Unit Testing with Mockito & Firebase

Question

I am attempting to implement testing for an android app that I'm building. One of the tests involves an interactor file I wrote (DatabaseInteractor.java) that connects to Google Firestore. The default constructor sets the FirestoreFirebase object to FirebaseFirestore.getInstance();.

I am using Mockito & JUnit for my unit tests. When I attempt to create a DataBaseInteractor object in my Unit Test, I get the following error:

java.lang.IllegalStateException: Default FirebaseApp is not initialized in this process null. Make sure to call FirebaseApp.initializeApp(Context) first.

Apparently this error occurs when the default constructor is called in my Unit Test. I'm not sure why this is happening. I am able to create a DataBaseInteractor object in other parts of my android app without issue. Are there perhaps limitations to when and where I can initialize a class that utilizes FireBaseFireStore objects? If I want to mock the behavior of FireBase for unit testing, how do I get around these restrictions?

EDIT: A truncated version of my DataBaseInteractor.java file:

public class DatabaseInteractor {
    private static User theUser;
    private FirebaseFirestore db;
    private DocumentReference userData;


    public DatabaseInteractor() {
        db = FirebaseFirestore.getInstance();
        theUser = new User();
    }
 // ... various methods that add/retrieve Users from FireStore ... //
}

Answer 1

You need to inject the FirebaseFirestore instance in your constructor so that it can be mocked, something like:

public class DatabaseInteractor {
    private static User theUser;
    private FirebaseFirestore db;
    private DocumentReference userData;


    public DatabaseInteractor(FirebaseFirestore firestore) {
        db = firestore
        theUser = new User();
    }
 // ... various methods that add/retrieve Users from FireStore ... //
}  

The in your test you can mock FirebaseFirestore with Mockito:

@Test
public void someTest() {
  FirebaseFirestore mockFirestore = Mockito.mock(FirebaseFirestore.class)
  Mockito.when(mockFirestore.someMethodCallYouWantToMock()).thenReturn(something)

  DatabaseInteractor interactor = new DatabaseInteractor(mockFirestore)

  // some assertion or verification
}

I'd suggest you read about dependency injection, it will make unit testing a lot easier if you can pass in mocks of your dependencies in the constructor instead of instantiating them inside it.