If you have more than a passing interest in reading and writing concurrent code on the Java platform, hanging out on the concurrency-interest mailing list can be very educational. Occasionally you get to see upcoming revisions to Java libraries, or the mechanics of analyzing a potential change to JDK code.

For instance, this thread recently is an interesting review of CopyOnWriteArrayList's addIfAbsent() method and a potential change to it. Sure, you might not use CopyOnWriteArrayList all the time, and the difference between the two methods might not make a big deal to you, but it's interesting to see some of the kind of discussion and analysis that goes into this kind of a change.