How to install jdk in ubuntu

From : http://www.blogs.digitalworlds.net/softwarenotes/?p=41

Install Java JDK 7 on Ubuntu 12.04 (JavaFX is included in the installation)

Posted by ig on Jan 28, 2013 in Java, JavaFX, Ubuntu | 40 comments

This package provides Oracle Java JDK 7 (which includes JRE, the Java browser plugin and JavaFX). However, you can’t only install Oracle JRE – the PPA only provides the full Oracle JDK7 package.

1. To add our PPA and install the latest Oracle Java (JDK) 7 in Ubuntu, use the commands below:

#sudo add-apt-repository ppa:webupd8team/java
#sudo apt-get update && sudo apt-get install oracle-jdk7-installer

2. After the installation you have enable the jdk:

#update-alternatives –display java

3. Check if Ubuntu uses Java JDK 7

#java -version

If all went right the answer should be something like this:
java version “1.7.0_07″
Java(TM) SE Runtime Environment (build 1.7.0_07-b10)
Java HotSpot(TM) Server VM (build 23.3-b01, mixed mode)

4. Check what compiler is used:

#javac -version

The correct answer should be like this:
javac 1.7.0_07

5. Adding JAVA_HOME to environment
Edit /etc/environment and add JAVA_HOME=/usr/lib/jvm/java-7-oracle to the end of the file.
First open a Terminal (Applications → Accessories → Terminal), then enter:

#sudo gedit /etc/environment

Append to the end of the file:
JAVA_HOME=/usr/lib/jvm/java-7-oracle

6. Removing Oracle JDK 7

If you don’t want to use Oracle Java (JDK) 7 anymore and want to go back to OpenJDK, all you have to do is remove the Oracle JDK7 Installer and the previous Java (OpenJDK, etc.) version will be used:

#sudo apt-get remove oracle-jdk7-installer