It took a while for me to look through the book, but it is an excellent resource for those who want to get the most out of PMD. I really appreciate Tom's efforts on the PMD project, and think it is definitely the strongest "style checking" static analysis tool around for Java code. We also use Checkstyle in our grading system. While there is substantial overlap between the checks provided by Checkstyle and PMD, they each offer some features that the other one does not.

PMD is still my favorite, however. I really like the ability to write new checks as XPATH expressions and just add them to the XML config file for PMD, without having to write any new code. PMD even comes with an interactive XPATH expression tester that allows you to enter code fragments together with your XPATH rule definition so you can interactively see what your new rule matches and what it doesn't. This is a powerhouse feature, and makes it very easy to extend PMD with your own checks. Try it out for yourself.