I've been a longtime linux user (early 90s :) but all the cool kids have been using the mac these days. Apple sure knows how to make superb hardware and the UI isn't quite all that bad either though I do miss the extreme customiation I could do with some of the linux window managers. But I digress ...
Anyways, this came up recently so I thought I'd blog about it. Generally most OSes will install Java at the OS level and scatter the various parts in different places depending on the OS. Linux distro will typically put stuff in /usr/share or some other location, Windows will put stuff in c:\Program Files\ and the Mac folks seem to prefer /System/Library/... I generally find it more beneficial to just install various versions of the JDK to my user's home folder and then to just switch between them via $JAVA_HOME and $PATH.
Well, here's how you do that with the dmg mac installer: