If it was nice and intuitive to install Java on Debian (and other Debian based distros such as Ubuntu), I wouldn’t need to write this post because it would just be "apt-get install java". But no, I always forget what I need to install, hence this post…
Install openjdk-7-jre or openjdk-7-jre-headless
If you just want to install the runtime so you can run Java applications, install either openjdk-7-jre or openjdk-7-jre-headless. The first one will install a bunch of Gnome stuff if not already installed, so if you are doing this on a server without a GUI then install openjdk-7-jre-headless.
sudo apt-get install openjdk-7-jre
sudo apt-get install openjdk-7-jre-headless
Right, now maybe I’ll remember this next time 🙂
This is correct for (at least) Debian 7 & 8, possibly older versions, probably future versions, and possibly other Debian based distributions.