Howdy! A class-to-source decompiler doesn't ship with the Java SDK, but there is a disassembler. Try this at the command-line:
javap -c TheClassName
This gives you the bytecodes in a readable form; it doesn't reverse it back to .java. There are programs that *can* try to get the program back to a useful .java file, but I can't remember what they are at the moment.
The bytecodes can be quite tedious, but also really cool to study. Occasionally, it can be useful to see what the compiler is really doing with your code.
cheers,
Kathy