Interfaces

No multiple inheritance in Java

Interfaces allow the programmer to associate multiple attributes with a given blueprint, such that the user of the blueprint can choose just the one she wants.

An interface looks like this:

      public interface Dimensonality {
        int getDimensionality ();
      }
    

Using such a declaration we can change the first line of the classes above to look like this:

        public class Position implements Dimensonality {
      

and

        public class Position3D extends Position implements Dimensonality {
      

We can then refer to instances of Position or Position3D like so:

        Dimensonality obj = new Position (0, 0);
      

..so as far as the user is concerned in this usage, it isn't interesting that the object also has specific coordinates.