$ sudo apt-get update
$ sudo apt-get install openjdk-7-jre-headless 

openjdk-7-jre-headless is installed in your system.

Make ensure the openjdk-7-jre-headless package were installed using the commands given below,

$ sudo dpkg-query -l | grep openjdk-7-jre-headless *

You will get with openjdk-7-jre-headless package name, version, architecture and description in a table

openjdk-7-jre-headless package basic information: