Java Interview Questions : Series 2

 12. What is the difference between

byte and char data types in Java?

Both byte and char are numeric data types in Java. They are used to

represent numbers in a specific range.

Major difference between them is that a byte can store raw binary

data where as a char stores characters or text data.

Usage of char is E.g. char ch = ‘x’;

Byte values range from -128 to 127.

A byte is made of 8 bits. But a char is made of 16 bits. So it is

equivalent to 2 bytes.

13. What are the main principles of

Object Oriented Programming?

Main principles of Object Oriented Programming (OOPS) are:

1. Abstraction

2. Encapsulation

3. Inheritance

4. Polymorphism

14. What is the difference between

Object Oriented Programming

language and Object Based

Programming language?

Object Oriented Programming languages like Java and C++ follow

concepts of OOPS like- Encapsulation, Abstraction, Polymorphism

and Inheritance etc.

Object Based Programming languages follow some features of

OOPS but they do not provide support for Polymorphism and

Inheritance. Egg. JavaScript, VBScript etc.

Object Based Programming languages provide support for Objects

and you can build objects from constructor. They languages also

support Encapsulation. These are also known as Prototype-oriented

languages.

15. In Java what is the default value of

an object reference defined as an

instance variable in an Object?

All the instance variable object references in Java are null.

16. Why do we need constructor in

Java?

Java is an object-oriented language, in which we create and use

objects. A constructor is a piece of code similar to a method. It is

used to create an object and set the initial state of the object.

A constructor is a special function that has same name as class

name.

Without a constructor, there is no other way to create an object.

By default, Java provides a default constructor for every object. If

we overload a constructor then we have to implement default

constructor.

17. Why do we need default

constructor in Java classes?

Default constructor is the no-argument constructor that is

automatically generated by Java if no other constructor is defined.

Java specification says that it will provide a default constructor if

there is no overloaded constructor in a class. But it does not say

anything about the scenario in which we write an overloaded

constructor in a class.

We need at least one constructor to create an object, that’s why Java

provides a default constructor.

When we have overloaded constructor, then Java assumes that we

want some custom treatment in our code. Due to which it does not

provide default constructor. But it needs default constructor as per

the specification. So it gives error.

18. What is the value returned by

Constructor in Java?

When we call a constructor in Java, it returns the object created by

it. That is how we create new objects in Java.

19. Can we inherit a Constructor?

No, Java does not support inheritance of constructor.

20. Why constructors cannot be final,

static, or abstract in Java?

If we set a method as final it means we do not want any class to

override it. But the constructor (as per Java Language

Specification) cannot be overridden. So there is no use of marking it

final.

If we set a method as abstract it means that it has no body and it

should be implemented in a child class. But the constructor is called

implicitly when the new keyword is used. Therefore it needs a

body.

If we set a method as static it means that it belongs to the class, but

not a particular object. The constructor is always called to initialize

an object. Therefore, there is no use of marking constructor static.

Inheritance

21. What is the purpose of ‘this’

keyword in java?

In Java, ‘this’ keyword refers to current instance of the object.

It is useful for differentiating between instance variables and local

variables.

It can be used to call constructors. Or it can be used to refer to the

instance.

In case of method overriding, this is used for falling the method of

current class.

22. Explain the concept of

Inheritance?

Inheritance is an important concept in Object Oriented

Programming. Some objects share certain characteristics and

behavior. By using Inheritance, we can put the common behavior

and characteristics in a base class which also known as super class.

And then all the objects with common behavior inherit from this

base class.

It is also represented by IS-A relationship.

Inheritance promotes, code reuse, method overriding and polymorphism.

23. Which class in Java is superclass

of every other class?

Java is an object oriented programming language. In Java, Object

class is the superclass of every other class.

24. Why Java does not support

multiple inheritance?

Multiple Inheritance means that a class can inherit behavior from

two or more parent classes.

The issue with Multiple Inheritance is that both the parent classes

may have different implementation for the same method. So they

have different ways of doing the same thing. Now which

implementation should the child class choose?

This leads to ambiguity in Multiple Inheritance. This is the main

reason for Java not supporting Multiple Inheritance in

implementation.

Lets say you have a class TV and another class AtomBomb. Both

have method switchOn() but only TV has switchOff() method. If

your class inherits from both these classes then you have an issue

that you can switchOn() both parents, but switchOff will only

switchOff() TV.

But you can implement multiple interfaces in Java.

25. In OOPS, what is meant by

composition?

Composition is also known as “has-a” relationship. In composition,

“has-a” relation relates two classes. E.g. Class Car has a steering

wheel.

If a class holds the instance of another class, then it is called

composition.

26. How aggregation and composition

are different concepts?

In OOPS, Aggregation and Composition are the types of association

relations. A composition is a strong relationship. If the composite

object is destroyed, then all its parts are destroyed. E.g. A Car has a

Steering Wheel. If Car object is destroyed, then there is no meaning

of Steering Wheel.

In Aggregation, the relationship is weaker than Composition.

E.g. A Library has students. If a Library is destroyed, Students still

exist. So Library and Student are related by Aggregation. A Library

has Books. If Library is destroyed, the Books are also destroyed.

Books of a Library cannot exist without the Library. So Book and

Library are related by Composition.