This article has a pretty good answer as long as you still have your boot disk or can find a downloadable version of Java you need. Here are the instructions (copied and pasted from the article):
1 Recommend making a bootable backup before starting
2 Locate the required Java installers
- Leopard: download update 5
- Snow Leopard: grab your install DVD
3 Remove current installed java stuff (warning: no going back from this point on!). In terminal:
sudo rm -R /Applications/Utilities/Java\ Preferences.app/
sudo rm -R /System/Library/Frameworks/JavaVM.framework/
sudo rm -R /System/Library/Java/
sudo rm -R /System/Library/CoreServices/Jar\ Launcher.app
sudo rm -R /System/Library/CoreServices/Java\ Web\ Start.app/
sudo rm -R /System/Library/CoreServices/JavaVersion.plist
sudo rm -R /System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/
sudo rm -R /System/Library/Frameworks/JavaEmbedding.framework/
sudo rm -R /System/Library/CFMSupport/StubLibraries/JavaEmbeddingLib/
4.1 Reinstall java: Leopard
- run the installer that you downloaded in step 1
4.2 Reinstall java: Snow Leopard (haven't tested yet! but should work)
- insert installer DVD
- run terminal commands (change the paths in each command as necessary for both the OS X Install disc and your system hard drive):
installer -verbose -pkg "/Volumes/Mac OS X Install Disk/System/Installation/Packages/Java.pkg -target /Volumes/Macintosh HD"
installer -verbose -pkg "/Volumes/Mac OS X Install Disk/System/Installation/Packages/JavaTools.pkg -target /Volumes/Macintosh HD"
- (alternately you can use a tool like Pacifist to browse for and run the Java.pkg and JavaTools.pkg installers located in the main .pkg installer)
Note: we have confirmed this works for Leopard but haven't verified for Snow Leopard yet.