Your problem is this. Say you have a factory class which gives you concrete instances of some implenting sub class.
This class has been loaded by a classloader different from the default one.

So if you have class B that extends abstract class A then the classloader stores exactly this info (dunno how)....

My guess is that you are trying to typecast the object returned to the interface (which of course if perfectly legal)... BUT... your current class loader does not yet have the information that the class of your current object actually implements your interface.. that information is stored only in your URL class loader...ur current classloader has only your interface loaded thus the cast fails.

A possible solution to this would be to explicitly load the class of your object using Class.forName or not use another classloader for loading your factory class.

Hope this helps