http://download.oracle.com/javase/tutorial/java/IandI/usinginterface.html
The example given in section 'Implementing the Relatable Interface' doesn't suggest a good design and might confuse beginner programmers, in my opinion:

As RectanglePlus implements the Relatable interface, I would expect that I can pass any kind of Relatable instances in the isLargerThan method, however, I would encounter an ugly big exception if I tried it with objects other than RectanglePlus (or its subclasses).
Hard coding exact types in an interface implementation questions the whole point of using an interface.