Mac Java
From Software By Jeff
Sun Microsystems doesn't provide a Mac version of the JDK, but Apple does.
Installation
Installation of the Java Software Development Kit on Mac OSX is easy--it's already there. Apple has included the JDK in every installation of OSX as a means to ensure the features are available to all Mac users.
Updating the JDK included with the OS is done using the System Updater, and should be done automatically for most systems.
Note that the version of the J2SE included is 1.4, and that there is no J2EE or J2ME release for Mac. The features of J2EE must be added as extra libraries, and using a Mac to develop J2ME applications will simply use the target features of the JDK to ensure unavailable J2SE features are not used.
Additional Resources
Mac OSX Java Features (http://www.apple.com/macosx/features/java/)
Mac Java Developer Resources (http://developer.apple.com/java)