Basic use of interface

Suppose you have three classes Ferrari, Toyota and Ford. All these are Cars and can move().

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Ferrari scaglietti = new Ferrari(); Toyota yaris = new Toyota(); Ford focus = new Focus();

If you wanted to call move() for all 3 objects you should code like this:

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yaris.move(); scaglietti.move(); focus.move();

But if you had 1 thousand of cars it would be impossible. So you say

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interface Car {    void move(); }   class Ferrari implements Car { ... public void move() ... } class Toyota implements Car { ... public void move() ... } class Ford implements Car { ... public void move() ... }

And then you can:

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Car[] myCars = { ... }   for(each car in myCars) {    car.move(); }

It is called an UPCAST. This is one of the major goals of interfaces. Of course, there are lots of other things, you'd better read the Sun Tutorial linked by Fubarable