As you known, Sun JDK version 1.5 or 5 is deleted from Ubuntu 10.4 and 9.10 repositories and the version 6 has been replaced.
The easiest way to install Sun JDK 5 version is add its repository from Ubuntu 9.04 to the list of repositories in 9.10 and 10.04. For this purpose, follow the steps.
1- Open /etc/apt/sources.list with a text editor like gedit:
sudo gedit /etc/apt/sources.list
2- Add the following lines to the end of the file then save it and close:
## For sun-java5-jdk
deb http://ir.archive.ubuntu.com/ubuntu jaunty-updates main multiverse
3- Update the packages lists and install sun-java5-jdk:
sudo aptitude update
sudo aptitude install sun-java5-jdk
* Above method can be used for other applications.
Another way to install jdk 5 is download software package and its dependencies from packages.ubuntu.com.