I use the java-package make-jpkg to install java JVM's. I have mostly used sun's JVM's up to now.

I wanted to easily switch between them (which the alternatives mechanism handles well) but also to keep the relevant JAVA_HOME in sync (for things that still need this in the environment).

The following script I added to my .bashrc

switch_java () {
    case $1 in
    4 ) JAVA_HOME=/usr/lib/j2sdk1.4-sun ;;
    5 ) JAVA_HOME=/usr/lib/j2sdk1.5-sun ;;
    esac

    sudo update-alternatives --set java $JAVA_HOME/bin/java
    sudo update-alternatives --set javac $JAVA_HOME/bin/javac

    export JAVA_HOME
}

So I can now just run

$ switch_java 4

or

$ switch_java 5

to change. Note - will require sudo access for the alternatives change.