Josh Bloch has (in my opinion) one of the best Java books out there for improving your understanding of the Java Language – Effective Java
In the same vein Josh Bloch and Neal Gafter have been presenting ‘Java Puzzlers’ at JavaOne and other conferences, presenting an unusual snippet of code and challenging people to work out what the code does. JavaLobby have a recording of one of their sessions at JavaPolis.
Also, Bloch and Gafter released a book which is a collection of these code puzzles, which is well worth a look (it’s currently on my Amazon Wish List 🙂 – Java Puzzlers