Via LtU, JPie:
JPie is an interactive programming environment designed to make the power of Java software development accessible to a wider audience
In JPie, all aspects of a program's behavior can be modified while the program runs, without the edit-compile-test cycle that routinely bogs down software development. When a class is modified in JPie, even existing objects are affected by the change.
In JPie, you don't describe programs in "code," but instead you directly manipulate their functional components.
JPie maintains detailed information about the software being developed and uses this to provide a more fluid software development process. For example, JPie's integrated thread-oriented integrated debugger supports fine-grain inspection of the execution, run-time code modification, and on-the-fly exception handling.
All sounds interesting... 
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