Installing java 7 is a simple process in linux (debian etc). Follow these simple steps to get it installed.
sudo su
echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a /etc/apt/sources.list
echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a /etc/apt/sources.list
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EEA14886
apt-get update
apt-get install oracle-java7-installer
exit
To set Java 7 as the default version run (you may have to sudo):
apt-get install oracle-java7-set-default
And run the following to check the java version
java -version