Add the following to /etc/apt/sources.list:
deb http://us.archive.ubuntu.com/ubuntu/ jaunty multiverse
deb http://us.archive.ubuntu.com/ubuntu/ jaunty-updates multiverse
Update the package list:
sudo apt-get update
Install Sun Java 1.5:
sudo apt-get install sun-java5-jdk
Depending on your setup you might need to set this as the default. Check what version is default:
java -version
And list all Java versions installed:
sudo update-java-alternatives -l
Then if necessary:
sudo update-java-alternatives -s java-1.5.0-sun
If you want to get Sun Java 1.6 installed, instead of OpenJDK, just enable the partner repositories in the Package Manager > Software Sources and it will show up in the list for you to install.