1.Add the PPA:

$ sudo add-apt-repository ppa:webupd8team/java

2.Update the repo index:

$ sudo apt-get update

3.install jdk:

Install Java 8

$ sudo apt-get install oracle-java8-installer

Or, install Java 7:

$ sudo apt-get install oracle-java7-installer

Or, install Java 6:

$ sudo apt-get install oracle-java6-installer

ubuntu install jdk