Changing the Default Java Vendor (or Version) in Ubuntu

Run sudo update-java-alternatives -l to see the current configuration and possibilities.

Run sudo update-java-alternatives -s XXXX to set the XXX java version as default.
For Sun Java 6 this would be sudo update-java-alternatives -s java-6-sun

Run java -version to ensure that the correct version is being called.

Source: https://help.ubuntu.com/community/Java