The same as this question but for java
Update Based on the comments and responses of a few people, Its clear that Java has very little undefined behaviour.
So I'd like to ask as well what behaviour is not obvious. Please when answering make the distinction between the two :)
I think that Java(TM) Puzzlers: Traps, Pitfalls, and Corner Cases book will be very useful, it explains many hidden points and undefined behaviours of java.