[codingvelocity]

I believe older java versions are already incompatible with newer jvms. I couldn't find it explicitly stated but i'd be surprised if 1.4 code worked 100% correct on a 1.7 jvm.

My experience so far is that the list of incompatibilities (http://www.oracle.com/technetwork/java/javase/compatibility-...) between versions makes it nearly impossible to just drop code between versions. It will run, but often implementation details have changed causing significant bugs. Your mileage may vary

[wener]

I think so, the implementation is hard to control than jvms.As long as jvm can run older(e.g. 1.4) version bytecodes, jvms is compatibility, but what if jvm drop that, then support real type generic like C#, etc, that's would be wonderful.