what is the actual use of interface in java? [duplicate]

Question

Possible Duplicates:
Abstract class and Interface class?
Java: interface / abstract classes / abstract method

In Java, whatever use of interface is fulfilled by abstract class. I know one advantage of interfaces is that if we implement an interface then we can also extend another class. Is there any other use or advantage of interface in Java?

Answer 1

Interfaces allow you to use classes in different hierarchies, polymorphically.

For example, say you have the following interface:

public interface Movable {
    void move();
}

Any number of classes, across class hierarchies could implement Movable in their own specific way, yet still be used by some caller in a uniform way.

So if you have the following two classes:

public class Car extends Vehicle implements Movable {
    public void move() {
       //implement move, vroom, vroom!
    }
}

public class Horse extends Animal implements Movable {
    public void move() {
       //implement move, neigh!
    }
}

From the perspective of the caller, it's just a Movable

Movable movable = ...;
movable.move();  //who am I?

I hope this helps.

Answer 2

What you like : thousands of abstract methods in one Abstract Class and inherit this class OR make as many interfaces for specific abstract methods and use those only you want by inheriting as many interfaces as needed...

abstract class A
{
 //thousands of abstract method();
 abstract methodA();
 abstract methodB();
 abstract methodC();
}

//OR
interface ForOnlymethodA
{
 void methodA();
}
interface FormethodBandmethodC
{
 void methodB();
 void methodC();
}

So, use that method only what you just need by inheriting particular interface, if you are inheriting Abstract classes then you are unnecessarily inheriting all methods that you don't need in one class and may be needed in some other classes..