I've just installed Mac OS 10.4.0 and added the J2SE 5.0 R1 available from Apple.
The 5.0 release installs alongside the 1.4.2 release. However, after that installation, I had problems with the 1.4.2 producing error messages:
$ java -version
HotSpot Error: update_sharing has failed
java version "1.4.2_07"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_07-215)