Damn dude, I usually don't do this sort of sh.t in public, especially
on USENET.  But here it is, I got it directly from sun.  It has all the
f.cking answers!!!  You'll be certified in no time.  P.S. remember
don't tell anyone I gave this to you...

http://java.sun.com/docs/books/jls/download/langspec-3.0.pdf

For Java Programmer certification.
Buy the certification guide book. Most of the questions in the exam
will be from the guide.
javaranch web site has some good mock exams that you can go through.
Advanced topics like Threads depend on your comprehension. You can mug
up the answers. Hope this helps

Mohan Kakulavarapu
Freelance Java Developer