For time to time people produce "obfuscators" which attempt to make the contents of class files less readable, but you can never prevent decompilation completely. Why do you feel you need to do this ?
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You can use a professional obfuscation tool such as Dash-O-Pro which will render any decompiled class pretty much un-readable (apart from to the odd UberGeek or two) but there are drawbacks 1) Very Expensive 2) For every Obfuscator - there is a De-Obfuscator in the pipeline
Its not what you do, its the way you say you've done it.
Evacuate the building! Here, take this tiny ad with you:
a bit of art, as a gift, that will fit in a stocking