Open source java compiler and virtual machine. It is 1.1 java and not 1.2/1.3 java . This is very new and needs lots of work. Also it is for linux only! http://sources.redhat.com/java/ So I would not use it to help you study for the test. www.blackdown.org this is the official sun java 1.2/1.3 java compiler and vm for linux . http://www.blackdown.org/ This is pure 100% java and 100% compatible with java for solaris and win32 /win95/98/nt/2000/me . ps: Multi-threading is not cross-platform. But everything else is. The threading issue is being addressed with solaris to have the swapping threads instead of priority threads.