There are lots of great free pracitce exams. You should probably do
them first.
Are you studying for SCJA or SCJP? Head First Java is good for SCJP,
but doesn't cover all the SCJA objectives.
I have a few free online practice exams at my site. Check them out:
www.scja.com
It all comes down to your confidence. If you're not confident enough,
take some exams.
Cheers!
-Cameron McKenzie
Author of the SCJA Certification Guide and What is WebSphere?
Certification Resources:
www.examscam.com
Free WebSphere Tutorials:
www.pulpjava.com
Free Mock Java Certification Exams:
www.scja.com
Free J2EE and Java Multimedia Tutorials:
www.mcnz.com