Install Java on Ubuntu:

Installing Java on Ubuntu does not require any expertise  
as such, it is easy.

By default the Ubuntu Operating System has
some Java packages that comes with it:

sun-java6-jdk - Contains the JDK

sun-java6-jre - Contains the JRE

sun-java6-plugin - Contains the plug-in for Mozilla-based
browsers

sun-java6-demo - Contains demos and examples

Installing the (Java Runtime Environment) JRE or (Java Development
Kit) JDK :

But first you need to ensure that the multiverse repository is enabled:

GO to System > Administration > Software Sources, under Ubuntu Software, check the first four check boxes to enable software from different sources. 

Then open a terminal window. To install the JRE, the browser plug-in, and the JDK , use the following command from a terminal:

sudo apt-get install sun-java6-jre sun-java6-plugin sun-java6-jdk

Once it downloads the packages and begins the installation, you’ll get a screen that contains the Sun Operating System Distributor License for Java and hit Enter to continue. You’ll see a dialog that asks you if you agree with the DLJ license terms. Select Yes, and hit Enter; the JRE will finish installing.

Testing JRE:

First, check that the JRE is properly installed by running the following command from a terminal.

java -version

You should get similar outputjava version “1.6.0″
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

Testing Java Plugin for Firefox:

open Firefox and typing about:plugins in the address bar and check for java plugin

Testing the JDK:

Issue the following command from the terminal:

Javac -version

You should see a similar output like that of the JRE's output.