I need to analyze a java application (a .jar file) for a war game. I know java can be decompiled, but the coder apparently did a lot of obfuscation to prevent the reverse engineers from understanding how his/her application works. So I thought it would be better if I used a debugger. Would you guys recommend me a debugger like ollydbg but for debugging java applets?
jdb/jdebugtool seem to fit your requirements.
Though a decompile (http://java.decompiler.free.fr/), pulling into your favourite IDE and then stepping through may be a neater approach? That way you can isolate methods and play with them individually. If you have a behaviour you want performed, that you can identify, such as incrementing a specific counter, you can cut through a lot of obfuscation by stepping through from something else (such as click button 'A') that you know should have that ultimate effect.