I have several versions of java installed on my ubuntu 9.04. I installed a couple manually and others I used repositories. I have java 6 set as my default but I use scripts for other applications like older versions of eclipse. After you install another version of java if you want to change your default use the update-java-alternatives command and it's options. A file worth mentioning is /etc/jvm.
I don't know what happened, I must have missed a decimal point or something. I was never good at Math.