I installed Ubuntu 10.04 and noticed Sun Java in not in default repository any more. I checked Ubuntu 10.04 LTS Release Notes and found
Easy, but it's wrong. Here is how to enable partner's repository.
Sun Java moved to the Partner repository
For Ubuntu 10.04 LTS, the sun-java6 packages have been dropped from the Multiverse section of the Ubuntu archive. It is recommended that you use openjdk-6 instead.If you can not switch from the proprietary Sun JDK/JRE to OpenJDK, you can install sun-java6 packages from the Canonical Partner Repository. You can configure your system to use this repository via command-line:add-apt-repository "deb http://archive.canonical.com/ lucid partner"