Installing Java on Ubuntu

Update the apt repository with the following lines.

deb http://us.archive.ubuntu.com/ubuntu dapper main restricted
deb http://us.archive.ubuntu.com/ubuntu dapper universe multiverse

After this have apt updates its repository

Run the following command 
 sudo apt-get update

Once the above command is run. Its time to install java.
Run the following command to install java.
 sudo apt-get install sun-java5-jdk