When we updated the core Java Tutorial to JDK 6, we included a page designed to help those who are studying for Java Certification.
This study guide calls out which portions of the tutorial are relevant to the testing objectives -- it is by no means comprehensive. We still sometimes receive questions on the tutorial feedback alias asking how to prepare for the Java Programmer Certification test.
The book strongly recommended by our Java instructors and course developers is the Complete
Java 2 Certification Study Guide by Philip Heller.