Can't install anything via apt-get - openjdk errors

Fixed with the following two lines!

sudo apt-get purge openjdk-7-jre-headless
sudo apt-get install openjdk-7-jre-headless

Don't totally understand why... but oh well!
EDIT - this broke some other packages having something to do with me only installing the headless version.
Fixed with this:

sudo apt-get install openjdk-7-jre