Is there more to an interface than having the correct methods

Question

So lets say I have this interface:

public interface IBox
{
   public void setSize(int size);
   public int getSize();
   public int getArea();
  //...and so          


        

Answer 1

Normally Interfaces define the interface you should use (as the name says it ;-) ). Sample

public void foo(List l) {
   ... do something
}

Now your function foo accepts ArrayLists, LinkedLists, ... not only one type.

The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class! Sample:

class Test extends Foo implements Comparable, Serializable, Formattable {
...
}

is possible but

class Test extends Foo, Bar, Buz {
...
}

is not!

Your code above could also be: IBox myBox = new Rectangle();. The important thing is now, that myBox ONLY contains the methods/fields from IBox and not the (possibly existing) other methods from Rectangle.