How do I make java default to a manually installed JRE/JDK?

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http://askubuntu.com – I have Ubuntu 12.04 amd64 installed on my machine, on the previous versions of Ubuntu it was deadly easy, now there is this command update-java-alternatives with a really bad man page. I just have my JDK unpacked on a mounted partition like /media/mydisk/jdk, how i can force the use of that JDK instead of the one that comes in the Ubuntu repository? What is the logic behind this update-java-alternatives ? (HowTos)