What your talking about sounds more like some form of scripting exploit or the like.......

If you tried to hack with Java, I believe the largest problem you would face would be that java runs in a virtual machine.. so even if you were able to find an exploit in a Java program, it would still only be interfacing with the VM.. not the actual OS..

I could be wrong though.. All I've learnt about Java is by discussing it with friends.. I've never done any real work on it...