There are free tools out there. I can tell you such a thing called mocha exists, but there's enough legal trouble surrounding its distribution that
you should search for it yourself. In short, Symantec has been trying for years to apply legal pressure on mocha's author. You can get it, though, if you look.
For something less controversial, try
JODE.
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Michael Ernest, co-author of:
The Complete Java 2 Certification Study Guide