Any idea how to reverse obfuscated java code !?
Decompilation is useful for reverse engineering, but in most cases, you won't be able to recompile the results, because compilation and decompilation are lossy processes and obfuscation exacerbates that.
If you understand Java bytecode, a good way to edit JARs is to use a bytecode assembler/disassembler such as Krakatau. Since this operates directly at the level of compiled bytecode, you can edit any Java classfiles, no matter how obfuscated, and you don't have to worry about compiler errors.