interfaces..

[sireesha]

Hi all,
I saw this code in a book.

Code:

interface Lion extends Animal {}
	Lion king = ...;

In the above code Lion is an interface which extends Animal interface.
But in the second line ,
Is King is the object of type Lion ?
Can we create Objects for interfaces ?
I am very confused about it.
Please tell me...
Thankq very much.

[tim]

Hello sireesha.

An interface is like a list of methods. Those classes that implement that interface must define those methods. An interface on it own is not an object because it is not a subclass of the Object class. A nice feature about interfaces is that you can give functionality to classes that are not related but have similar behavior. Lets say we have the interface Movable:

Code:

public interface Moveble{
    public abstract void move();
}

Note that method definitions of interfaces are empty. Now, lets say we have a class Car and a class Animal that implements the Moveble interface. They are not related but they can both move.
So now we have

Code:

public class Car implements Moveble {
    private int position = 0;
    public Car(){
    }
    public void move(){
        position += 5;
    }
}

and

Code:

public class Animal implements Moveble {
    private double position = 0;
    public Animal (){
    }
    public void move(){
        position += 2.5;
    }
}

Note that the classes are very different but have the move() method in common. Now to answer this:

Quote:

Originally Posted by sireesha

Can we create Objects for interfaces ?

No, but you can create a variable for classes that implement that interface and use the methods of that interface. Any object that is an instance of a class that implements that interface can be assigned to that variable:

Code:

Moveble myMovebleObject = new Animal();
myMovebleObject.move();

or

Code:

Moveble myMovebleObject = new Car();
myMovebleObject.move();

I hope this cleared things up.
This code has been checked by hand. Please correct me if I made a mistake.

[sireesha]

Thanks for your explanation,
i have one more doubt,
what is the difference between

Code:

Moveble myMovebleObject = new Animal();
myMovebleObject.move();

and

Code:

Animal a=new Animal();
a.move();

In both cases we are calling move method of Animal class.
then what is the difference between them ?

[roots]

I have always felt learning language comes with this baggage that "You need to understand all to start with". I expect this question be raised again and again, for now Dr. Dobb's | Design by Interface | February 1, 1999 , Keywords: Design By Contract, Loose Coupling might give you some hints in understanding the difference between your codes above.

[tim]

Quote:

Originally Posted by sireesha

Thanks for your explanation,
i have one more doubt,
what is the difference between

Code:

Moveble myMovebleObject = new Animal();
myMovebleObject.move();

and

Code:

Animal a=new Animal();
a.move();

In both cases we are calling move method of Animal class.
then what is the difference between them ?

Hello

The difference is that the variable, a, can only keep instances of the Animal class, including all its subclasses, while the variable, myMovebleObject, can only keep classes that implement the Moveble interface, including all the subclasses of those classes.

Hope this clears things up.

[sireesha]

thankq very much,now i got it.