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
What I did to solve this problem was to add a new source
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
sudo apt-get update
sudo apt-get install sun-java6-jdk
It worked fine for me.