Question

7. What are abstract classes and interfaces in Java? What are they used for, and what are they comprised of? Clearly explain the difference between the two.

Answer 1

Abstract classes in Java

• They are so many modifiers in java abstarct is one of the access modifier in java and also a keyword
• "Abstract" means no implementation and no code nothing it just declares method .
• If the keyword abstract is used infront of class then that class is called as abstract class
• It provides abstraction but not 100% bcz it may contains the concrete method

Synatax: abstract class classname{}

• If you want to use it you need to be extends by other class otherwise it has no use.
• A class is called concrete if it does not contain any abstract method and implements all abstract method inherited from abstract class or interface it has implemented or extended
• Abstract class can have abstract methods and concrete methods it may or maynot contains the abstract method but any class having abstract method then it must be declared as abstract

AbstractMethod

• IF a method that doesnot contains any body or if we simply declares the method and not defined it is abstract method and it can be implemented by extending it in subclass

Syntax:

Public abstract returntype methodname();

Example

abstract class Abex{

public void NM(){

System.out.println("Normal method");

}

abstract public void AM();

}

class example extends Abex

public void AM()

{

System.out.println("Implementing Abstarct method in subclass");

}

public static void main(String args[])

{

example obj = new example();

obj.AM();

}

}

Output :   Implementing Abstarct method in subclass

Interface in Java

• It is the another way to provide abstraction in java and by default interfaces are abstract and provides 100% abstraction
• It looks like a class but it exactly not ,it can have methods and variables just like a classbut here methods are by default abstract methods and variables in this interface are public static final by default
• It can bee declared by a keyword "interface"

Syntax interface

interface-name{
void m1();
void m2();
}

Purpose

• As it provides 100% abstraction all method in that interface doesnt have body
• If we want to acess the method in interface they have to be implemented in another class
• A class can implements interface by extending another interface

Example

interface Inter
{
public void m1();
public void m2();
}

class sam implements Inter
{

public void m1()
{
system.out.println("implementation of m1");
}
public void m2()
{
system.out.println("implementation of m2");
}
public static void main(String args[])
{
Inter obj = new sam();
obj.m2();
}
}
Output: implementation of m2

Difference between the two

Abstract Class	Interface
It can have both abstract methods and normal methods	By default all methods here are abstract
It can be declared by using keyword abstract	It can be declared by using keyword interface
It can have final,static,non-final,non-static variables	it can only have final and static variables
A class can extend only one abstract class	An class can implements by any number of inerfaces