Ubuntu安装JDK7（Oracle JDK） 

Preparing your system

First you need to remove openjdk for this run the following command from your terminal

sudo apt-get purge openjdk*

If you installed java 7 from any other PPA and you are having problem with java then you have to do following steps before installing the PPA menctioned here

sudo rm /var/lib/dpkg/info/oracle-java7-installer*

sudo apt-get purge oracle-java7-installer*

sudo rm /etc/apt/sources.list.d/*java*

sudo apt-get update

Install oracle java 7 in ubuntu 12.04

Open the terminal and run the following commands

sudo add-apt-repository ppa:webupd8team/java

sudo apt-get update

sudo apt-get install oracle-java7-installer

You can check the java version by searching java in dashboard