FireStore v5

public class MainActivity extends AppCompatActivity {

    private static final String NAME_KEY = "Name";
    private static final String EMAIL_KEY = "Email";
    private static final String PHONE_KEY = "Phone";
    FirebaseFirestore db;
    TextView textDisplay;
    TextView message;
    EditText name, email, phone;
    Button save;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        db = FirebaseFirestore.getInstance();
        textDisplay = findViewById(R.id.textDisplay);
        message = findViewById(R.id.displayMessage);
        save    = findViewById(R.id.save);
}