EDIT: This is for those that haven’t (yet) upgraded to Mac OS X 10.6 (Snow Leopard) where Java SE 6 is now default and comes with sources and documentation by default.
Mac OS X 10.5 comes with Java 5 and 6 (v1.5 and v1.6 respectively). By default, however, only the documentation and source for Java 5 is provided. If you want to develop for Java 6 you’ll need to download the Java for Mac OS X 10.5 Update 5 Developer Documentation (Update 5 was at time of writing most current).
Where you get it? Log into Apple Developer Connection website (using your Apple ID or free registration) and go to Downloads > Java. That section should have always the most current update. Don’t make the mistake going Documentation as it will have older updates.