A sample project showing how an Android App and the Google App Engine can communicate with a Firebase database.
This project defines a coniguration for a WebServlet running on the Google App Engine, and a simple Android application, which both use Firebase to listen for push notifications generated on changes made to a database entry.
In order to connect to your own Firebase Database, you'll have to supply your own private key for admin access (see: Add Firebase to your app) and a package-compatible googe-services.json (see: Maually add Firebase). Additionally, you'll want to change the default authorisation settings to allow R/W access to the database.