There are also non-Android related Java projects there, for example from the guys from Netflix, SpringSource, LinkedIn, Twitter, etc... but I think that you're right, a lot of Java projects are Android related.
An interesting phenomenon is a lot of Java projects on Github if they are not Android related, very likely they are from big guys like Twitter, Netflix, LinkedIn, Yammer etc. Definitely they all have the reasons (e.g. performance, static typing) to use Java or JVM technologies, but if you count the number of projects from indie developers, Ruby, JavaScript & Python are much popular - and they are the target audience of Discourse.
Java is still the No.1 language but it is unhealthy (IMO) if most indie developers prefer to ignore it, despite the benefits it bring.
