This is the source code that accompanies the Firestore Android Codelab: https://codelabs.developers.google.com/codelabs/firestore-android
The codelab will walk you through developing an Android restaurant recommendation app powered by Cloud Firestore.
If you don't want to do the codelab and would rather view the completed sample code, see the Firebase Android Quickstart repository: https://github.com/firebase/quickstart-android