You can not easily change default JVM on your machine, simply by installing your prefered JVM. It needs a special step to make that possible. Fortunately, there is a convenient way to change default JVM on Ubuntu, it is using update-java-alternatives command.

Here is the basic functionality :

- Run [sudo update-java-alternatives -l] to see the current configuration

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