As mentioned here, by running this command you can install Java 7 JRE.
$ sudo apt-get install openjdk-7-jre
But sometimes I can't find the package in the official repository of Ubuntu even after updating my /etc/apt/sources.list . Am I the only one who has this problem or is it a problem in my version of Ubuntu.
openjdk version 7 was only available in the standard ubuntu repositories for Oneiric (11.10).
Both maverick (10.10) and natty (11.04), only version 6 of openjdk is available.
You have a couple of choices.
First consider upgrading through 11.04 to 11.10. You are probably already considering this since you are aware Maverick is just about to lose its official support.
Alternatively, you may wish to consider installing Oracle Java - either v6 or v7
