Peter Norvig has an excellent list of Java related IAQ's (Infrequently Answered Questions) on his website. He talks about stuff that cover various subtleties in Java. Here's an example:
Q:Within a method m in a class C, isn't this.getClass() always C?No. It's possible that for some object x that is an instance of some subclass C1 of C either there is no C1.m() method, or some method on x called super.m(). In either case, this.getClass() is C1, not C within the body of C.m(). If C is final, then you're ok.
-- Peter Norvig