Sun Java packages have been dropped from Ubuntu 10.04 LTS from the Multiverse section of Ubuntu archive. Ubuntu team is encouraging users to use OpenJDK instead of Sun JDK/JRE but still if you want to install Sun JDK on your Ubuntu then here is the solution for the same.
OpenJDK Appearing – No Sun Java in Synaptic
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner" sudo apt-get update sudo apt-get install sun-java6-jre sun-java6-plugin sudo update-alternatives --config java
This will install sun-java6 packages from Canonical Partner Repository. Enjoy Sun Java on Ubunu 10.04 Lucid.
The information is provided in the Ubuntu 10.04 LTS Lucid release note. You can read the details about Sun Java installation in Ubuntu and other details as well.