Instalar JDK 6 / JDK 7 Ubuntu

sudo apt-get install software-properties-common

Add the PPA:
$ sudo add-apt-repository ppa:webupd8team/java

Update the repo index:
$ sudo apt-get update

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