1.4 version
Bitwise operators
1.5 version
Generics (like previously mentioned)
Auto-boxing (especially when combined with Widening) box then widen
Enums (not that they are very hard - but since they are new - I haven't had the chance to use them on a real project yet - 1.4.2 is still our production Java).
And you didn't ask about this, but as will happen people tend to given additional advice. Whats the rush? You have been working with Java for a year and you want to be ready in 5 weeks? I've been working with Java for 5 years and decide to take this
test. Figured I could take a month or so to get ready. Once I started reading the k&B book - I knew I was Dead Wrong. I've learned so much I never even thought about before. Some I question the value of, like picking apart source code to find small compiler errors (public abstract void foo() {}), but others have been great insights.
So my final piece of advice - is take your time. Study, Code, Do Mocks, until you know when you walk into the exam you will pass. I can see this is a hard test, but this is also enough information about what is on it and what is not, one should know if one is ready or not.