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1.What is a file?


A file is a named location which stores data or information permanently. A file is always stored inside a storage device using file name (e.g. STUDENT.MARKS). A file name normally has primary and secondary name separated by a “.”(DOT). 

2.What is a class?


A class is a blueprint from which objects are created. A class contains methods and variables associated with an instance of a class. 

3.What is an object?


An object is an instance of a class. For example

class Abc{ —– This is a class

int a; —— This is a variable

public Abc(); —- This is contractor

public static void main (String args[]) ——- This is a method


Abc a= new Abc(); —— This is object creation where ‘a’ is the reference variable or object name



4.What is a constructor?


A constructor is methods which are used to create an Object of class. There are two types of constructor Default & Parameterized constructor. 

5.What is the different OOPS principle?


The basic OOPS principle are as follows,

• Encapsulation

• Abstraction

• Inheritance

• Polymorphism

6.What is inheritance?


Inheritance is property in which the property of a parent class(Superclass) is passed on to child class(Subclass). For example

class Abc{ —– This is a class

int a; —— This is a variable

public void abc(){} — Methods


class Xyz extends Abc —–(Extend is the keyword, Xyz is the subclass which inherits the properties of ABC parent class.)


public static void main (String args[]) ——- This is a method


Abc a= new Abc(); —— This is object creation where ‘a’ is the reference variable or object name



7.What is polymorphism?


Polymorphism is the ability of an object to take on multiple forms. Most commonly polymorphism is used in OOP when a parent class reference is used to refer to a child class object. 

8.What are instance and class variables?


Instance variable belongs to a particular instance of that class whereas Class variable. A class variable is also known as static variables. For example 

public class Abc{

public int a; …….. This is an instance variable

public static int a1;…….. This is a static or class variable




9.Compare method and constructor?


Constructor: Used to initialize the instance of a class.

Method: Used to perform some function or operation.

Constructor: Doesn’t have a return type.

Method: Has a return type.

10.What is a singleton class?


Singleton class limits the number of objects created for a class to one but gives the flexibility of creating more objects if the situation changes. 

11.What are the steps for creating the object?


An object is first declared then instantiated and at last declared. For example

Abc a= new Abc();

12.What is the different type of access modifiers?


There are four type of access modifiers as given below:-

• Visible to the overall package. No modifier needed.

• Private – Visible to class only.

• Public – Visible to the world.

• Protected – Visible to package and subclass.

13.Which is the highest operator precedence in Java


The operator with the highest preference is Postfix operators i.e () [].

14.What is an array?


The array is a container which holds the fixed number of similar data types.

15.What is the difference between equals() and method and == operator?


The equals() is a method and it matches the content of the strings whereas == is an operator and matches object or reference of the strings. 

16.Is string class final?



17.What is a wrapper class?


To access the primitive data type as an object we use wrapper class. They are following:-

Primitive Type Wrapper class

boolean Boolean

char Character

byte Byte

short Short

int Integer

long Long

float Float

double Double

18.Difference between overloading and overriding?


Overloading is when two or more methods in the same class have the same method name but different parameters(i.e different method signatures). 

Overriding is when two methods having the same method name and parameters (i.e., method signature) but one of the methods is in the parent class and the other is in the child class. 

19.What are multiple inheritances in Java?


Java supports multiple inheritances i.e the ability of a class to implement more than one interface. A class can implement multiple Interfaces but cannot extends multiple classes. 

20.What is a stream?


A stream can be defined as the sequence of data. There is two type of streams.

InputStream: Used to read a data from a source.

OutPut Stream: Used to write a data into a destination.

21.What is a Character stream?


Java Character stream is basically used to perform input and output for 16 bit Unicode. The main classes users are FileReader and FileWriter which internally uses FileInputStream and FileOutputStream so the basic difference is that FileReader and FileWriter read and writes two bites at a time respectively. 

22.What is a Byte stream?


Java Byte stream is basically used to perform input and output for 8 bit Unicode.

The main classes related to byte streams are FileInputStream and FileOutputStream.

23.What is an Interface?


The interface is a reference type in Java, similar to the class but its collection of abstract methods. A class can implement multiple interfaces. 

24.Difference between class and interface?


Below are the difference between Interface and class:-

• The interface cannot be instantiated.

• An interface doesn’t have any constructors.

• Interface only have abstract methods.

• A class implements an interface and extends a class.

• An interface can extend multiple interfaces.

25.What is an abstract class?


A class which contains the abstract keyword in a declaration is called abstract class. The properties of the abstract class are as follows:- 

• Abstract classes may or may not contain abstract methods but, if a class has at least one abstract method, then it must be declared abstract. 

• The abstract class cannot be instantiated.

• To use an abstract class, we have to inherit it from another class.

• If we inherit an abstract class, then we have to provide implementations to all the abstract methods in it.

• Question 1. What Is An Object In C++?

Answer :

An object is a package that contains related data and instructions. The data relates to what the object represents, while the instructions define how this object relates to other objects and itself. 

• Question 2. What Is A Message?

Answer :

A message is a signal from one object to another requesting that a computation take place. It is roughly equivalent to a function call in other languages. 

• Question 3. What Is A Class?

Answer :

A class defines the characteristics of a certain type of object. It defines what its members will remember, the messages to which they will respond, and what form the response will take. 

• Question 4. What Is An Instance?

Answer :

An individual object that is a member of some class.

• Question 5. What Is A Super-class?

Answer :

Given a class, a super-class is the basis of the class under consideration. The given class is defined as a subset (in some respects) of the super-class. Objects of the given class potentially posses all the characteristics belonging to objects of the super-class. 

• Question 6. What Is Inheritance?

Answer :

Inheritance is property such that a parent (or super) class passes the characteristics of itself to children (or sub) classes that are derived from it. The sub-class has the option of modifying these characteristics in order to make a different but fundamentally related class from the super-class. 

• Question 7. To What Does Message Protocol Refer?

Answer :

An object’s message protocol is the exact form of the set of messages to which the object can respond.

• Question 8. What Is Polymorphism?

Answer :

Polymorphism refers to the ability of an object to respond in a logically identical fashion to messages of the same protocol, containing differing types of objects. Consider 1 + 5 and 1 + 5.1. In the former, the message “+ 5” is sent to an object of class integer (1). In the later, the message “+ 5.1” is sent to the same integer object. The form of the message (its protocol) is identical in both cases. What differs is the type of object on the right-hand side of these messages. The former is an integer object (5) while the later is a floating point object (5.1). The receiver (1) appears (to other objects) to respond in the same way to both messages. Internally, however, it knows that it must treat the two types of objects differently in order to obtain the same overall response. 

• Question 9. What Are Instance Variables?

Answer :

These represent an object’s private memory. They are defined in an object’s class.

• Question 10. What Are Class Variables?

Answer :

These represent a class’s memory which it shares with each of its instances.

• Question 11. What Is A Method?

Answer :

A method is a class’s procedural response to a given message protocol. It is like the definition of a procedure in other languages. 

• Question 12. In C++ What Is A Constructor? A Destructor?

Answer :

A constructors and destructors are methods defined in a class that are invoked automatically when an object is created or destroyed. They are used to initialize a newly allocated object and to cleanup behind an object about to be removed. 

• Question 13. Compare And Contrast C And C++.?

Answer :

Comparison: C++ is an extension to the C language. When C++ is used as a procedural language, there are only minor syntactical differences between them. 

Contrast: When used as a procedural language, C++ is a better C because:

• It vigorously enforces data typing conventions.

• It allows variables to be defined where they are used.

• It allows the definition of real (semantically significant) constants.

• It allows for automatic pointer dereferencing.

• It supports call-by-reference in addition to call-by-value in functions.

• It supports tentative variable declarations (when the type and location of a variable cannot be known before hand. 

As an object oriented language, C++ introduces much of the OOP paradigm while allowing a mixture of OOP and procedural styles. 

• Question 14. What Is Operator Overloading?

Answer :

It is the process of, and ability to redefine the way an object responds to a C++ operator symbol. This would be done in the object’s class definition. 

• Question 15. What Is Cin And Cout?

Answer :

They are objects corresponding to a program’s default input and output files.

Contrast procedural and object oriented programming.

The procedural paradigm performs computation through a step-by-step manipulation of data items. Solving problems this way is akin to writing a recipe. ie: All the ingredients (data items) are defined. Next a series of enumerated steps (statements) are defined to transform the raw ingredients into a finished meal. 

The object oriented model, in contrast, combines related data and procedural information into a single package called an object. Objects are meant to represent logically separate entities (like real world objects). Objects are grouped together (and defined by) classes. (This is analogous to user defined data types in procedural languages.) Classes may pass-on their “makeup” to classes derived from them. In this way, Objects that are of a similar yet different nature need not be defined from scratch. 

Computation occurs though the intercommunication of objects. Programming this way is like writing a play. First the characters are defined with their attributes and personalities. Next the dialog is written so that the personalities interact. The sum total constitutes a drama. 

• Question 16. How Do You Link A C++ Program To C Functions?

Answer :

By using the extern “C” linkage specification around the C function declarations.

You should know about mangled function names and type-safe linkages. Then you should explain how the extern “C” linkage specification statement turns that feature off during compilation so that the linker properly links function calls to C functions.

Source: Contents are provided by Technicalsymposium Google Group Members. 
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