The subject of decompiling Java - or trying to protect it with obfuscators - has come up in these forums previously.
Now, there's a book that covers the subject in depth. Decompiling Java, by Godfrey Nolan, is published by Apress.
It goes deep into compilation and reverse-engineering and even walks through how to build an obfuscator and a decompiler.
Check out the sample code
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