If all you have is the java class files than there isn't an easy way to open it (decompile time). If you have the .java files than you can open it in any text editor or java ide and then recvompile with the javac command. You will need a copy of the Java jdk or j2se (they changed lettering between version, jdk is for 1.1 to 1.3, j2se is 1.4). The jdk/j2se will have your compiler, libraries, and so on. If you don't know java already this may not be a simple task.
-tarwn