How to force update-alternatives to update jdk?

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http://askubuntu.com – I've got java-6-openjdk and java-7-openjdk running on Natty (11.04) on one machine and successfully toggle between them using update-alternatives --config java (or javac). For some reason this doesn't seem to work on a second machine of identical spec. The sym links in /etc/alternatives gets updated when i run update-alternatives but when i check 'java -version' or 'which java' it's always java-6-openjdk. I've been running java 7 using the entire path but this isn't ideal as I don't want to clutter build files with path names. Any suggestions on what else I need to do to get update-alterna (HowTos)