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

Don't forget that at a later date you can take an existing class, and make it implement IBox, and it will then become available to all your box-aware code.

This becomes a bit clearer if interfaces are named -able. e.g.

public interface Saveable {
....

public interface Printable {
....

etc. (Naming schemes don't always work e.g. I'm not sure Boxable is appropriate here)