Here's what I did. It worked.
First I installed the jdk for Java 7 like this:
sudo apt-get install openjdk-7-jdk
That might be enough: check and see if javac in your PATH by running javac -version
If not, then follow Nicholas' answer except that instead of sudo update-alternatives --config javac
use this:
sudo update-alternatives --config java
And selected Java 7 at at the the prompt by typing 2
:
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
* 2 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1051 manual mode
Press enter to keep the current choice[*], or type selection number: