Mac OS X Update Java

Mac OS X 10.5.2+ Install Java 1.6
http://www.apple.com/downloads/macosx/apple/application_updates/javaformacosx105update1.html
57MB Download

Mac OS X 10.5.4+ Update your java.
This update will upgrade your version to a later release. ie if you 1.4.2 it will be the latest 1.4.2_18. 1.5 to the latest 1.5_16.
http://support.apple.com/downloads/Java_for_Mac_OS_X_10_5_Update_2
135MB Download

Now change your default java
/Applications/Utilities/Java/Java Preferences.app

Apple Java Info
http://developer.apple.com/java/javaleopard.html