I just had trouble to install the Sun Java6 JDK after updating to Ubuntu 10.04 for my project on Faculty. The problem was that the system couldn’t find the package sun-java6-sdk and apt-get gave me the following message:

Package sun-java6-jdk is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package sun-java6-jdk has no installation candidate

I found solution on one blog. It was to add a new source for apt-get :

sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"

After that I used normal command :

sudo apt-get update sudo apt-get install sun-java6-jdk

And it worked for me.

You can also update the/etc/apt/sources.list file and add/uncomment the following lines:

deb http://archive.canonical.com/ubuntu lucid partner
deb-src http://archive.canonical.com/ubuntu lucid partner

Chears :)