A bytecode decompile is a tool which usually is seldom needed but when needed you would want to have it handy. There seems to be a lot of time since I’ve used one and when I needed one now I didn’t know what to choose, there are a lot outdated ones, which don’t decompile Java 1.5 correctly.
After a web search I’ve found Java Decompiler which has even a GUI and is quite good.