This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Is there a way to list all classes available in the class path? If so, you can use reflection to check for each of the classes if they implement a given interface / extend a given (super) class.
However, I'm not exactly sure if there is a way of listing all classes in the class path - afraid there isn't. But you might be able to read the class path and search it for .class files, and also search all the .jar files you find for .class entries. (I do some plugin loading by searching jars for implementations of specific interfaces).
What do you intend to do with the result of your search? Maybe there's a different way of achieving the same functionality.