Ya know, I couldn't tell you. On a class-by-class basis, I don't know. I imagine IBM came out with their JVM in part because the were/are the biggest corporate backer in the Eclipse project. It is an ECMA standard, to my recollection.
Kinda like Novell and Mono...with C# being an ECMA standard, Novell and a community took some initiative and ported .NET and C# to the *nix platforms (Unix, Linux, Mac OS X, and even Windows) So now with something like 98% compatibility .NET programmers can compile their code to run on Mono too.