android - 即使在定义 @NonNull 之后，mLastKnownLocation 也是空的

Question

我正在尝试从这段代码中获取设备位置

private void getDeviceLocation() {
    /*
     * Get the best and most recent location of the device, which may be null in rare
     * cases when a location is not available.
     */
    try {
      if (mLocationPermissionGranted) {
        Task<Location> locationResult = mFusedLocationProviderClient.getLastLocation();
        locationResult.addOnCompleteListener(getActivity(), new OnCompleteListener<Location>() {
          @Override
          public void onComplete(@NonNull Task<Location> task) {
            if (task.isSuccessful()) {
              mMap.clear();
              // Set the map's camera position to the current location of the device.
              mLastKnownLocation = task.getResult();
              mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(
                  new LatLng(mLastKnownLocation.getLatitude(),
                      mLastKnownLocation.getLongitude()), DEFAULT_ZOOM));


            } else {
              Log.d(TAG, "Current location is null. Using defaults.");
              Log.e(TAG, "Exception: %s", task.getException());
                            mMap.moveCamera(CameraUpdateFactory
                                    .newLatLngZoom(mDefaultLocation, DEFAULT_ZOOM));
                            mMap.getUiSettings().setMyLocationButtonEnabled(false);
            }
          }
        });

通常工作正常，但有时会抛出错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'double android.location.Location.getLatitude()' on a null object reference
    at SecondFragment$1.onComplete(SecondFragment.java:230)

代码的第230行是:

      mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(
          new LatLng(mLastKnownLocation.getLatitude(),  //Line230
              mLastKnownLocation.getLongitude()), DEFAULT_ZOOM));

即使在使用@NonNull 屏蔽之后，我也不明白代码是如何到这里并给出空点异常的。

此外，此问题仅在模拟器中出现。在真实设备中，我没有发现这个错误(即应用程序没有像在模拟器中那样崩溃)

Answer 1

NonNull 表示返回的值不为空。这并不意味着其上的函数返回值(如 getResult())不能为 null。

此外，getLastKnownLocation 可以始终返回 null，无论您如何包装它。如果您想确保获得位置，请使用 getSingleUpdate 并等待回调。