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