Most of the stuff should be pretty familiar to you. I'd just download eclipse and google a tutorial site. Familiarize yourself with classloading, keywords. One tricky thing a lot of C++ guys run into is how to run a java app so that it finds its library classes(sort of analogous to dynamic linking). Learn the difference between the JRE and JDK. If you can get a few hello world type apps working you ought to be able to get a start on hadoop if you follow the tutorials.
