You're looking for a decompiler? Any IDE has one built in....just Ctrl click the classes and their decompiled source comes up.
IntelliJ or Eclipse. Choose one or both. If you ask anywhere it's going to be a war between fans of each IDE.
hello there, i am interested on the job. feel free to pm me with your contact details.
I can help. whats the file format you have with you. Is it a .exe file? Are you sure the software is written in JAVA?