Multiple polymorphism is a powerful feature, unfortunately not available with standard Java, consider example
if multiple polymorphism would be available, depending on Currency type appropriate method would be invoked, although IBankAccount has only one method declared. With current Java only deposit(Currency curr) is invoked always.
You can look into http://www.cyberdyne-object-sys.com/oofaq2/DynamicDispatch.htm
(the above example is taken from their site), I once needed quite badly to have this functionality and stumbled onto this site. They say they've done Java implementation with this feature.
I think Sun could consider this as a future feature.