class maintain a list of Observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

15) Which java.util classes and interfaces support event handling?

ANSWER : The EventObject class and the EventListener interface support event processing.

16) Does java provide standard iterator functions for inspecting a collection of objects?

ANSWER : The Enumeration interface in the java.util package provides a framework for stepping once through a collection of objects. We have two methods in that interface.

public interface Enumeration {

boolean hasMoreElements();

Object nextElement();

}

17) The Math.random method is too limited for my needs- How can I generate random numbers more flexibly?

ANSWER : The random method in Math class provide quick, convienient access to random numbers, but more power and flexibility use the Random class in the java.util package.

double doubleval = Math.random();

The Random class provide methods returning float, int, double, and long values.

nextFloat() // type float; 0.0 <= value < 1.0

nextDouble() // type double; 0.0 <= value < 1.0

nextInt() // type int; Integer.MIN_VALUE <= value <= Integer.MAX_VALUE

nextLong() // type long; Long.MIN_VALUE <= value <= Long.MAX_VALUE

nextGaussian() // type double; has Gaussian("normal") distribution with mean 0.0 and standard deviation 1.0)

Eg. Random r = new Random();

float floatval = r.nextFloat();

18) How can we get all public methods of an object dynamically?

ANSWER : By using getMethods(). It return an array of method objects corresponding to the public methods of this class.

getFields() returns an array of Filed objects corresponding to the public Fields(variables) of this class.

getConstructors() returns an array of constructor objects corresponding to the public constructors of this class.

JDBC

1) What are the steps involved in establishing a connection?

ANSWER : This involves two steps: (1) loading the driver and (2) making the connection.

2) How can you load the drivers?

ANSWER : Loading the driver or drivers you want to use is very simple and involves just one line of code. If, for example, you want to use the JDBC-ODBC Bridge driver, the following code will load it:

Eg.

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

Your driver documentation will give you the class name to use. For instance, if the class name is jdbc.DriverXYZ , you would load the driver with the following line of code:

Eg.

Class.forName("jdbc.DriverXYZ");

3) What Class.forName will do while loading drivers?

ANSWER : It is used to create an instance of a driver and register it with the DriverManager.

When you have loaded a driver, it is available for making a connection with a DBMS.

4) How can you make the connection?

ANSWER : In establishing a connection is to have the appropriate driver connect to the DBMS. The following line of code illustrates the general idea:

Eg.

String url = "jdbc:odbc:Fred";

Connection con = DriverManager.getConnection(url, "Fernanda", "J8");

5) How can you create JDBC statements?

ANSWER : A Statement object is what sends your SQL statement to the DBMS. You simply create a Statement object and then execute it, supplying the appropriate execute method with the SQL statement you want to send. For a SELECT statement, the method to use is executeQuery. For statements that create or modify tables, the method to use is executeUpdate.

Eg.

It takes an instance of an active connection to create a Statement object. In the following example, we use our Connection object con to create the Statement object stmt :

Statement stmt = con.createStatement();

6) How can you retrieve data from the ResultSet?

ANSWER : Step 1.

JDBC returns results in a ResultSet object, so we need to declare an instance of the class ResultSet to hold our results. The following code demonstrates declaring the ResultSet object rs.

Eg.

ResultSet rs = stmt.executeQuery("SELECT COF_NAME, PRICE FROM COFFEES");

Step2.

String s = rs.getString("COF_NAME");

The method getString is invoked on the ResultSet object rs , so getString will retrieve (get) the value stored in the column COF_NAME in the current row of rs

7) What are the different types of Statements?

ANSWER : 1.Statement (use createStatement method) 2. Prepared Statement (Use prepareStatement method) and 3. Callable Statement (Use prepareCall)

8) How can you use PreparedStatement?

ANSWER : This special type of statement is derived from the more general class, Statement.If you want to execute a Statement object many times, it will normally reduce execution time to use a PreparedStatement object instead.

The advantage to this is that in most cases, this SQL statement will be sent to the DBMS right away, where it will be compiled. As a result, the PreparedStatement object contains not just an SQL statement, but an SQL statement that has been precompiled. This means that when the PreparedStatement is executed, the DBMS can just run the PreparedStatement 's SQL statement without having to compile it first.

Eg.

PreparedStatement updateSales = con.prepareStatement("UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ?");

9) What setAutoCommit does?

ANSWER : When a connection is created, it is in auto-commit mode. This means that each individual SQL statement is treated as a transaction and will be automatically committed right after it is executed. The way to allow two or more statements to be grouped into a transaction is to disable auto-commit mode

Eg.

con.setAutoCommit(false);

Once auto-commit mode is disabled, no SQL statements will be committed until you call the method commit explicitly.

Eg.

con.setAutoCommit(false);

PreparedStatement updateSales = con.prepareStatement(

"UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ?");

updateSales.setInt(1, 50);

updateSales.setString(2, "Colombian");

updateSales.executeUpdate();

PreparedStatement updateTotal = con.prepareStatement("UPDATE COFFEES SET TOTAL = TOTAL + ? WHERE COF_NAME LIKE ?");

updateTotal.setInt(1, 50);

updateTotal.setString(2, "Colombian");

updateTotal.executeUpdate();

con.commit();

con.setAutoCommit(true);

10) How to call a Strored Procedure from JDBC?

ANSWER : The first step is to create a CallableStatement object. As with Statement an and PreparedStatement objects, this is done with an open Connection

object. A CallableStatement object contains a call to a stored procedure;

Eg.

CallableStatement cs = con.prepareCall("{call SHOW_SUPPLIERS}");

ResultSet rs = cs.executeQuery();

11) How to Retrieve Warnings?

ANSWER : SQLWarning objects are a subclass of SQLException that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something did not happen as planned.

A warning can be reported on a Connection object, a Statement object (including PreparedStatement and CallableStatement objects), or a ResultSet object. Each of these classes has a getWarnings method, which you must invoke in order to see the first warning reported on the calling object

Eg.

SQLWarning warning = stmt.getWarnings();

if (warning != null) {

System.out.println("\n---Warning---\n");

while (warning != null) {

System.out.println("Message: " + warning.getMessage());

System.out.println("SQLState: " + warning.getSQLState());

System.out.print("Vendor error code: ");

System.out.println(warning.getErrorCode());

System.out.println("");

warning = warning.getNextWarning();

}

}

12) How can you Move the Cursor in Scrollable Result Sets ?

ANSWER : One of the new features in the JDBC 2.0 API is the ability to move a result set's cursor backward as well as forward. There are also methods that let you move the cursor to a particular row and check the position of the cursor.

Eg.

Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,

ResultSet.CONCUR_READ_ONLY);

ResultSet srs = stmt.executeQuery("SELECT COF_NAME, PRICE FROM COFFEES");

The first argument is one of three constants added to the ResultSet API to indicate the type of a ResultSet object: TYPE_FORWARD_ONLY, TYPE_SCROLL_INSENSITIVE , and TYPE_SCROLL_SENSITIVE .

The second argument is one of two ResultSet constants for specifying whether a result set is read-only or updatable: CONCUR_READ_ONLY and CONCUR_UPDATABLE . The point to remember here is that if you specify a type, you must also specify whether it is read-only or updatable. Also, you must specify the type first, and because both parameters are of type int , the compiler will not complain if you switch the order.

Specifying the constant TYPE_FORWARD_ONLY creates a nonscrollable result set, that is, one in which the cursor moves only forward. If you do not specify any constants for the type and updatability of a ResultSet object, you will automatically get one that is TYPE_FORWARD_ONLY and CONCUR_READ_ONLY

13) What’s the difference between TYPE_SCROLL_INSENSITIVE , and TYPE_SCROLL_SENSITIVE?

ANSWER : You will get a scrollable ResultSet object if you specify one of these ResultSet constants.The difference between the two has to do with whether a result set reflects changes that are made to it while it is open and whether certain methods can be called to detect these changes. Generally speaking, a result set that is TYPE_SCROLL_INSENSITIVE does not reflect changes made while it is still open and one that is TYPE_SCROLL_SENSITIVE does. All three types of result sets will make changes visible if they are closed and then reopened

Eg.

Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);

ResultSet srs = stmt.executeQuery("SELECT COF_NAME, PRICE FROM COFFEES");

srs.afterLast();

while (srs.previous()) {

String name = srs.getString("COF_NAME");

float price = srs.getFloat("PRICE");

System.out.println(name + " " + price);

}

14) How to Make Updates to Updatable Result Sets?

ANSWER : Another new feature in the JDBC 2.0 API is the ability to update rows in a result set using methods in the Java programming language rather than having to send an SQL command. But before you can take advantage of this capability, you need to create a ResultSet object that is updatable. In order to do this, you supply the ResultSet constant CONCUR_UPDATABLE to the createStatement method.

Eg.

Connection con = DriverManager.getConnection("jdbc:mySubprotocol:mySubName");

Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,

ResultSet.CONCUR_UPDATABLE);

ResultSet uprs = stmt.executeQuery("SELECT COF_NAME, PRICE FROM COFFEES");

Networking Concepts

1) The API doesn't list any constructors for InetAddress- How do I create an InetAddress instance?

ANSWER : In case of InetAddress the three methods getLocalHost, getByName, getByAllName can be used to create instances.

E.g.

InetAddress add1;

InetAddress add2;

try{

add1 = InetAddress.getByName("java.sun.com");

add2 = InetAddress.getByName("199.22.22.22");

}catch(UnknownHostException e){}

2) Is it possible to get the Local host IP?

ANSWER : Yes. Use InetAddress's getLocalHost method.

3) What's the Factory Method?

ANSWER : Factory methods are merely a convention whereby static methods in a class return an instance of that class. The InetAddress class has no visible constructors. To create an InetAddress object, you have to use one of the available factory methods. In InetAddress the three methods getLocalHost, getByName, getByAllName can be used to create instances of InetAddress.

4) What’s the difference between TCP and UDP?

ANSWER : These two protocols differ in the way they carry out the action of communicating. A TCP protocol establishes a two way connection between a pair of computers, while the UDP protocol is a one-way message sender. The common analogy is that TCP is like making a phone call and carrying on a two-way communication, while UDP is like mailing a letter.

5) What is the Proxy Server?

ANSWER : A proxy server speaks the client side of a protocol to another server. This is often required when clients have certain restrictions on which servers they can connect to. And when several users are hitting a popular web site, a proxy server can get the contents of the web server's popular pages once, saving expensive internetwork transfers while providing faster access to those pages to the clients.

Also, we can get multiple connections for a single server.

6) What are the seven layers of OSI model?

ANSWER : Application, Presentation, Session, Transport, Network, DataLink, Physical Layer.

What Transport Layer does?

ANSWER : It ensures that the mail gets to its destination. If a packet fails to get its destination, it handles the process of notifying the sender and requesting that another packet be sent.

8) What is DHCP?

ANSWER : Dynamic Host Configuration Protocol, a piece of the TCP/IP protocol suite that handles the automatic assignment of IP addresses to clients.

9) What is SMTP?

ANSWER : Simple Mail Transmission Protocol, the TCP/IP Standard for Internet mails. SMTP exchanges mail between servers; contrast this with POP, which transmits mail between a server and a client.

10) In OSI N/w architecture, the dialogue control and token management are responsibilities of...

Answer : Network b) Session c) Application d) DataLink

ANSWER : b) Session Layer.

11) In OSI N/W Architecture, the routing is performed by ______

Answer : Network b) Session c) Application d) DataLink

ANSWER : Answer : Network Layer.

Networking

What is the difference between URL instance and URLConnection instance?

ANSWER : A URL instance represents the location of a resource, and a URLConnection instance represents a link for accessing or communicating with the resource at the location.

2) How do I make a connection to URL?

ANSWER : You obtain a URL instance and then invoke openConnection on it.

URLConnection is an abstract class, which means you can't directly create instances of it using a constructor. We have to invoke openConnection method on a URL instance, to get the right kind of connection for your URL.

Eg. URL url;

URLConnection connection;

try{ url = new URL("...");

conection = url.openConnection();

}catch (MalFormedURLException e) { }

3) What Is a Socket?

A socket is one end-point of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent.Socket classes are used to represent the connection between a client program and a server program. The java.net package provides two classes--Socket and ServerSocket--which implement the client side of the connection and the server side of the connection, respectively.

What information is needed to create a TCP Socket?

ANSWER : The Local System’s IP Address and Port Number.

And the Remote System's IPAddress and Port Number.

5) What are the two important TCP Socket classes?

ANSWER : Socket and ServerSocket.

ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through the sockets.

getInputStream() and getOutputStream() are the two methods available in Socket class.

When MalformedURLException and UnknownHostException throws?

ANSWER : When the specified URL is not connected then the URL throw MalformedURLException and If InetAddress’ methods getByName and getLocalHost are unabletoresolve the host name they throwan UnknownHostException.

Servlets

1) What is the servlet?

ANSWER : Servlets are modules that extend request/response-oriented servers, such as Java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company's order database.

Servlets are to servers what applets are to browsers. Unlike applets, however, servlets have no graphical user interface.

2) Whats the advantages using servlets than using CGI?

ANSWER : Servlets provide a way to generate dynamic documents that is both easier to write and faster to run. Servlets also address the problem of doing server-side programming with platform-specific APIs: they are developed with the Java Servlet API, a standard Java extension.

3) What are the uses of Servlets?

ANSWER : A servlet can handle multiple requests concurrently, and can synchronize requests. This allows servlets to support systems such as on-line conferencing.

Servlets can forward requests to other servers and servlets.Thus servlets can be used to balance load among several servers that mirror the same content, and to partition a single logical service over several servers, according to task type or organizational boundaries.

4) Which pakage provides interfaces and classes for writing servlets?

ANSWER : javax

5) Whats the Servlet Interfcae?

ANSWER : The central abstraction in the Servlet API is the Servlet interface. All servlets implement this interface, either directly or, more commonly, by extending a class that implements it such as HttpServlet.

Servlets-->Generic Servlet-->HttpServlet-->MyServlet.

The Servlet interface declares, but does not implement, methods that manage the servlet and its communications with clients. Servlet writers provide some or all of these methods when developing a servlet.

6) When a servlet accepts a call from a client, it receives two objects- What are they?

ANSWER : ServeltRequest: Which encapsulates the communication from the client to the server.

ServletResponse: Whcih encapsulates the communication from the servlet back to the client.

ServletRequest and ServletResponse are interfaces defined by the javax.servlet package.

7) What information that the ServletRequest interface allows the servlet access to?

ANSWER : Information such as the names of the parameters passed in by the client, the protocol (scheme) being used by the client, and the names of the remote host that made the request and the server that received it.

The input stream, ServletInputStream.Servlets use the input stream to get data from clients that use application protocols such as the HTTP POST and PUT methods.

8) What information that the ServletResponse interface gives the servlet methods for replying to the client?

ANSWER : It Allows the servlet to set the content length and MIME type of the reply.

Provides an output stream, ServletOutputStream and a Writer through which the servlet can send the reply data.

9) What is the servlet Lifecycle?

ANSWER : Each servlet has the same life cycle:

A server loads and initializes the servlet (init())

The servlet handles zero or more client requests (service())

The server removes the servlet (destroy())

(some servers do this step only when they shut down)

10) How HTTP Servlet handles client requests?

ANSWER : An HTTP Servlet handles client requests through its service method. The service method supports standard HTTP client requests by dispatching each request to a method designed to handle that request. 1

Encapsulation :

Encapsulation is the mechanism that binds together code and the data it manipulates and keeps both safe from outside interference and misuse.

Inheritance:

Inheritance is the process by which one object acquires the properties of another object.

Polymorphism :

Polymorphism is a feature that allows one interface to be used for a general class of actions. The specific action is determined by the exact nature of actions.

Code Blocks :

Two or more statements which is allowed to be grouped into blocks of code is otherwise called as Code Blocks.This is done by enclosing the statements between opening and closing curly braces.

Floating-point numbers:

Floating-point numbers which is also known as real numbers, are used when evaluating expressions that require fractional precision.

Unicode:

Unicode defines a fully international character set that can represent all of the characters found in all human languages. It is a unification of dozens of character sets, such as Latin, Greek, Arabic and many more.

Booleans:

Java has a simple type called boolean, for logical values. It can have only on of two possible values, true or false.

Casting:

A cast is simply an explicit type conversion. To create a conversion between two incompatible types, you must use a cast.

Arrays:

An array is a group of like-typed variables that are referred to by a common name. Arrays offer a convenient means of grouping related information. Arrays of any type can be created and may have one or more dimension.

Relational Operators:

The relational operators determine the relationship that one operand has to the other. They determine the equality and ordering.

11.Short-Circuit Logical Operators:

The secondary versions of the Boolean AND and OR operators are known as short-

circuit logical operators. It is represented by || and &&..

12. Switch:

The switch statement is Java’s multiway branch statement. It provides an easy way to

dispatch execution to different parts of your code based on the value of an

experession.

13. Jump Statements:

Jump statements are the statements which transfer control to another part of your

program. Java Supports three jump statements: break, continue, and return.

14. Instance Variables:

The data, or variable, defined within a class are called instance variable.

1. What is a Marker Interface? - An interface with no methods. Example: Serializable, Remote, Cloneable

2. What interface do you implement to do the sorting? - Comparable

3. What is the eligibility for a object to get cloned? - It must implement the Cloneable interface

4. What is the purpose of abstract class? - It is not an instantiable class. It provides the concrete implementation for some/all the methods. So that they can reuse the concrete functionality by inheriting the abstract class.

5. What is the difference between interface and abstract class? - Abstract class defined with methods. Interface will declare only the methods. Abstract classes are very much useful when there is a some functionality across various classes. Interfaces are well suited for the classes which varies in functionality but with the same method signatures.

6. What do you mean by RMI and how it is useful? - RMI is a remote method invocation. Using RMI, you can work with remote object. The function calls are as though you are invoking a local variable. So it gives you a impression that you are working really with a object that resides within your own JVM though it is somewhere.

7. What is the protocol used by RMI? - RMI-IIOP

8. What is a hashCode? - hash code value for this object which is unique for every object.

9. What is a thread? - Thread is a block of code which can execute concurrently with other threads in the JVM.

10. What is the algorithm used in Thread scheduling? - Fixed priority scheduling.

11. What is hash-collision in Hashtable and how it is handled in Java? - Two different keys with the same hash value. Two different entries will be kept in a single hash bucket to avoid the collision.

12. What are the different driver types available in JDBC? - 1. A JDBC-ODBC bridge 2. A native-API partly Java technology-enabled driver 3. A net-protocol fully Java technology-enabled driver 4. A native-protocol fully Java technology-enabled driver For more information: Driver Description

13. Is JDBC-ODBC bridge multi-threaded? - No

14. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection? - No

15. What is the use of serializable? - To persist the state of an object into any perminant storage device.

16. What is the use of transient? - It is an indicator to the JVM that those variables should not be persisted. It is the users responsibility to initialize the value when read back from the storage.

17. What are the different level lockings using the synchronization keyword? - Class level lock Object level lock Method level lock Block level lock

18. What is the use of preparedstatement? - Preparedstatements are precompiled statements. It is mainly used to speed up the process of inserting/updating/deleting especially when there is a bulk processing.

19. What is callable statement? Tell me the way to get the callable statement? - Callablestatements are used to invoke the stored procedures. You can obtain the callablestatement from Connection using the following methods prepareCall(String sql) prepareCall(String sql, int resultSetType, int resultSetConcurrency)

20. In a statement, I am executing a batch. What is the result of the execution? - It returns the int array. The array contains the affected row count in the corresponding index of the SQL.

21. Can a abstract method have the static qualifier? - No

22. What are the different types of qualifier and what is the default qualifier? - public, protected, private, package (default)

23. What is the super class of Hashtable? - Dictionary

24. What is a lightweight component? - Lightweight components are the one which doesn’t go with the native call to obtain the graphical units. They share their parent component graphical units to render them. Example, Swing components

25. What is a heavyweight component? - For every paint call, there will be a native call to get the graphical units. Example, AWT.

26. What is an applet? - Applet is a program which can get downloaded into a client environment and start executing there.

27. What do you mean by a Classloader? - Classloader is the one which loads the classes into the JVM.

28. What are the implicit packages that need not get imported into a class file? - java.lang

29. What is the difference between lightweight and heavyweight component? - Lightweight components reuses its parents graphical units. Heavyweight components goes with the native graphical unit for every component. Lightweight components are faster than the heavyweight components.

30. What are the ways in which you can instantiate a thread? - Using Thread class By implementing the Runnable interface and giving that handle to the Thread class.

31. What are the states of a thread? - 1. New 2. Runnable 3. Not Runnable 4. Dead

32. What is a socket? - A socket is an endpoint for communication between two machines.

33. How will you establish the connection between the servlet and an applet? - Using the URL, I will create the connection URL. Then by openConnection method of the URL, I will establish the connection, through which I can be able to exchange data.

34. What are the threads will start, when you start the java program? - Finalizer, Main, Reference Handler, Signal Dispatcher

What is garbage collection? What is the process that is responsible
for doing that in java? - Reclaiming the unused memory by the invalid
objects. Garbage collector is responsible for this process
# What kind of thread is the Garbage collector thread? - It is a
daemon thread.
# What is a daemon thread? - These are the threads which can run
without user intervention. The JVM can exit when there are daemon
thread by killing them abruptly.
# How will you invoke any external process in Java? -
Runtime.getRuntime().exec(….)
# What is the finalize method do? - Before the invalid objects get
garbage collected, the JVM give the user a chance to clean up some
resources before it got garbage collected.


What is Collection API ?
The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements of a Collection.

What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
Interfaces provide a form of multiple inheritance. A class can extend only one other class. Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.
Similarities:

Neither Abstract classes or Interface can be instantiated.

Java Interview Questions - How to define an Abstract class?
A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
public abstract string anyAbstractFunction();
}

How to define an Interface in Java ?
In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
public void functionOne();

public long CONSTANT_ONE = 1000;
}

If a class is located in a package, what do you need to change in the OS environment to be able to use it?

You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee

How many methods in the Serializable interface?
There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable.

How many methods in the Externalizable interface?
There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal().

What is the difference between Serializalble and Externalizable interface?
When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process.

What is a transient variable in Java?
A transient variable is a variable that may not be serialized. If you don't want some field to be serialized, you can mark that field transient or static.

Which containers use a border layout as their default layout?
The Window, Frame and Dialog classes use a border layout as their default layout.

How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

What is Java?
Java is an object-oriented programming language developed initially by James Gosling and colleagues at Sun Microsystems. The language, initially called Oak (named after the oak trees outside Gosling's office), was intended to replace C++, although the feature set better resembles that of Objective C. Java should not be confused with JavaScript, which shares only the name and a similar C-like syntax. Sun Microsystems currently maintains and updates Java regularly.

What does a well-written OO program look like?
A well-written OO program exhibits recurring structures that promote abstraction, flexibility, modularity and elegance.

Can you have virtual functions in Java?
Yes, all functions in Java are virtual by default. This is actually a pseudo trick question because the word "virtual" is not part of the naming convention in Java (as it is in C++, C-sharp and VB.NET), so this would be a foreign concept for someone who has only coded in Java. Virtual functions or virtual methods are functions or methods that will be redefined in derived classes.

Jack developed a program by using a Map container to hold key/value pairs. He wanted to make a change to the map. He decided to make a clone of the map in order to save the original data on side. What do you think of it? ?
If Jack made a clone of the map, any changes to the clone or the original map would be seen on both maps, because the clone of Map is a shallow copy. So Jack made a wrong decision.

What is more advisable to create a thread, by implementing a Runnable interface or by extending Thread class?
Strategically speaking, threads created by implementing Runnable interface are more advisable. If you create a thread by extending a thread class, you cannot extend any other class. If you create a thread by implementing Runnable interface, you save a space for your class to extend another class now or in future.

What is NullPointerException and how to handle it?

When an object is not initialized, the default value is null. When the following things happen, the NullPointerException is thrown:
--Calling the instance method of a null object.
--Accessing or modifying the field of a null object.
--Taking the length of a null as if it were an array.
--Accessing or modifying the slots of null as if it were an array.
--Throwing null as if it were a Throwable value.
The NullPointerException is a runtime exception. The best practice is to catch such exception even if it is not required by language design.

An application needs to load a library before it starts to run, how to code?
One option is to use a static block to load a library before anything is called. For example,
class Test {
static {
System.loadLibrary("path-to-library-file");
}
....
}
When you call new Test(), the static block will be called first before any initialization happens. Note that the static block position may matter.

How could Java classes direct program messages to the system console, but error messages, say to a file?
The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

What's the difference between an interface and an abstract class?
An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

Name the containers which uses Border Layout as their default layout?
Containers which uses Border Layout as their default are: window, Frame and Dialog classes.

What do you understand by Synchronization?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value.
Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}

What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often causes dirty data and leads to significant errors.

What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to a method or an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.

What's the difference between J2SDK 1.5 and J2SDK 5.0?
There's no difference, Sun Microsystems just re-branded this version.

What would you use to compare two String variables - the operator == or the method equals()?
I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

What is thread?
A thread is an independent path of execution in a system.

What is multi-threading?
Multi-threading means various threads that run in a system.

How does multi-threading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

How to create a thread in a program?
You have two ways to do so. First, making your class "extends" Thread class. Second, making your class "implements" Runnable interface. Put jobs in a run() method and call start() method to start the thread.

Can Java object be locked down for exclusive use by a given thread?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.

Can each Java object keep track of all the threads that want to exclusively access to it?
Yes. Use Thread.currentThread() method to track the accessing thread.

Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

What invokes a thread's run() method?
After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.

What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems, a process gets its own memory address space; a thread doesn't. Threads typically share the heap belonging to their parent process. For instance, a JVM runs in a single process in the host O/S. Threads in the JVM share the heap belonging to that process; that's why several threads may access the same object. Typically, even though they share a common heap, threads have their own stack space. This is how one thread's invocation of a method is kept separate from another's. This is all a gross oversimplification, but it's accurate enough at a high level. Lots of details differ between operating systems. Process vs. Thread A program vs. similar to a sequential program an run on its own vs. Cannot run on its own Unit of allocation vs. Unit of execution Have its own memory space vs. Share with others Each process has one or more threads vs. Each thread belongs to one process Expensive, need to context switch vs. Cheap, can use process memory and may not need to context switch More secure. One process cannot corrupt another process vs. Less secure. A thread can write the memory used by another thread

Can an inner class declared inside of a method access local variables of this method?
It's possible if these variables are final.

What can go wrong if you replace &emp;&emp; with &emp; in the following code: String a=null; if (a!=null && a.length()>10) {...}
A single ampersand here would lead to a NullPointerException.

What is the Vector class?
The Vector class provides the capability to implement a growable array of objects

What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

What's the main difference between a Vector and an ArrayList?
Java Vector class is internally synchronized and ArrayList is not.

What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.

Does garbage collection guarantee that a program will not run out of memory?
No, it doesn't. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Name Component subclasses that support painting ?
The Canvas, Frame, Panel, and Applet classes support painting.

What is a native method?
A native method is a method that is implemented in a language other than Java.

How can you write a loop indefinitely?
for(;;)--for loop; while(true)--always true, etc.

Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

When should the method invokeLater()be used?
This method is used to ensure that Swing components are updated through the event-dispatching thread.

How many methods in Object class?
This question is not asked to test your memory. It tests you how well you know Java. Ten in total.
clone()
equals() & hashcode()
getClass()
finalize()
wait() & notify()
toString()

How does Java handle integer overflows and underflows?
It uses low order bytes of the result that can fit into the size of the type allowed by the operation.

What is the numeric promotion?
Numeric promotion is used with both unary and binary bitwise operators. This means that byte, char, and short values are converted to int values before a bitwise operator is applied.
If a binary bitwise operator has one long operand, the other operand is converted to a long value.
The type of the result of a bitwise operation is the type to which the operands have been promoted. For example:
short a = 5;
byte b = 10;
long c = 15;
The type of the result of (a+b) is int, not short or byte. The type of the result of (a+c) or (b+c) is long.

Is the numeric promotion available in other platform?
Yes. Because Java is implemented using a platform-independent virtual machine, bitwise operations always yield the same result, even when run on machines that use radically different CPUs.

What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above.

When is the ArithmeticException throwQuestion: What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.

What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.

How can a subclass call a method or a constructor defined in a superclass?
Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.

What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass. Or, a method that has no implementation.

What is the difference between interface and abstract class?
interface contains methods that must be abstract; abstract class may contain concrete methods. interface contains variables that must be static and final; abstract class may contain non-final and final variables. members in an interface are public by default, abstract class may contain non-public members. interface is used to "implements"; whereas abstract class is used to "extends". interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance. interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces. interface is absolutely abstract; abstract class can be invoked if a main() exists. interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces. If given a choice, use interface instead of abstract class.

What is a static method?
A static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated.

What is a protected method?
A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its class.

What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

When can an object reference be cast to an interface reference?
An object reference can be cast to an interface reference when the object implements the referenced interface.

What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.

What is the difference between a Window and a Frame?
Heavy weight components like Abstract Window Toolkit (AWT), depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight.

Which package has light weight components?
javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.

What are peerless components?
The peerless components are called light weight components.

What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy.
The throws keyword is a modifier of a method that designates that exceptions may come out of the method, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw.

If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Does a class inherit the constructors of its super class?
A class does not inherit constructors from any of its superclasses.

Name primitive Java types.
The primitive types are byte, char, short, int, long, float, double, and Boolean.

Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.

How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.

Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.

What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Explain the usage of the keyword transient?
This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.

How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

How is it possible for two String objects with identical values not to be equal under the == operator? How are this() and super() used with constructors?
The == operator compares two objects to determine if they are the same objects in memory. It is possible for two String objects to have the same value, but located in different areas of memory.

What is an IO filter?
An IO filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

How can you force garbage collection?
You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

How do you know if an explicit object casting is needed?
If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

How do you restrict a user to cut and paste from the html page?
Using Servlet or client side scripts to lock keyboard keys. It is one of solutions.

? How could Java classes direct program messages to the system console, but error messages, say to a file?

The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st =   new Stream(new FileOutputStream("techinterviews_com.txt")); System.setErr(st); 
 
System.setOut(st); 

? What’s the difference between an interface and an abstract class?

An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

? Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

? Explain the usage of the keyword transient?

This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

? How can you force garbage collection?

You can’t force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

? How do you know if an explicit object casting is needed?

If you assign a superclass object to a variable of a subclass’s data type, you need to do explicit casting. For example:

Object a;Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

? What’s the difference between the methods sleep() and wait()

The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

? Can you write a Java class that could be used both as an applet as well as an application ?

Yes. Add a main() method to the applet.

? What’s the difference between constructors and other methods?

Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

? Can you call one constructor from another if a class has multiple constructors

Yes. Use this() syntax.

? Explain the usage of Java packages.

This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

? If a class is located in a package, what do you need to change in the OS environment to be able to use it?

You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let’s say a class Employee belongs to a package com.xyz.hr; and is located in the file c:/dev/com.xyz.hr.Employee.java. In this case, you’d need to add c:/dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\&gt;java com.xyz.hr.Employee

? What’s the difference between J2SDK 1.5 and J2SDK 5.0?

There’s no difference, Sun Microsystems just re-branded this version.

? What would you use to compare two String variables - the operator == or the method equals()?

I’d use the method equals() to compare the values of the Strings and the = = to check if two variables point at the same instance of a String object.

? Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception’s subclasses have to be caught first.

? Can an inner class declared inside of a method access local variables of this method?

It’s possible if these variables are final.

? What can go wrong if you replace && with & in the following code:

String a=null;
 
if (a!=null && a.length()&gt;10)
 
{...}

A single ampersand here would lead to a NullPointerException.

? What’s the main difference between a Vector and an ArrayList

Java Vector class is internally synchronized and ArrayList is not.

? When should the method invokeLater()be used?

This method is used to ensure that Swing components are updated through the event-dispatching thread.

? How can a subclass call a method or a constructor defined in a superclass?

Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass’s constructor.

? What’s the difference between a queue and a stack?

Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule.

? You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?

Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

? What comes to mind when you hear about a young generation in Java?

Garbage collection.

? What comes to mind when someone mentions a shallow copy in Java?

Object cloning.

? If you’re overriding the method equals() of an object, which other method you might also consider?

hashCode()

? You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?

ArrayList

? How would you make a copy of an entire Java object with its state?

Have this class implement Cloneable interface and call its method clone().

? How can you minimize the need of garbage collection and make the memory use more effective?

Use object pooling and weak object references.

? There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

If these classes are threads I’d consider notify() or notifyAll(). For regular classes you can use the Observer interface.

? What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?

You do not need to specify any access level, and Java will use a default package access level.

What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

What is Serialization and deserialization ?
Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

Explain the usage of Java packages.
This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

Does the code in finally block get executed if there is an exception and a return statement in a catch block?
If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.

Is Java a super set of JavaScript?
No. They are completely different. Some syntax may be similar.

What is a Container in a GUI?
A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.

How the object oriented approach helps us keep complexity of software development under control?
We can discuss such issue from the following aspects:
Objects allow procedures to be encapsulated with their data to reduce potential interference.
Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places.
The well-defined separations of interface and implementation allow constraints to be imposed on inheriting classes while still allowing the flexibility of overriding and overloading.

What is polymorphism?
Polymorphism means "having many forms". It allows methods (may be variables) to be written that needn't be concerned about the specifics of the objects they will be applied to. That is, the method can be specified at a higher level of abstraction and can be counted on to work even on objects of un-conceived classes.

What is design by contract?
The design by contract specifies the obligations of a method to any other methods that may use its services and also theirs to it. For example, the preconditions specify what the method required to be true when the method is called. Hence making sure that preconditions are. Similarly, postconditions specify what must be true when the method is finished, thus the called method has the responsibility of satisfying the post conditions.
In Java, the exception handling facilities support the use of design by contract, especially in the case of checked exceptions. The assert keyword can be used to make such contracts.

What are use cases?
A use case describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance. It is part of the analysis of a program. The collection of use cases should, ideally, anticipate all the standard circumstances and many of the extraordinary circumstances possible so that the program will be robust.

What is scalability and performance?
Performance is a measure of "how fast can you perform this task." and scalability describes how an application behaves as its workload and available computing resources increase.

What is the benefit of subclass?
Generally: The sub class inherits all the public methods and the implementation.
The sub class inherits all the protected methods and their implementation.
The sub class inherits all the default(non-access modifier) methods and their implementation.
The sub class also inherits all the public, protected and default member variables from the super class.
The constructors are not part of this inheritance model.

How to add menushortcut to menu item?
If you have a button instance called aboutButton, you may add menu short cut by calling aboutButton.setMnemonic('A'), so the user may be able to use Alt+A to click the button.

In System.out.println(),what is System,out and println,pls explain?
System is a predefined final class,out is a PrintStream object acting as a field member and println is a built-in overloaded method in the out object.

Can you write a Java class that could be used both as an applet as well as an application?
A. Yes. Add a main() method to the applet.

Can you make an instance of an abstract class? For example - java.util.Calender is an abstract class with a method getInstance() which returns an instance of the Calender class.
No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method.

What is the output of x > y? a:b = p*q when x=1,y=2,p=3,q=4?
When this kind of question has been asked, find the problems you think is necessary to ask back before you give an answer. Ask if variables a and b have been declared or initialized. If the answer is yes. You can say that the syntax is wrong. If the statement is rewritten as: x

What is the difference between Swing and AWT components?
AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button.

Why Java does not support pointers?
Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C-sharp shine.

Parsers? DOM vs SAX parser
Parsers are fundamental xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema.
DOM
1. Tree of nodes
2. Memory: Occupies more memory, preffered for small XML documents
3. Slower at runtime
4. Stored as objects
5. Programmatically easy
6. Ease of navigation
SAX
1. Sequence of events
2. Doesn't use any memory preferred for large documents
3. Faster at runtime
4. Objects are to be created
5. Need to write code for creating objects
6. Backward navigation is not possible as it sequentially processes the document

Can you declare a class as private?
Yes, we can declare a private class as an inner class. For example,

class MyPrivate {
private static class MyKey {
String key = "12345";
}
public static void main(String[] args) {
System.out.println(new MyKey().key);//prints 12345
}
}

What is the difference between shallow copy and deep copy?
Shallow copy shares the same reference with the original object like cloning, whereas the deep copy get a duplicate instance of the original object. If the shallow copy has been changed, the original object will be reflected and vice versa.

Can one create a method which gets a String and modifies it?
No. In Java, Strings are constant or immutable; their values cannot be changed after they are created, but they can be shared. Once you change a string, you actually create a new object. For example:
String s = "abc"; //create a new String object representing "abc"
s = s.toUpperCase(); //create another object representing "ABC"

Why is multiple inheritance not possible in Java?
It depends on how you understand "inheritance". Java can only "extends" one super class, but can "implements" many interfaces; that doesn't mean the multiple inheritance is not possible. You may use interfaces to make inheritance work for you. Or you may need to work around. For example, if you cannot get a feature from a class because your class has a super class already, you may get that class's feature by declaring it as a member field or getting an instance of that class. So the answer is that multiple inheritance in Java is possible.

What's the difference between constructors and other methods?
Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

What is the relationship between synchronized and volatile keyword?
The JVM is guaranteed to treat reads and writes of data of 32 bits or less as atomic.(Some JVM might treat reads and writes of data of 64 bits or less as atomic in future) For long or double variable, programmers should take care in multi-threading environment. Either put these variables in a synchronized method or block, or declare them volatile.

This class (IncrementImpl) will be used by various threads concurrently; can you see the inherent flaw(s)? How would you improve it?
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public int getCounter() {
return counter;
}
}

The counter is static variable which is shared by multiple instances of this class. The increment() method is synchronized, but the getCounter() should be synchronized too. Otherwise the Java run-time system will not guarantee the data integrity and the race conditions will occur. The famous producer/consumer example listed at Sun's thread tutorial site will tell more.
one of solutions
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public synchronized int getCounter() {
return counter;
}
}

What are the drawbacks of inheritance?
Since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation. In addition, the inheritance may make peers hardly understand your code if they don't know how your super-class acts and add learning curve to the process of development.
Usually, when you want to use a functionality of a class, you may use subclass to inherit such function or use an instance of this class in your class. Which is better, depends on your specification.

Is there any other way that you can achieve inheritance in Java?
There are a couple of ways. As you know, the straight way is to "extends" and/or "implements". The other way is to get an instance of the class to achieve the inheritance. That means to make the supposed-super-class be a field member. When you use an instance of the class, actually you get every function available from this class, but you may lose the dynamic features of OOP

Two methods have key words static synchronized and synchronized separately. What is the difference between them?
Both are synchronized methods. One is instance method, the other is class method. Method with static modifier is a class method. That means the method belongs to class itself and can be accessed directly with class name and is also called Singleton design. The method without static modifier is an instance method. That means the instance method belongs to its object. Every instance of the class gets its own copy of its instance method.
When synchronized is used with a static method, a lock for the entire class is obtained. When synchronized is used with a non-static method, a lock for the particular object (that means instance) of the class is obtained.
Since both methods are synchronized methods, you are not asked to explain what is a synchronized method. You are asked to tell the difference between instance and class method. Of course, your explanation to how synchronized keyword works doesn't hurt. And you may use this opportunity to show your knowledge scope.

How do you create a read-only collection?
The Collections class has six methods to help out here:
1. unmodifiableCollection(Collection c)
2. unmodifiableList(List list)
3. unmodifiableMap(Map m)
4. unmodifiableSet(Set s)
5. unmodifiableSortedMap(SortedMap m)
6. unmodifiableSortedSet(SortedSet s)
If you get an Iterator from one of these unmodifiable collections, when you call remove(), it will throw an UnsupportedOperationException.

Can a private method of a superclass be declared within a subclass?
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.

Why Java does not support multiple inheritance ?
This is a classic question. Yes or No depends on how you look at Java. If you focus on the syntax of "extends" and compare with C++, you may answer 'No' and give explanation to support you. Or you may answer 'Yes'. Recommend you to say 'Yes'.
Java DOES support multiple inheritance via interface implementation. Some people may not think in this way. Give explanation to support your point.

What is the difference between final, finally and finalize?
Short answer:
final - declares constant
finally - relates with exception handling
finalize - helps in garbage collection
If asked to give details, explain:
final field, final method, final class
try/finally, try/catch/finally
protected void finalize() in Object class

What kind of security tools are available in J2SE 5.0?
There are three tools that can be used to protect application working within the scope of security policies set at remote sites.
keytool -- used to manage keystores and certificates.
jarsigner -- used to generate and verify JAR signatures.
policytool -- used for managing policy files.
There are three tools that help obtain, list and manage Kerberos tickets.
kinit -- used to obtain Kerberos V5 tickets.
tklist -- used to list entries in credential cache and key tab.
ktab -- used to help manage entries in the key table.

How to make an array copy from System?
There is a method called arraycopy in the System class. You can do it:
System.arraycopy(sourceArray, srcOffset, destinationArray, destOffset, numOfElements2Copy);
When you use this method, the destinationArray will be filled with the elements of sourceArray at the length specified.

Can we use System.arraycopy() method to copy the same array?
Yes, you can. The source and destination arrays can be the same if you want to copy a subset of the array to another area within that array.

What is shallow copy or shallow clone in array cloning?
Cloning an array invloves creating a new array of the same size and type and copying all the old elements into the new array. But such copy is called shallow copy or shallow clone because any changes to the object would be reflected in both arrays.

When is the ArrayStoreException thrown?
When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.

How to check two arrays to see if contents have the same types and contain the same elements?
One of options is to use the equals() method of Arrays class.
Arrays.equals(a, b);
If the array types are different, a compile-time error will happen.

Can you call one constructor from another if a class has multiple constructors?
Yes. Use this() syntax.

What are the different types of inner classes?
There are four different types of inner classes in Java. They are: a)Static member classes , a static member class has access to all static methods of the parent, or top-level, class b) Member classes, the member class is instance specific and has access to any and all methods and members, even the parent's this reference c) Local classes, are declared within a block of code and are visible only within that block, just as any other method variable. d) Anonymous classes, is a local class that has no name

In which case would you choose a static inner class?
Interesting one, static inner classes can access the outer class's protected and private fields. This is both a positive and a negative point for us since we can, in essence, violate the encapsulation of the outer class by mucking up the outer class's protected and private fields. The only proper use of that capability is to write white-box tests of the class -- since we can induce cases that might be very hard to induce via normal black-box tests (which don't have access to the internal state of the object). Second advantage,if I can say, is that, we can this static concept to impose restriction on the inner class. Again as discussed in earlier point, an Inner class has access to all the public, private and protected members of the parent class. Suppose you want to restrict the access even to inner class, how would you go ahead? Making the inner class static enforces it to access only the public static members of the outer class( Since, protected and private members are not supposed to be static and that static members can access only other static members). If it has to access any non-static member, it has to create an instance of the outer class which leads to accessing only public members.

What is weak reference in Java
A weak reference is one that does not prevent the referenced object from being garbage collected. You might use them to manage a HashMap to look up a cache of objects. A weak reference is a reference that does not keep the object it refers to alive. A weak reference is not counted as a reference in garbage collection. If the object is not referred to elsewhere as well, it will be garbage collected.

What is the difference between final, finally and finalize?
final is used for making a class no-subclassable, and making a member variable as a constant which cannot be modified. finally is usually used to release all the resources utilized inside the try block. All the resources present in the finalize method will be garbage collected whenever GC is called. Though finally and finalize seem to be for a similar task there is an interesting tweak here, usually I prefer finally than finalize unless it is unavoidable. This is because the code in finally block is guaranteed of execution irrespective of occurrence of exception, while execution of finalize is not guarenteed.finalize method is called by the garbage collector on an object when the garbage collector determines that there are no more references to the object. Presumably the garbage collector will, like its civil servant namesake, visit the heap on a regular basis to clean up resources that are no longer in use. Garbage collection exists to prevent programmers from calling delete. This is a wonderful feature. For example, if you can't call delete, then you can't accidentally call delete twice on the same object. However, removing delete from the language is not the same thing as automatically cleaning up. To add to it, Garbage collection might not ever run. If garbage collection runs at all, and an object is no longer referenced, then that object's finalize will run. Also, across multiple objects, finalize order is not predictable. The correct approach to resource cleanup in Java language programs does not rely on finalize. Instead, you simply write explicit close methods for objects that wrap native resources. If you take this approach, you must document that the close method exists and when it should be called. Callers of the object must then remember to call close when they are finished with a resource.

What's the difference between the methods sleep() and wait()
The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

The following statement prints true or false, why?
byte[] a = { 1, 2, 3 };,
byte[] b = (byte[]) a.clone();
System.out.println(a == b);
The false will be printed out. Because the two arrays have distinctive memory addresses. Starting in Java 1.2, we can use java.util.Arrays.equals(a, b) to compare whether two arrays have the same contents.

Why do we need to use getSystemResource() and getSystemResources() method to load resources?
Because we want to look for resources strictly from the system classpath, These methods use the system ClassLoader to locate resources, which gives you stricter control of the resources used by the application.

ArithmeticException?
The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.

What is a transient variable?
A transient variable is a variable that may not be serialized.

Which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.

Why do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.

What is the output from System.out.println("Hello"+null);?
Hellonull

What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

Is null a keyword?
The null value is not a keyword.

What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.

What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.

Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.

What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.

What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.

Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

What is the List interface?
The List interface provides support for ordered collections of objects.

How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

What is the Vector class?
The Vector class provides the capability to implement a growable array of objects

What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

Which method of the Component class is used to set the position and size of a component?
setBounds()

How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.

Is sizeof a keyword?
The sizeof operator is not a keyword.

What are wrapper classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.

Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).

Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.

What is the immediate superclass of the Applet class?
Panel

What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.

What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.

What is the immediate superclass of the Dialog class?
Window.

What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.

What is a native method?
A native method is a method that is implemented in a language other than Java.

Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left

When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.

To what value is a variable of the String type automatically initialized?
The default value of a String type is null.

What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

What is a task's priority and how is it used in scheduling?
A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.

Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

What is the range of the short type?
The range of the short type is -(2^15) to 2^15 - 1.

What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.

In which package are most of the AWT events that support the event-delegation model defined?
Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.

What is the immediate super class of Menu?
What is the immediate super class of Menu? MenuItem

What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Which class is the immediate super class of the MenuComponent class.
Object

What invokes a thread's run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.

Which Container method is used to cause a container to be laid out and redisplayed?
validate()

What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.

How many times may an object's finalize() method be invoked by the garbage collector?
An object's finalize() method may only be invoked once by the garbage collector.

What is the purpose of the finally clause of a try-catch-finally statement? garbage collector?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

What is the argument type of a program's main() method?
A program's main() method takes an argument of the String[] type.

Which Java operator is right associative?
The = operator is right associative.

What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.

What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.

What method is invoked to cause an object to begin executing as a separate thread?
The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

Name two subclasses of the TextComponent class.
TextField and TextArea

What is the advantage of the event-delegation model over the earlier event-inheritance model?
The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

Which containers may have a MenuBar?
Frame

How are commas used in the initialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.

What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods.

What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.

How are Java source code files named?
A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.

What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.

Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.

What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.

If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.

What is an object's lock and which objects have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.

How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.

What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.

Which class is extended by all other classes?
The Object class is extended by all other classes.

Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..

Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.

What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.

What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.

What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its super classes.

For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.

What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

Which TextComponent method is used to set a TextComponent to the read-only state?
setEditable()

How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

Is &&= a valid Java operator?
No, it is not.

Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.

Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.

What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

Is "abc" a primitive value?
The String literal "abc" is not a primitive value. It is a String object.

What is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.

Is a class a subclass of itself?
A class is a subclass of itself.

What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.

How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.

What modifiers can be used with a local inner class?
A local inner class may be final or abstract.

What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.

Can an exception be rethrown?
Yes, an exception can be rethrown.

Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.

How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.

When is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

Which class is the immediate superclass of the Container class?
Component

If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.

Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier

What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.

What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

What is the return type of a program's main() method?
A program's main() method has a void return type.

Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane

What is the difference between a Choice and a List?
A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.

What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.

What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

Under what conditions is an object's finalize() method invoked by the garbage collector?
The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.

How are this () and super () used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?
A method's throws clause must declare any checked exceptions that are not caught within the body of the method.

What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?
The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried. In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.

How is it possible for two String objects with identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.

What Checkbox method allows you to tell if a Checkbox is checked?
getState()

What state is a thread in when it is executing?
An executing thread is in the running state.

What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.

How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.

What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.

Are true and false keywords?
The values true and false are not keywords.

What is a void return type?
A void return type indicates that a method does not return a value.

What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

What happens when you add a double value to a String?
The result is a String object.

What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..

Which package is always imported by default?
The java.lang package is always imported by default.

What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

What is a compilation unit?
A compilation unit is a Java source code file.

What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.

What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

How can a dead thread be restarted?
A dead thread cannot be restarted.

What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

What is a layout manager?
A layout manager is an object that is used to organize components in a container.

Which arithmetic operations can result in the throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.

What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Can an abstract class be final?
An abstract class may not be declared as final.

What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.

Can try statements be nested?
Try statements may be tested.

What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.

What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.

What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.

What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in which it is used.

What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()

Which Component subclass is used for drawing and painting?
Canvas

What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a Boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.

Can there be an abstract class with no abstract methods in it?
yes.

Can an Interface be final?
yes.

Can an Interface have an inner class?
Yes public interface abc { static int i=0; void dd(); class a1 { a1() { int j; System.out.println("in interfia"); }; public static void main(String a1[]) { System.out.println("in interfia"); } } }

Can we define private and protected modifiers for variables in interfaces?
Yes.

What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

What is a local, member and a class variable?
Variables declared within a method are "local" variables.
Variables declared within the class i.e not within any methods are "member" variables (global variables).
Variables declared within the class i.e not within any methods and are defined as "static" are class variables

I made my class Cloneable but I still get 'Can't access protected method clone. Why?
Yeah, some of the Java books, in particular "The Java Programming Language", imply that all you have to do in order to have your class support clone() is implement the Cloneable interface. Not so. Perhaps that was the intent at some point, but that's not the way it works currently. As it stands, you have to implement your own public clone() method, even if it doesn't do anything special and just calls super.clone().

What are the different identifier states of a Thread?
The different identifiers of a Thread are:
R - Running or runnable thread
S - Suspended thread
CW - Thread waiting on a condition variable
MW - Thread waiting on a monitor lock
MS - Thread suspended waiting on a monitor lock

What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn't even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().

What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work. out is a static field in the java.lang.System class.

Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Is null a keyword?
The null value is not a keyword.

Which characters may be used as the second character of an identifier,but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

What is the difference between notify() and notifyAll()?
notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a "writer" lock on a file might permit all "readers" to resume).

Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()?
The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That's just the way it works, you'll get used to it. It's really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn't need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can't use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at all, because Math is a "final" class which means it can't be extended.

Why are there no global variables in Java?
Global variables are considered bad form for a variety of reasons: · Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables).
· State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state.
· When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once.
For these reasons, Java decided to ban global variables.

What does it mean that a class or member is final?
A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass.
Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared. For example,
public final double c = 2.998;
It's also possible to make a static field final to get the effect of C++'s const statement or some uses of C's #define, e.g. public static final double c = 2.998;

What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the current class. It exists only to be overridden in subclasses. It has no body. For example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract.

What is the main difference between Java platform and other platforms?
The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms.
The Java platform has three elements:
Java programming language
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)

What is the Java Virtual Machine?
The Java Virtual Machine is a software that can be ported onto various hardware-based platforms.

What is the Java API?
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.

What is the package?
The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.

What is native code?
The native code is code that after you compile it, the compiled code runs on a specific hardware platform.

Explain the user defined Exceptions?
User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}

Is Java code slower than native code?
Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.

Can main() method be overloaded?
Yes. the main() method is a special method for a program entry. You can overload main() method in any ways. But if you change the signature of the main method, the entry point for the program will be gone.

What is the serialization?
The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.

Explain the new Features of JDBC 2.0 Core API?
The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:

Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
Java applications can now use the ResultSet.updateXXX methods.
New data types - interfaces mapping the SQL3 data types
Custom mapping of user-defined types (UTDs)
Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.

How you can force the garbage collection?
Garbage collection automatic process and can't be forced.

Explain garbage collection?
Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

Describe the principles of OOPS.
There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.

Explain the Encapsulation principle.
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.

Explain the Inheritance principle.
Inheritance is the process by which one object acquires the properties of another object.

Explain the Polymorphism principle.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".

Explain the different forms of Polymorphism.
From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:
Method overloading
Method overriding through inheritance
Method overriding through the Java interface

What are Access Specifiers available in Java?
ccess specifiers are keywords that determines the type of access to the member of a class. These are:
Public
Protected
Private
Defaults

Describe the wrapper classes in Java.
Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive Wrapper
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
double java.lang.Double
float java.lang.Float
int java.lang.Integer
long java.lang.Long
short java.lang.Short
void java.lang.Void

Question: Read the following program:
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}

What is the result?
A. The output is “Equal”
B. The output in “Not Equal”
C. An error at " if (x = y)" causes compilation to fall.
D. The program executes but no output is show on console.


Answer: C

Use the Externalizable interface when you need complete control over your Bean's serialization (for example, when writing and reading a specific file format).
No. Earlier order is maintained.

he superclass constructor runs before the subclass constructor. The subclass's version of the overridable method will be invoked before the subclass's constructor has been invoked. If the subclass's overridable method depends on the proper initialization of the subclass (through the subclass constructor), the method will most likely fail. Is that true?
Yes. It is true

Why are the interfaces more flexible than abstract classes?
--An interface-defined type can be implemented by any class in a class hierarchy and can be extended by another interface. In contrast, an abstract-class-defined type can be implemented only by classes that subclass the abstract class.
--An interface-defined type can be used well in polymorphism. The so-called interface type vs. implementation types.
--Abstract classes evolve more easily than interfaces. If you add a new concrete method to an abstract class, the hierarchy system is still working. If you add a method to an interface, the classes that rely on the interface will break when recompiled.
--Generally, use interfaces for flexibility; use abstract classes for ease of evolution (like expanding class functionality).

What are new language features in J2SE 5.0?
Generally:
1. generics
2. static imports
3. annotations
4. typesafe enums
5. enhanced for loop
6. autoboxing/unboxing
7. varargs
8. covariant return types

What is covariant return type?
A covariant return type lets you override a superclass method with a return type that subtypes the superclass method's return type. So we can use covariant return types to minimize upcasting and downcasting.
class Parent {
Parent foo () {
System.out.println ("Parent foo() called");
return this;
}
}

class Child extends Parent {
Child foo () {
System.out.println ("Child foo() called");
return this;
}
}

class Covariant {
public static void main(String[] args) {
Child c = new Child();
Child c2 = c.foo(); // c2 is Child
Parent c3 = c.foo(); // c3 points to Child
}
}

What is the result of the following statement?
int i = 1, float f = 2.0f;
i += f; //ok, the cast done automatically by the compiler
i = i + f; //error
The compound assignment operators automatically include cast operations in their behaviors.

What is externalization? Where is it useful?
Use the Externalizable interface when you need complete control over your Bean's serialization (for example, when writing and reading a specific file format).

What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc);
}
}

A Error array not initialized
B 5
C null
D Print some junk characters


Answer : D

It will print some junk characters to the output. Here it will not give any compile time or runtime error because we have declared and initialized the array properly. Event if we are not assigning a value to the array, it will always initialized to its defaults.

What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc[0]);
}
}
A Error array not initialized
B 5
C 0
D Print some junk characters


Answer : C.

What is a marker interface ?
An interface that contains no methods. E.g.: Serializable, Cloneable, SingleThreadModel etc. It is used to just mark java classes that support certain capability.

What are tag interfaces?
Tag interface is an alternate name for marker interface.

What are the restrictions placed on static method ?
We cannot override static methods. We cannot access any object variables inside static method. Also the this reference also not available in static methods.

What is JVM?
JVM stands for Java Virtual Machine. It is the run time for java programs. All are java programs are running inside this JVM only. It converts java byte code to OS specific commands. In addition to governing the execution of an application's byte codes, the virtual machine handles related tasks such as managing the system's memory, providing security against malicious code, and managing multiple threads of program execution.

What is JIT?
JIT stands for Just In Time compiler. It compiles java byte code to native code.

What are ClassLoaders?
A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a "class file" of that name from a file system.
Every Class object contains a reference to the ClassLoader that defined it.
Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader.
Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes.

What is Service Locator pattern?
The Service Locator pattern locates J2EE (Java 2 Platform, Enterprise Edition) services for clients and thus abstracts the complexity of network operation and J2EE service lookup as EJB (Enterprise JavaBean) Interview Questions - Home and JMS (Java Message Service) component factories. The Service Locator hides the lookup process's implementation details and complexity from clients. To improve application performance, Service Locator caches service objects to eliminate unnecessary JNDI (Java Naming and Directory Interface) activity that occurs in a lookup operation.

What is Session Facade pattern?
Session facade is one design pattern that is often used while developing enterprise applications. It is implemented as a higher level component (i.e.: Session EJB), and it contains all the iteractions between low level components (i.e.: Entity EJB). It then provides a single interface for the functionality of an application or part of it, and it decouples lower level components simplifying the design. Think of a bank situation, where you have someone that would like to transfer money from one account to another. In this type of scenario, the client has to check that the user is authorized, get the status of the two accounts, check that there are enough money on the first one, and then call the transfer. The entire transfer has to be done in a single transaction otherwise is something goes south, the situation has to be restored.
As you can see, multiple server-side objects need to be accessed and possibly modified. Multiple fine-grained invocations of Entity (or even Session) Beans add the overhead of network calls, even multiple transaction. In other words, the risk is to have a solution that has a high network overhead, high coupling, poor reusability and mantainability.
The best solution is then to wrap all the calls inside a Session Bean, so the clients will have a single point to access (that is the session bean) that will take care of handling all the rest.

What is Data Access Object pattern?
The Data Access Object (or DAO) pattern: separates a data resource's client interface from its data access mechanisms adapts a specific data resource's access API to a generic client interface
The DAO pattern allows data access mechanisms to change independently of the code that uses the data.
The DAO implements the access mechanism required to work with the data source. The data source could be a persistent store like an RDBMS, an external service like a B2B exchange, a repository like an LDAP database, or a business service accessed via CORBA Internet Inter-ORB Protocol (IIOP) or low-level sockets. The business component that relies on the DAO uses the simpler interface exposed by the DAO for its clients. The DAO completely hides the data source implementation details from its clients. Because the interface exposed by the DAO to clients does not change when the underlying data source implementation changes, this pattern allows the DAO to adapt to different storage schemes without affecting its clients or business components. Essentially, the DAO acts as an adapter between the component and the data source.

Can we make an EJB singleton?
This is a debatable question, and for every answer we propose there can be contradictions. I propose 2 solutions of the same. Remember that EJB's are distributed components and can be deployed on different JVM's in a Distributed environment
i) Follow the steps as given below
Make sure that your serviceLocator is deployed on only one JVM.
In the serviceLocator create a HashTable/HashMap(You are the right judge to choose between these two)
When ever a request comes for an EJB to a serviceLocator, it first checks in the HashTable if an entry already exists in the table with key being the JNDI name of EJB. If key is present and value is not null, return the existing reference, else lookup the EJB in JNDI as we do normally and add an entry into the Hashtable before returning it to the client. This makes sure that you maintain a singleton of EJB.
ii) In distributed environment our components/Java Objects would be running on different JVM's. So the normal singleton code we write for maintaining single instance works fine for single JVM, but when the class could be loaded in multiple JVM's and Instantiated in multiple JVM's normal singleton code does not work. This is because the ClassLoaders being used in the different JVM's are different from each other and there is no defined mechanism to check and compare what is loaded in another JVM. A solution could be(Not tested yet. Need your feedback on this) to write our own ClassLoader and pass this classLoader as argument, whenever we are creating a new Instance and make sure that only one instance is created for the proposed class. This can be done easily.

How can we make a class Singleton ?
A) If the class is Serializable

class Singleton implements Serializable
{
private static Singleton instance;

private Singleton() { }

public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}

/**
If the singleton implements Serializable, then this
* method must be supplied.
*/
protected Object readResolve() {
return instance;
}

/**
This method avoids the object fro being cloned
*/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}

B) If the class is NOT Serializable


class Singleton
{
private static Singleton instance;
private Singleton() { }

public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}

/**
This method avoids the object from being cloned
**/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}

}

How is static Synchronization different form non-static synchronization?
When Synchronization is applied on a static Member or a static block, the lock is performed on the Class and not on the Object, while in the case of a Non-static block/member, lock is applied on the Object and not on class. [Trail 2: There is a class called Class in Java whose object is associated with the object(s) of your class. All the static members declared in your class will have reference in this class(Class). As long as your class exists in memory this object of Class is also present. Thats how even if you create multiple objects of your class only one Class object is present and all your objects are linked to this Class object. Even though one of your object is GCed after some time, this object of Class is not GCed untill all the objects associated with it are GCed.
This means that when ever you call a "static synchronized" block, JVM locks access to this Class object and not any of your objects. Your client can till access the non-static members of your objects.

What are class members and Instance members?
Any global members(Variables, methods etc.) which are static are called as Class level members and those which are non-static are called as Instance level members.

Name few Garbage collection algorithms?
Here they go:
Mark and Sweep
Reference counting
Tracing collectors
Copying collectors
Heap compaction
Mark-compact collectors

Can we force Garbage collection?
java follows a philosophy of automatic garbage collection, you can suggest or encourage the JVM to perform garbage collection but you can not force it. Once a variable is no longer referenced by anything it is available for garbage collection. You can suggest garbage collection with System.gc(), but this does not guarantee when it will happen. Local variables in methods go out of scope when the method exits. At this point the methods are eligible for garbage collection. Each time the method comes into scope the local variables are re-created.

Does Java pass by Value or reference?
Its uses Reference while manipulating objects but pass by value when sending method arguments. Those who feel why I added this simple question in this section while claiming to be maintaining only strong and interesting questions, go ahead and answer following questions.
a)What is the out put of:

import java.util.*;

class TestCallByRefWithObject
{
ArrayList list = new ArrayList(5);


public void remove(int index){
list.remove(index);
}

public void add(Object obj){
list.add(obj);
}

public void display(){
System.out.println(list);
}

public static void main(String[] args)
{
TestCallByRefWithObject test = new TestCallByRefWithObject();

test.add("1");
test.add("2");
test.add("3");
test.add("4");
test.add("5");

test.remove(4);
test.display();
}
}

b) And now what is the output of:


import java.util.*;

class TestCallByRefWithInt
{
int i = 5;


public void decrement(int i){
i--;
}

public void increment(int i){
i++;
}

public void display(){
System.out.println("\nValue of i is : " +i);
}

public static void main(String[] args)
{
TestCallByRefWithInt test = new TestCallByRefWithInt();

test.increment(test.i);

test.display();
}
}

Why Thread is faster compare to process?
A thread is never faster than a process. If you run a thread(say there's a process which has spawned only one thread) in one JVM and a process in another and that both of them require same resources then both of them would take same time to execute. But, when a program/Application is thread based(remember here there will be multiple threads running for a single process) then definetly a thread based appliation/program is faster than a process based application. This is because, when ever a process requires or waits for a resource CPU takes it out of the critical section and allocates the mutex to another process.
Before deallocating the ealier one, it stores the context(till what state did it execute that process) in registers. Now if this deallocated process has to come back and execute as it has got the resource for which it was waiting, then it can't go into critical section directly. CPU asks that process to follow scheduling algorithm. So this process has to wait again for its turn. While in the case of thread based application, the application is still with CPU only that thread which requires some resource goes out, but its co threads(of same process/apllication) are still in the critical section. Hence it directly comes back to the CPU and does not wait outside. Hence an application which is thread based is faster than an application which is process based.
Be sure that its not the competion between thread and process, its between an application which is thread based or process based.

When and How is an object considered as Garbage by a GC?
An object is considered garbage when it can no longer be reached from any pointer in the running program. The most straightforward garbage collection algorithms simply iterate over every reachable object. Any objects left over are then considered garbage.

What are generations in Garbage Collection terminology? What is its relevance?
Garbage Collectors make assumptions about how our application runs. Most common assumption is that an object is most likely to die shortly after it was created: called infant mortality. This assumes that an object that has been around for a while, will likely stay around for a while. GC organizes objects into generations (young, tenured, and perm). This tells that if an object lives for more than certain period of time it is moved from one generation to another generations( say from young -> tenured -> permanent). Hence GC will be run more frequently at the young generations and rarely at permanent generations. This reduces the overhead on GC and gives faster response time.

What is a Throughput Collector?
The throughput collector is a generational collector similar to the default collector but with multiple threads used to do the minor collection. The major collections are essentially the same as with the default collector. By default on a host with N CPUs, the throughput collector uses N garbage collector threads in the collection. The number of garbage collector threads can be controlled with a command line option.

When to Use the Throughput Collector?
Use the throughput collector when you want to improve the performance of your application with larger numbers of processors. In the default collector garbage collection is done by one thread, and therefore garbage collection adds to the serial execution time of the application. The throughput collector uses multiple threads to execute a minor collection and so reduces the serial execution time of the application. A typical situation is one in which the application has a large number of threads allocating objects. In such an application it is often the case that a large young generation is needed

What is Aggressive Heap?
The -XX:+AggressiveHeap option inspects the machine resources (size of memory and number of processors) and attempts to set various parameters to be optimal for long-running, memory allocation-intensive jobs. It was originally intended for machines with large amounts of memory and a large number of CPUs, but in the J2SE platform, version 1.4.1 and later it has shown itself to be useful even on four processor machines. With this option the throughput collector (-XX:+UseParallelGC) is used along with adaptive sizing (-XX:+UseAdaptiveSizePolicy). The physical memory on the machines must be at least 256MB before Aggressive Heap can be used.

What is a Concurrent Low Pause Collector?
The concurrent low pause collector is a generational collector similar to the default collector. The tenured generation is collected concurrently with this collector. This collector attempts to reduce the pause times needed to collect the tenured generation. It uses a separate garbage collector thread to do parts of the major collection concurrently with the applications threads. The concurrent collector is enabled with the command line option -XX:+UseConcMarkSweepGC. For each major collection the concurrent collector will pause all the application threads for a brief period at the beginning of the collection and toward the middle of the collection. The second pause tends to be the longer of the two pauses and multiple threads are used to do the collection work during that pause. The remainder of the collection is done with a garbage collector thread that runs concurrently with the application. The minor collections are done in a manner similar to the default collector, and multiple threads can optionally be used to do the minor collection.

When to Use the Concurrent Low Pause Collector?
Use the concurrent low pause collector if your application would benefit from shorter garbage collector pauses and can afford to share processor resources with the garbage collector when the application is running. Typically applications which have a relatively large set of long-lived data (a large tenured generation), and run on machines with two or more processors tend to benefit from the use of this collector. However, this collector should be considered for any application with a low pause time requirement. Optimal results have been observed for interactive applications with tenured generations of a modest size on a single processor.

What is Incremental Low Pause Collector?
The incremental low pause collector is a generational collector similar to the default collector. The minor collections are done with the same young generation collector as the default collector. Do not use either -XX:+UseParallelGC or -XX:+UseParNewGC with this collector. The major collections are done incrementally on the tenured generation. This collector (also known as the train collector) collects portions of the tenured generation at each minor collection. The goal of the incremental collector is to avoid very long major collection pauses by doing portions of the major collection work at each minor collection. The incremental collector will sometimes find that a non-incremental major collection (as is done in the default collector) is required in order to avoid running out of memory.

When to Use the Incremental Low Pause Collector?
Use the incremental low pause collector when your application can afford to trade longer and more frequent young generation garbage collection pauses for shorter tenured generation pauses. A typical situation is one in which a larger tenured generation is required (lots of long-lived objects), a smaller young generation will suffice (most objects are short-lived and don't survive the young generation collection), and only a single processor is available.

How do you enable the concurrent garbage collector on Sun's JVM?
-Xconcgc options allows us to use concurrent garbage collector (1.2.2_07+)we can also use -XX:+UseConcMarkSweepGC which is available beginning with J2SE 1.4.1.

What is a platform?
A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000 and XP, Linux, Solaris, and MacOS.

What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

How to make a class or a bean serializable?
By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.

What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.

Name Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane

What is the List interface?
The List interface provides support for ordered collections of objects.

What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

What is tunnelling?
Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data.

What is meant by "Abstract Interface"?
First, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.

Can Java code be compiled to machine dependent executable file?
Yes. There are many tools out there. If you did so, the generated exe file would be run in the specific platform, not cross-platform.

Do not use the String contatenation operator in lengthy loops or other places where performance could suffer. Is that true?
Yes.

What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.

Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.

What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.

What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.

What is the List interface?
The List interface provides support for ordered collections of objects.

Is sizeof a keyword?
The sizeof operator is not a keyword in Java.

Which class is the superclass for every class.
Object.

Which Container method is used to cause a container to be laid out and redisplayed?
validate()

What's the difference between a queue and a stack?
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

What comes to mind when you hear about a young generation in Java?
Garbage collection.

You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

What comes to mind when someone mentions a shallow copy in Java?
Object cloning.

If you're overriding the method equals() of an object, which other method you might also consider?
hashCode()

You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?
ArrayList

How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().

How can you minimize the need of garbage collection and make the memory use more effective?
Use object pooling and weak object references.

There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?
If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.

What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
You do not need to specify any access level, and Java will use a default package access level.

What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.

Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.

Difference between Swing and AWT?
AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.

What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected). It means that it is visible to all within a particular package.

What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a super class can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a no static method. In other words, you can't change a static method into an instance method in a subclass.

What is final?
A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.

What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".

What if I write static public void instead of public static void?
Program compiles and runs properly.

What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".

What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
It is empty. But not null.

How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.

Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.

Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes

Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;

Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

What is the default value of an object reference declared as an instance variable?
Null unless we define it explicitly.

Can a top level class be private or protected?
No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.

What type of parameter passing does Java support?
In Java the arguments are always passed by value .

Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.

Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .

What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

How can I customize the serialization process? i.e. how can one have a control over the serialization process?
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilized.

What is Externalizable interface?
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

When you serialize an object, what happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.

What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

Does Java provide any construct to find out the size of an object?
No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc

Give a simplest way to find out the time a method takes for execution without using any profiling tool?
Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amount of processing.

Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these reasons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.

If I want an object of my class to be thrown as an exception object, what should I do?
The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
One can not do anything in this scenario. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

What are the different ways to handle exceptions?
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.

What is the basic difference between the 2 approaches to exception handling.
1. try catch block and
2. specifying the candidate exceptions in the throws clause?
When should you use which approach?

In the first approach as a programmer of the method, you yourself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

If I write System.exit (0); at the end of the try block, will the finally block still execute?
No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection .

What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.

What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

How are this() and super() used with constructors?
This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

What is daemon thread and which method is used to create the daemon thread?
Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

Can applets communicate with each other?
At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

What are the steps in the JDBC connection?
While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();

How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

Is Empty .java file a valid source file?
Yes, an empty .java file is a perfectly valid source file.

Can a .java file contain more than one java classes?
Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

Is String a primitive data type in Java?
No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

Is main a keyword in Java?
No, main is not a keyword in Java.

Is next a keyword in Java?
No, next is not a keyword.

Is delete a keyword in Java?
No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

Is exit a keyword in Java?
No. To exit a program explicitly you use exit method in System object.

What happens if you don't initialize an instance variable of any of the primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a Boolean will be initialized to false.

What will be the initial value of an object reference which is defined as an instance variable?
The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerException everywhere you try to use such default initialized references.

What are the different scopes for Java variables?
The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessible only during the course of method execution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain about the local variable not being initialized..

How many objects are created in the following piece of code? MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();

Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

Can a public class MyClass be defined in a source file named YourClass.java?
No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.

Can main method be declared final?
Yes, the main method can be declared final, in addition to being public static.

What will be the output of the following statement?
System.out.println ("1" + 3);

It will print 13.

What will be the default values of all the elements of an array defined as an instance variable?
If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of Boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

1. What is the difference between URL instance and URLConnection instance? - A URL instance represents the location of a resource, and a URLConnection instance represents a link for accessing or communicating with the resource at the location.

2. What are the two important TCP Socket classes? - Socket and ServerSocket. ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.

3. What technologies are included in J2EE? - The primary technologies in J2EE are: Enterprise JavaBeansTM (EJBsTM), JavaServer PagesTM (JSPsTM), Java Servlets, the Java Naming and Directory InterfaceTM (JNDITM), the Java Transaction API (JTA), CORBA, and the JDBCTM data access API.

4. What is the Java Authentication and Authorization Service (JAAS) 1.0? - The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. JAAS is a Java programing language version of the standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.

5. What’s the difference between JNDI lookup(), list(), listBindings(), and search()? - lookup() attempts to find the specified object in the given context. I.e., it looks for a single, specific object and either finds it in the current context or it fails. list() attempts to return an enumeration of all of the NameClassPair’s of all of the objects in the current context. I.e., it’s a listing of all of the objects in the current context but only returns the object’s name and the name of the class to which the object belongs. listBindings() attempts to return an enumeration of the Binding’s of all of the objects in the current context. I.e., it’s a listing of all of the objects in the current context with the object’s name, its class name, and a reference to the object itself. search() attempts to return an enumeration of all of the objects matching a given set of search criteria. It can search across multiple contexts (or not). It can return whatever attributes of the objects that you desire. It’s by far the most complex and powerful of these options but is also the most expensive.

6. Components of JNDI - Naming Interface- The naming interface organizes information hierarchically and maps human-friendly names to addresses or objects that are machine-friendly. It allows access to named objects through multiple namespaces. Directory Interface - JNDI includes a directory service interface that provides access to directory objects, which can contain attributes, thereby providing attribute-based searching and schema support. Service Provider Interface - JNDI comes with the SPI, which supports the protocols provided by third parties.

7. What is the Max amount of information that can be saved in a Session Object? - As such there is no limit on the amount of information that can be saved in a Session Object. Only the RAM available on the server machine is the limitation. The only limit is the Session ID length(Identifier), which should not exceed more than 4K. If the data to be store is very huge, then it’s preferred to save it to a temporary file onto hard disk, rather than saving it in session. Internally if the amount of data being saved in Session exceeds the predefined limit, most of the servers write it to a temporary cache on Hard disk.

8. Must my bean-managed persistence mechanism use the WebLogic JTS driver? - BEA recommend that you use the TxDataSource for bean-managed persistence.

9. Do EJBs have to be homogeneously deployed across a cluster? Why? - Yes. Beginning with WebLogic Server version 6.0, EJBs must be homogeneously deployed across a cluster for the following reasons:

o To keep clustering EJBs simple

o To avoid cross server calls which results in more efficiency. If EJBs are not deployed on all servers, cross server calls are much more likely.

o To ensure that every EJB is available locally

o To ensure that all classes are loaded in an undeployable way

o Every server must have access to each EJB’s classes so that it can be bound into the local JNDI tree. If only a subset of the servers deploys the bean, the other servers will have to load the bean’s classes in their respective system classpaths which makes it impossible to undeploy the beans.

10. Is an XSLT processor bundled in WebLogic Server? - Yes, an XSLT processor, based on Apache’s Xalan 2.0.1 processor, in WebLogic Server 6.1.

11. I plugged in a version of Apache Xalan that I downloaded from the Apache Web site, and now I get errors when I try to transform documents. What is the problem? - You must ensure that the version of Apache Xalan you download from the Apache Web site is compatible with Apache Xerces version 1.3.1. Because you cannot plug in a different version of Apache Xerces , the only version of Apache Xerces that is compatible with WebLogic Server 6.1 is 1.3.1. The built-in parser (based on version 1.3.1 of Apache Xerces) and transformer (based on version 2.0.1 of Apache Xalan) have been modified by BEA to be compatible with each other.

12. How do I increase WebLogic Server memory? - Increase the allocation of Java heap memory for WebLogic Server. (Set the minimum and the maximum to the same size.) Start WebLogic Server with the -ms32m option to increase the allocation, as in this example:

13.                 $ java ... -ms32m -mx32m ...

This allocates 32 megabytes of Java heap memory to WebLogic Server, which improves performance and allows WebLogic Server to handle more simultaneous connections. You can increase this value if necessary.

14. What causes Java.io exceptions in the log file of WebLogic Server? - You may see messages like these in the log file:

15.     (Windows NT)
16.                 java.io.IOException Connection Reset by Peer
17.                 java.io.EOFException Connection Reset by Peer
18.     (Solaris)
19.                 java.io.Exception: Broken pipe

These messages occur when you are using servlets. A client initiates an HTTP request, and then performs a series of actions on the browser:

o Click Stop or enter equivalent command or keystrokes

o Click Refresh or enter equivalent command or keystrokes

o Send a new HTTP request.

The messages indicate that WebLogic Server has detected and recovered from an interrupted HTTP request.

20. What is the function of T3 in WebLogic Server? - T3 provides a framework for WebLogic Server messages that support for enhancements. These enhancements include abbreviations and features, such as object replacement, that work in the context of WebLogic Server clusters and HTTP and other product tunneling. T3 predates Java Object Serialization and RMI, while closely tracking and leveraging these specifications. T3 is a superset of Java Object. Serialization or RMI; anything you can do in Java Object Serialization and RMI can be done over T3. T3 is mandated between WebLogic Servers and between programmatic clients and a WebLogic Server cluster. HTTP and IIOP are optional protocols that can be used to communicate between other processes and WebLogic Server. It depends on what you want to do. For example, when you want to communicate between a browser and WebLogic Server-use HTTP, or an ORB and WebLogic Server-IIOP.

21. What are the enhancements in EJB 2.0 specification with respect to Asynchronous communication? - EJB 2.0 mandates integration between JMS and EJB. We have specified the integration of Enterprise JavaBeans with the Java Message Service, and have introduced message-driven beans. A message-driven bean is a stateless component that is invoked by the container as a result of the arrival of a JMS message. The goal of the message-driven bean model is to make developing an enterprise bean that is asynchronously invoked to handle the processing of incoming JMS messages as simple as developing the same functionality in any other JMS MessageListener.

22. What are the enhancements in EJB 2.0 with respect to CMP? - EJB 2.0 extends CMP to include far more robust modeling capability, with support for declarative management of relationships between entity EJBs. Developers no longer need to re-establish relationships between the various beans that make up their application — the container will restore the connections automatically as beans are loaded, allowing bean developers to navigate between beans much as they would between any standard Java objects.
EJB 2.0 also introduces for the first time a portable query language, based on the abstract schema, not on the more complex database schema. This provides a database and vendor-independent way to find entity beans at run time, based on a wide variety of search criteria.

23. Can you briefly describe local interfaces? - EJB was originally designed around remote invocation using the Java Remote Method Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for these calls using RMI/IIOP. This design allowed for maximum flexibility in developing applications without consideration for the deployment scenario, and was a strong feature in support of a goal of component reuse in J2EE. Many developers are using EJBs locally - that is, some or all of their EJB calls are between beans in a single container. With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism. The local interface may be defined for a bean during development, to allow streamlined calls to the bean if a caller is in the same container. This does not involve the overhead involved with RMI like marshalling etc. This facility will thus improve the performance of applications in which co-location is planned. Local interfaces also provide the foundation for container-managed relationships among entity beans with container-managed persistence.

24. What are the special design care that must be taken when you work with local interfaces? - It is important to understand that the calling semantics of local interfaces are different from those of remote interfaces. For example, remote interfaces pass parameters using call-by-value semantics, while local interfaces use call-by-reference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up front, then decide which interfaces can be local and which remote, and finally, develop the application code with these choices in mind. While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision.

25. What happens if remove( ) is never invoked on a session bean? - In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container. In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool.

26. What is the difference between creating a distributed application using RMI and using a EJB architecture? - It is possible to create the same application using RMI and EJB. But in case of EJB the container provides the requisite services to the component if we use the proper syntax. It thus helps in easier development and lesser error and use of proven code and methodology. But the investment on application server is mandatory in that case. But this investment is warranted because it results in less complex and maintainable code to the client, which is what the end client wants. Almost all the leading application servers provide load balancing and performance tuning techniques. In case of RMI we have to code the services and include in the program the way to invoke these services.

27. Why would a client application use JTA transactions? - One possible example would be a scenario in which a client needs to employ two (or more) session beans, where each session bean is deployed on a different EJB server and each bean performs operations against external resources (for example, a database) and/or is managing one or more entity beans. In this scenario, the client’s logic could required an all-or-nothing guarantee for the operations performed by the session beans; hence, the session bean usage could be bundled together with a JTA UserTransaction object. In the previous scenario, however, the client application developer should address the question of whether or not it would be better to encapsulate these operations in yet another session bean, and allow the session bean to handle the transactions via the EJB container. In general, lightweight clients are easier to maintain than heavyweight clients. Also, EJB environments are ideally suited for transaction management.

28.     Context c = new InitialContext(); UserTransaction ut = (UserTransaction)
29.     c.lookup(\"java:comp/UserTransaction\");
30.     ut.begin();
31.                 // perform multiple operations...
32.     ut.commit() ...

33. Can the bean class implement the EJBObject class directly? If not why? - It is better not to do it will make the Bean class a remote object and its methods can be accessed without the containers? security, and transaction implementations if our code by mistake passed it in one of its parameters. Its just a good design practice.

34. What does isIdentical() method return in case of different type of beans? - Stateless - true always. Stateful - depends whether the references point to the same session object. Entity - Depends whether the primary key is the same and the home is same.

35. How should you type cast a remote object? Why? - A client program that is intended to be interoperable with all compliant EJB Container implementations must use the javax.rmi.PortableRemoteObject.narrow(…) method to perform type-narrowing of the client-side representations of the remote home and remote interfaces. Programs using the cast operator for narrowing the remote and remote home interfaces are likely to fail if the Container implementation uses RMI-IIOP as the underlying communication transport.

36. What should you do in a passive method? - You try to make all nontransient variables, which are not one of the following to null. For the given list the container takes care of serializing and restoring the object when activated. Serializable objects, null, UserTransaction, SessionContext, JNDI contexts in the beans context, reference to other beans, references to connection pools.
Things that must be handled explicitly are like a open database connection etc. These must be closed and set to null and retrieved back in the activate method.

What is a platform?         
Answer:
        A platform is the hardware or software environment in which a 
program runs. Most 
platforms can be described as a combination of the operating system and 
hardware, like Windows 
2000 and XP, Linux, Solaris, and MacOS. 
What is the main difference between Java platform and other platforms?         
Answer:
The Java platform differs from most other platforms in that it's a 
software-only platform that 
runs on top of other hardware-based platforms. The Java platform has 
two components: 1. The Java 
Virtual Machine (Java VM) 2. The Java Application Programming Interface 
(Java API) 
  What is the Java Virtual Machine?         
Answer:
The Java Virtual Machine is a software that can be ported onto various 
hardware-based platforms. 
 
        What is the Java API?         
Answer:
The Java API is a large collection of ready-made software components 
that provide many useful 
capabilities, such as graphical user interface (GUI) widgets
What is the package?         
Answer:
The package is a Java namespace or part of Java libraries. The Java API 
is grouped into 
libraries of related classes and interfaces; these libraries are known 
as packages. 
What is native code?         
Answer:
The native code is code that after you compile it, the compiled code 
runs on a specific 
hardware platform. 
Is Java code slower than native code?         Can main() method be 
overloaded?         
Answer:
Yes. the main() method is a special method for a program entry. You can 
overload main() 
method in any ways. But if you change the signature of the main method, 
the entry point for 
the program will be gone.
What is the serialization?         
Answer:
The serialization is a kind of mechanism that makes a class or a bean 
persistence by having 
its properties or fields and state information saved and restored to 
and from storage. 
What is a transient variable?         
Answer:
A transient variable is a variable that may not be serialized. If you 
don\'t want some field 
to be serialized, you can mark that field transient or static. 
Which containers use a border layout as their default layout?         
Answer:
The Window, Frame and Dialog classes use a border layout as their 
default layout. 
        
Answer:
Not really. As a platform-independent environment, the Java platform 
can be a bit slower 
than native code. However, smart compilers, well-tuned interpreters, 
and just-in-time bytecode 
compilers can bring performance close to that of native code without 
threatening portability. 
What are three ways in which a thread can enter the waiting state?         
Answer:
A thread can enter the waiting state by invoking its sleep() method, by 
blocking on IO, by 
unsuccessfully attempting to acquire an object's lock, or by invoking 
an object's wait() method.
 It can also enter the 
waiting state by invoking its (deprecated) suspend() method. 
Can a lock be acquired on a class?         
Answer:
Yes, a lock can be acquired on a class. This lock is acquired on the 
class's Class object. 
What method is used to specify a container's layout?       
Answer:
The setLayout() method is used to specify a container's layout. 
Which containers use a FlowLayout as their default layout?       
Answer:
The Panel and Applet classes use the FlowLayout as their default 
layout. 
Can Java object be locked down for exclusive use by a given thread?       
Answer:
Yes. You can lock an object by putting it in a "synchronized" block. 
The locked object is 
inaccessible to any thread other than the one that explicitly claimed 
it. 
What state does a thread enter when it terminates its processing?       
Answer:
When a thread terminates its processing, it enters the dead state. 
What is the difference between Process and Thread?(donated in April 
2005)       
Answer:
A process can contain multiple threads. In most multithreading 
operating systems, a process 
gets its own memory address space; a thread doesn't. Threads typically 
share the heap belonging 
to their parent process. For instance, a JVM runs in a single process 
in the host O/S. 
Threads in the JVM share the heap belonging to that process; that's why 
several threads 
may access the same object.Typically, even though they share a common 
heap, threads have 
their own stack space. This is how one thread's invocation of a method 
is kept separate 
from another's.This is all a gross oversimplification, but it's 
accurate enough at a high level. 
Lots of details differ between operating systems.Process vs. Thread A 
program Similar to a 
sequential program  vs.  Can run on its own Cannot run on its own Unit 
of allocation Unit of 
execution       vs.  Have its own memory space Share with others Each 
process has one or more 
threads       vs.  Each thread belongs to one process Expensive, need 
to context switch   
 vs.  Cheap, can use process memory and may not need to context switch 
More secure. One process 
cannot corrupt another process Less secure.vs. A thread can write the 
memory used by another 
thread 
If a method is declared as protected, where may the method be accessed?       
Answer:
A protected method may only be accessed by classes or interfaces of the 
same package or by 
subclasses of the class in which it is declared. 
What is the purpose of finalization?       
Answer:
The purpose of finalization is to give an unreachable object the 
opportunity to perform any 
cleanup processing before the object is garbage collected. 
Which class is the superclass for every class.       
Answer:
Object. 
What is the purpose of the finally clause of a try-catch-finally 
statement?       
Answer:
The finally clause is used to provide the capability to execute code no 
matter whether or 
not an exception is thrown or caught. 
What is a static method?       
Answer:
A static method is a method that belongs to the class rather than any 
object of the class 
and doesn't apply to an object or even require that any objects of the 
class have been 
instantiated. 
What is the difference between a Window and a Frame?       
Answer:
The Frame class extends Window to define a main application window that 
can have a menu bar.
 
 
What do heavy weight components mean? 
       
Answer: 
 
Heavy weight components like Abstract Window Toolkit (AWT), depend on 
the local windowing 
toolkit. For example, java.awt.Button is a heavy weight component, when 
it is running on the 
Java platform for Unix platform, it maps to a real Motif button. In 
this relationship, the 
Motif button is called the peer to the java.awt.Button. If you create 
two Buttons, two peers 
and hence two Motif Buttons are also created. The Java platform 
communicates with the Motif 
Buttons using the Java Native Interface. For each and every component 
added to the application, 
there is an additional overhead tied to the local windowing system, 
which is why these 
components are called heavy weight. 
 
Which package has light weight components? 
       
Answer: 
 
javax.Swing package. All components in Swing, except JApplet, JDialog, 
JFrame and JWindow are 
lightweight components. 
 
Does a class inherit the constructors of its superclass? 
       
Answer: 
 
A class does not inherit constructors from any of its superclasses. 
 
Name primitive Java types. 
       
Answer: 
 
The primitive types are byte, char, short, int, long, float, double, 
and boolean. 
 
What restrictions are placed on method overloading? 
       
Answer: 
 
Two methods may not have the same name and argument list but different 
return types. 
 
What restrictions are placed on method overriding? 
       
Answer: 
 
Overridden methods must have the same name, argument list, and return 
type. The overriding 
method may not limit the access of the method it overrides. The 
overriding method may not 
throw any exceptions that may not be thrown by the overridden method. 
 
What is casting? 
       
Answer: 
 
There are two types of casting, casting between primitive numeric types 
and casting between 
object references. Casting between numeric types is used to convert 
larger values, such as 
double values, to smaller values, such as byte values. Casting between 
object references is 
used to refer to an object by a compatible class, interface, or array 
type reference. 
       
What is the difference between a Scrollbar and a ScrollPane? 
       
Answer: 
 
A Scrollbar is a Component, but not a Container. A ScrollPane is a 
Container. A ScrollPane 
handles its own events and performs its own scrolling. 
 
What is tunnelling? 
       
Answer: 
 
Tunnelling is a route to somewhere. For example, RMI tunnelling is a 
way to make RMI 
application get through firewall. In CS world, tunnelling means a way 
to transfer data. 
 
Java Language 
Question: 
 
Does the code in finally block get executed if there is an exception 
and a return statement 
in a catch block? 
       
Answer: 
 
If an exception occurs and there is a return statement in catch block, 
the finally block is 
still executed. The finally block will not be executed when the 
System.exit(1) statement is 
executed earlier or the system shut down earlier or the memory is used 
up earlier before the 
thread goes to finally block. 
 
What are use cases? 
       
Answer: 
 
A use case describes a situation that a program might encounter and 
what behavior the program 
should exhibit in that circumstance. It is part of the analysis of a 
program. The collection of 
use cases should, ideally, anticipate all the standard circumstances 
and many of the 
extraordinary circumstances possible so that the program will be 
robust. 
 
What is scalability and performance? 
       
Answer: 
 
Performance is a measure of "how fast can you perform this task." and 
scalability describes how 
an application behaves as its workload and available computing 
resources increase.
 
Java Language 
Question: 
 
What is the benefit of subclass? 
       
Answer: 
 
Generally: 
 
The sub class inherits all the public methods and the implementation. 
The sub class inherits all the protected methods and their 
implementation. 
The sub class inherits all the default(non-access modifier) methods and 
their implementation. 
The sub class also inherits all the public, protected and default 
member variables from the 
super class. 
The constructors are not part of this inheritance model. 
 
In System.out.println(),what is System,out and println,pls explain? 
       
Answer: 
 
System is a predefined final class,out is a PrintStream object acting 
as a field member and 
println is a built-in overloaded method in the out object.
 
What is meant by "Abstract Interface"? 
       
Answer: 
 
First, an interface is abstract. That means you cannot have any 
implementation in an interface.
 All the methods declared in an interface are abstract methods or 
signatures of the methods. 
 
Why Java does not support pointers? 
       
Answer: 
 
Because pointers are unsafe. Java uses reference types to hide pointers 
and programmers feel
 easier to deal with reference types without pointers. This is why Java 
and C-sharp shine.
 
Can you declare a class as private? (donated in April, 2005) 
       
Answer: 
 
Yes, we can declare a private class as an inner class. For example,
 
class MyPrivate {
    private static class MyKey {
        String key = "12345";
    }
    public static void main(String[] args) {
        System.out.println(new MyKey().key);//prints 12345
    }
}
 
Can Java code be compiled to machine dependent executable file? 
       
Answer: 
 
Yes. There are many tools out there. If you did so, the generated exe 
file would be run in the
 specific platform, not cross-platform.  
What are the drawbacks of inheritance? (donated by RS in Mar. 2005) 
       
Answer: 
 
Since inheritance inherits everything from the super class and 
interface, it may make the 
subclass too clustering and sometimes error-prone when dynamic 
overriding or dynamic overloading
 in some situation. In addition, the inheritance may make peers hardly 
understand your code if 
they don't know how your super-class acts.
 
Usually, when you want to use a functionality of a class, you may use 
subclass to inherit such 
function or use an instance of this class in your class. Which is 
better, depends on your 
specification.

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