Android getGeopPoint（）方法在调用时返回null。Firebase集合中有多个文档

我正在学习一个关于构建聊天室应用程序的教程，该应用程序包括谷歌地图API的用户位置功能，然而，我学习它只是为了学习如何使用谷歌地图API，我正在尝试构建一个跟踪应用程序，可以记录旅行路线并按日期访问它们，等等

有一个由地理位置、时间戳和用户对象组成的UserLocation对象，其实例存储在Firestore集合中。数据库工作正常：

但是，在进行身份验证后，我得到以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.trackingapp, PID: 13386
    java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.firestore.GeoPoint com.example.trackingapp.models.UserLocation.getGeo_point()' on a null object reference
        at com.example.trackingapp.ui.MainActivity.setCameraView(MainActivity.java:109)
        at com.example.trackingapp.ui.MainActivity.onMapReady(MainActivity.java:410)
        at com.google.android.gms.maps.zzac.zza(Unknown Source)
        at com.google.android.gms.maps.internal.zzaq.dispatchTransaction(Unknown Source)
        at com.google.android.gms.internal.maps.zzb.onTransact(Unknown Source)
        at android.os.Binder.transact(Binder.java:499)
        at ds.b(:com.google.android.gms.dynamite_mapsdynamite@203615052@20.36.15 (040700-0):2)
        at com.google.maps.api.android.lib6.impl.bf.run(:com.google.android.gms.dynamite_mapsdynamite@203615052@20.36.15 (040700-0):2)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

我猜我在尝试将原始应用程序的逻辑实现到我的应用程序中时犯了一个错误

我几乎可以肯定问题在于如何使用MuserListener。这个变量实际上保存了将UserLocations放入ArrayList的所有逻辑，但它只在OnDestroy方法中调用

本教程的源代码就是这样编写的，MuserListener没有其他用途。我曾尝试在其他地方实现UserLocations逻辑，但无法正确实现

以下是相关的主活动代码。这是我的应用程序中除登录和注册之外的唯一活动：

public class MainActivity extends AppCompatActivity implements OnMapReadyCallback {

    //    Variables Google Maps
    private boolean mLocationPermissionGranted = false;
    private MapView mMapView;
    private FusedLocationProviderClient mFusedLocationClient;
    private UserLocation mUserLocation;
    private ArrayList<UserLocation> mUserLocations = new ArrayList<>();
    private GoogleMap mGoogleMap;
    private LatLngBounds mMapBoundary;
    private UserLocation mUserPosition;

    //    Variables Firebase
    private FirebaseFirestore mDb;
    private ListenerRegistration mUserListEventListener;
    private Set<String> mUserIds = new HashSet<>();


    //    Otras Variables
    private RecyclerView rvUsers;
    private UserRecyclerAdapter adapter;
    private ArrayList<User> mUserList = new ArrayList<>();
    private static final String TAG = "MainActivity";

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

        initGoogleMap(savedInstanceState);

        mFusedLocationClient = LocationServices.getFusedLocationProviderClient(this);

        // DATA DE FIREBASE
        //
        mDb = FirebaseFirestore.getInstance();
        rvUsers = (RecyclerView) findViewById(R.id.user_list_recycler_view);
        getUsers();
        initUserRecyclerView();
        setUserPosition();
    }

    private void setCameraView(){

        double bottomBoundary = mUserPosition.getGeo_point().getLatitude() - .1;
        double leftBoundary = mUserPosition.getGeo_point().getLongitude() - .1;
        double topBoundary = mUserPosition.getGeo_point().getLatitude() + .1;
        double rightBoundary = mUserPosition.getGeo_point().getLongitude() + .1;

        mMapBoundary = new LatLngBounds(
                new LatLng(bottomBoundary,leftBoundary),
                new LatLng(topBoundary,rightBoundary)
        );

        mGoogleMap.moveCamera(CameraUpdateFactory.newLatLngBounds(mMapBoundary,0));
    }

    private void setUserPosition(){
        for (UserLocation userLocation : mUserLocations){
            if (userLocation.getUser().getUser_id().equals(FirebaseAuth.getInstance().getUid())){
                mUserPosition = userLocation;
            }
        }
    }

    private void getUserDetails(){
        if (mUserLocation == null){
            mUserLocation = new UserLocation();

            DocumentReference userRef = mDb.collection("Users")
                    .document(Objects.requireNonNull(FirebaseAuth.getInstance().getUid()));

            userRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
                @Override
                public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                    if (task.isSuccessful()){
                        Log.d(TAG, "onComplete: successfully set the user details.");

                        User user = task.getResult().toObject(User.class);
                        mUserLocation.setUser(user);
                        ((UserClient)getApplicationContext()).setUser(user);
                        getLastKnownLocation();
                    }
                }
            });
        }
        else {
            getLastKnownLocation();
        }
    }

    private void saveUserLocation(){
        if(mUserLocation != null){
            DocumentReference locationRef = mDb.
                    collection("User Locations")
                    .document(Objects.requireNonNull(FirebaseAuth.getInstance().getUid()));

            locationRef.set(mUserLocation).addOnCompleteListener(new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    if(task.isSuccessful()){
                        Log.d(TAG, "saveUserLocation: \ninserted user location into database." +
                                "\n latitude: " + mUserLocation.getGeo_point().getLatitude() +
                                "\n longitude: " + mUserLocation.getGeo_point().getLongitude());
                    }
                }
            });
        }
    }

    private void getLastKnownLocation() {
        Log.d(TAG, "getLastKnownLocation: called");

        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        mFusedLocationClient.getLastLocation().addOnCompleteListener(new OnCompleteListener<Location>() {
            @Override
            public void onComplete(@NonNull Task<Location> task) {
                if (task.isSuccessful()){
                    Location location = task.getResult();
                    GeoPoint geoPoint = new GeoPoint(location.getLatitude(), location.getLongitude());
                    Log.d(TAG, "onComplete: latitude: " + geoPoint.getLatitude());
                    Log.d(TAG, "onComplete: longitude" + geoPoint.getLongitude());

                    mUserLocation.setGeo_point(geoPoint);
                    mUserLocation.setTimestamp(null);
                    saveUserLocation();
                }
            }
        });
    }


private void getUsers() {

        CollectionReference usersCollection = mDb
                .collection("Users");

        mUserListEventListener = usersCollection.addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
                Log.d(TAG, "onEvent: called.");

                if (e != null) {
                    Log.e(TAG, "onEvent: Listen failed.", e);
                    return;
                }

                if(queryDocumentSnapshots != null){

                    // Clear the list and add all the users again
                    mUserList.clear();
                    mUserList = new ArrayList<>();

                    for (QueryDocumentSnapshot doc : queryDocumentSnapshots) {
                        User user = doc.toObject(User.class);
                        mUserList.add(user);
                        getUserLocation(user);
                    }

                    Log.d(TAG, "onEvent: user list size: " + mUserList.size());
                }

            }
        });

    }

    private void getUserLocation(User user){
        DocumentReference locationRef = mDb.collection("User Locations")
                .document(user.getUser_id() );

        locationRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                if (task.isSuccessful()){
                    if(task.getResult().toObject(UserLocation.class) != null){
                        mUserLocations.add(task.getResult().toObject(UserLocation.class));
                    }
                }
            }
        });
    }


    @Override
    protected void onResume() {
        super.onResume();
        if (checkMapServices()) {
            if (mLocationPermissionGranted) {
                getUsers();
                getUserDetails();
            } else {
                getLocationPermission();
            }
        }
        mMapView.onResume();
    }

    @Override
    protected void onStart() {
        super.onStart();
        mMapView.onStart();
    }

    @Override
    protected void onStop() {
        super.onStop();
        mMapView.onStop();
    }

    @Override
    public void onMapReady(GoogleMap map) {
        map.addMarker(new MarkerOptions().position(new LatLng(0, 0)).title("Marker"));
        if (ActivityCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_FINE_LOCATION)
                != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        map.setMyLocationEnabled(true);
        mGoogleMap = map;
        setCameraView();
    }

    @Override
    protected void onPause() {
        mMapView.onPause();
        super.onPause();
    }

    @Override
    protected void onDestroy() {
        mMapView.onDestroy();
        super.onDestroy();
        if(mUserListEventListener != null){
            mUserListEventListener.remove();
        }
    }

    @Override
    public void onLowMemory() {
        super.onLowMemory();
        mMapView.onLowMemory();
    }

}

拥有NullPointerException与返回null不同。 在这种情况下，如错误跟踪所示，在这里：

位于com.example.trackingapp.ui.MainActivity.setCameraViewMainActivity.java:109

有一个对空obj的调用

java.lang.NullPointerException: Attempt to invoke virtual method 
    'com.google.firebase.firestore.GeoPoint 
     com.example.trackingapp.models.UserLocation.getGeo_point()'
     on a null object reference

这里是mUserPosition.getGeo_point

该错误表示mUserPosition为null，您需要先初始化它

如果var mUserPosition为null，可能我不知道您需要调用getUserDetails

if (mUserPosition == null) {
    getUserDetails();
}