LatLng GPS位置的NullPointerException

Question

I am using Google Maps to obtain my current location and display a red marker on my location. So far it only the mMap.setMyLocation(true); works for my location. I am getting NullPointerException for the object userLocation towards the end of my code and the output to the console for userLocation is null too.

How do I set the userLocation as soon as I load the map ?

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    private GoogleMap mMap;

    LocationManager locationManager;

    LocationListener locationListener;
    LatLng userLocation;
    private FusedLocationProviderClient mFusedLocationClient;
    public  float DEFAULT_ZOOM = 15f;
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        if (requestCode == 1) {

            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED) {
                    {

                        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);

                    }

                }

            }

        }

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_maps);
        // Obtain the SupportMapFragment and get notified when the map is ready to be used.
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
            }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
        locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
        locationListener = new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {

                userLocation = new LatLng(location.getLatitude(), location.getLongitude());

                mMap.clear();
                mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));
                System.out.println("the latitude onLocationChanged is "+ userLocation.latitude);


            }

            @Override
            public void onStatusChanged(String s, int i, Bundle bundle) {

            }

            @Override
            public void onProviderEnabled(String s) {

            }

            @Override
            public void onProviderDisabled(String s) {

            }
        };



        if (Build.VERSION.SDK_INT < 23) {
        try {
            locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);
        }
        catch (SecurityException e){
            e.printStackTrace();
        }
        } else {

            if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {

                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);

            } else {

                locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1, 1, locationListener);
              //  LatLng latLng = new LatLng(40.741895,-73.989308);
                mMap.setMyLocationEnabled(true);
            //outputs the latitude is null 
                System.out.println("the latitude is "+userLocation);
              mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));


            }


        }


    }


}

Answer 1

You can't. Getting a location takes time. It won't be available until your callback is called, which means you can't load it until then. You could try calling getLastKnownLocation, but it will return null the majority of the time as well. The best solution is not to rely on it until the callback is made.

Answer 2

on method, OnMapReady use this code

if (!mMap.isMyLocationEnabled())
        mMap.setMyLocationEnabled(true);

    Location userLocation = getLocation();

and use this method to get the last location if you want it

public Location getLocation() {
        LocationManager lm = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
        Location myLocation = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
        if (myLocation == null) {
            Criteria criteria = new Criteria();
            criteria.setAccuracy(Criteria.ACCURACY_COARSE);
            String provider = lm.getBestProvider(criteria, true);
            myLocation = lm.getLastKnownLocation(provider);
        }
        return myLocation;
    }

Note : GPS_PROVIDER does not work indoors, so you should try it outdoor.

maybe you want to start the callback just if the last location before more than one minute. use this code

if (System.currentTimeMillis()-myLocation.getTime()>60000){
 locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1, 1, locationListener);

       };