As a Windows user, I was amused how not easy to install Java on Ubuntu Linux (10.04) nor installing Java (1.6) plugin into Firefox (3.6). It's not nearly a download-next-next-finish task. Additionally, digging into forums I found a lot outdated solutions which I tried and soon realized that they doubtlessly aren't working. Here is the one which worked for me:
First, open a terminal window:
Applications menu -> Accessories -> Terminal.
Type the followings (you will be promted for admin password, because sudo prefix means you want to do something in the name of the super (or sytem) user):
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
sudo aptitude update
sudo aptitude install sun-java6-jdk
Then you have to select things with arrows and pressing enter.
Then Timothy says in the forum you have to type this, but I didn't need it:
sudo update-alternatives --config java
Now you can check if java is installed by typing java -version.
Installing Java plugin into Firefox is no more than making a symbolic link with the ln command. I had to go in the /usr/lib/mozilla/plugins directory, then make the symbolic link with this:
sudo ln -s /usr/lib/jvm/java-6-sun/jre/lib/i386/libnpjp2.so
Restart Firefox if it's running. Java may work.
Sorry for the laxity, I'm still a beginner Ubuntu user.