An interesting thread started up on JavaRanch about the new Java 1.4.2 update for Mac OS X. I'll let you read it for yourself, but essentially I never realised that there is an additional Java developer package that you can download from the Apple Developer Connection (login required).
It's a 50Mb download and contains Javadocs, examples and additional tools. Notice from the screenshot what the prerequisites for this install are and what this install adds. I'm not sure that there are any further tools supplied (but I've not stopped looking and besides, the Java distributions contain javac anyway), but there is tons of documentation, including the Java 1.4.2 API. There's also loads of other stuff kicking around underneath /Developer that I've not looked at before. Anybody found the extra tools yet?
Read: Java 1.4.2 Developer Package - documentation and Javadocs