upgrade or downgrade java version ubuntu/linux

http://www.webupd8.org/2012/01/install-oracle-java-jdk-7-in-ubuntu-via.html

To add our PPA and install the latest Oracle Java 7 in Ubuntu (supports Ubuntu 13.04, 12.10, 12.04, 11.10, 11.04 and 10.04), use the commands below:

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java7-installer

After the installation finishes, if you wish to see if it was successful, you can run the following command:

java -version

It should return something like this:

java version "1.7.0_04"
Java(TM) SE Runtime Environment (build 1.7.0_04-b20)
Java HotSpot(TM) Server VM (build 23.0-b21, mixed mode)

The package installs all the Java binaries, so you can also try "javac -version" which should return "javac 1.7.0_04" and so on (the "_04" part of the version can be different because I'm constantly updating the PPA with the latest Oracle Java 7 version).

If for some reason, the Java version in use is not 1.7.0, you can try to run the following command:

sudo update-java-alternatives -s java-7-oracle

Update: the Oracle Java 7 (JDK & JRE) PPA is now available for Ubuntu 12.10 Quantal Quetzal and 13.04 Raring Ringtail.

Update 2: the installer now requires you accept the Oracle license before the installation begins. This is only required once. If for some reason you need the installation to be automated, you can run the following command to automatically accept the Oracle license:

sudo echo oracle-java7-installer shared/accepted-oracle-license-v1-1 select true | sudo /usr/bin/debconf-set-selections

Removing Oracle Java 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-java7-installer