MainActivity.java

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    RVAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        base = FirebaseFirestore.getInstance().collection("Doctors");
        recyclerView = findViewById(R.id.rv);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        FirestoreRecyclerOptions<Doctor_Class> options = new FirestoreRecyclerOptions.Builder<Doctor_Class>().setQuery(base, Doctor_Class.class).build();

        adapter = new RVAdapter(options);

        recyclerView.setAdapter(adapter);
    }
  
    // Function to tell the app to start getting
    // data from database on starting of the activity
    @Override protected void onStart()
    {
        super.onStart();
        adapter.startListening();
    }

    // Function to tell the app to stop getting
    // data from database on stoping of the activity
    @Override protected void onStop()
    {
        super.onStop();
        adapter.stopListening();
    }
}