The folks at mockobjects.com have done a good job of creating MockObjects for the Java API. I'm not sure how to use some of their objects yet, but once I learn a few of the basic patterns I'll put a tutorial out here.
They also have links to several MockObject code-generation tools, which may be helpful. These include EasyMock, MockCreator, and MockMaker. Which one is the best or most popular? Dunno yet. That's often an interesting part of the open source world.