3. What must I add to myu path? PATH=$PATH:/usr/java/jre1.7.0_13/bin ???Yes - basically just that but I do a couple of extra things. I make a soft link in /usr/java from /usr/java/jdk to /usr/java/jre1.7.0_13 and then modify the PATH at the end of /etc/bash.bashrc using PATH=$PATH:/usr/java/jre/bin .
6. If I go to Ubuntu Software Center -> Installed Software. I do not see that Java has been installed.You don't need to see it installed. It is in effect installed once you update the PATH.