This seems to be a good task for a new contributor, easy and potentially
with high payoff (uncovering bugs), and a good way to become cursorily
familiar with diverse parts of the codebase by fixing the bugs it finds.
http://errorprone.info/ is a good static checker that covers a number of
bugs not covered by FindBugs or Checkstyle. We use it internally at Google
and, when run on the Beam codebase, it occasionally uncovers issues missed
during PR review process.
It has Maven and Gradle plugins:
It would be good to integrate it into our Maven and Gradle builds.