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Run the following commands:

sudo apt-get install python-software-properties
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update

Then , to install a specific jdk run these:

jdk6:

sudo apt-get install oracle-java6-installer

jdk7:

sudo apt-get install oracle-java7-installer

jdk8:

sudo apt-get install oracle-java8-installer