你好，我的代码出现了这个错误。
java.lang.NullPointerException: Attempt to invoke virtual method 'double fr.uge.bluetoothle.MapsActivity.calculDistance(double, double, double, double)' on a null object reference
我在Android Studio上用Java开发。
我似乎不能从MyLocationListener.java访问MapsActivity.java。
我想我没有很好地实例化我的MpasActivity类。
我在下面的代码中出现了这个错误MyLocationListener.java。
package fr.uge.bluetoothle;
import android.location.Location;
import android.location.LocationListener;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;
import androidx.annotation.NonNull;
public class MyLocationListener implements LocationListener {
public String longitude;
public String latitude;
public double distance;
private static final String TAG = "Loca";
public MapsActivity mapsActivity;
@Override
public void onLocationChanged(@NonNull Location location) {
/*Toast.makeText(
mapsActivity.getBaseContext(),
"Location changed: Lat: " + location.getLatitude() + " Lng: "
+ location.getLongitude(), Toast.LENGTH_SHORT).show();*/
String longitude = "Longitude: " + location.getLongitude();
Log.v(TAG, longitude);
String latitude = "Latitude: " + location.getLatitude();
Log.v(TAG, latitude);
distance = mapsActivity.calculDistance(2.58622,48.84008,location.getLongitude(),location.getLatitude());
System.out.println(distance);
这里有我活动的名字MapsActivity.java。
package fr.uge.bluetoothle;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import android.Manifest;
import android.app.Activity;
import android.content.Context;
import android.content.pm.PackageManager;
import android.graphics.Color;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;
import com.mapbox.android.core.location.LocationEngine;
import com.mapbox.android.core.permissions.PermissionsListener;
import com.mapbox.android.core.permissions.PermissionsManager;
import com.mapbox.mapboxsdk.Mapbox;
import com.mapbox.mapboxsdk.location.LocationComponent;
import com.mapbox.mapboxsdk.location.modes.CameraMode;
import com.mapbox.mapboxsdk.maps.MapView;
import com.mapbox.mapboxsdk.maps.MapboxMap;
import com.mapbox.mapboxsdk.maps.OnMapReadyCallback;
import com.mapbox.mapboxsdk.maps.Style;
import java.util.List;
import java.lang.Math;
import com.mapbox.mapboxsdk.style.layers.CircleLayer;
import com.mapbox.mapboxsdk.style.layers.Layer;
import com.mapbox.mapboxsdk.style.layers.LineLayer;
import com.mapbox.mapboxsdk.style.layers.Property;
import com.mapbox.mapboxsdk.style.sources.VectorSource;
import static com.mapbox.mapboxsdk.style.layers.PropertyFactory.lineCap;
import static com.mapbox.mapboxsdk.style.layers.PropertyFactory.lineColor;
import static com.mapbox.mapboxsdk.style.layers.PropertyFactory.lineJoin;
import static com.mapbox.mapboxsdk.style.layers.PropertyFactory.lineWidth;
public class MapsActivity extends Activity implements OnMapReadyCallback, PermissionsListener {
private MapView mapView;
private MapboxMap mapboxMap;
private PermissionsManager permissionsManager;
private LocationComponent locationComponent;
MyLocationListener myLocationListener;
private static Context mContext;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Mapbox.getInstance(this, getString(R.string.mapbox_access_token));
setContentView(R.layout.activity_maps);
mapView = findViewById(R.id.mapView);
mapView.onCreate(savedInstanceState);
mapView.getMapAsync(this);
//Display Location
LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
LocationListener locationListener = new MyLocationListener();
if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
// TODO: Consider calling
// ActivityCompat#requestPermissions
// here to request the missing permissions, and then overriding
// public void onRequestPermissionsResult(int requestCode, String[] permissions,
// int[] grantResults)
// to handle the case where the user grants the permission. See the documentation
// for ActivityCompat#requestPermissions for more details.
ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);
return;
locationManager.requestLocationUpdates(
LocationManager.GPS_PROVIDER, 5000, 1, locationListener);
Toast.makeText(
getBaseContext(),
"Location changed: Lat: " + myLocationListener.getLatitude() + " Lng: "
+ myLocationListener.getLongitude(), Toast.LENGTH_SHORT).show();
@Override
public void onMapReady(@NonNull MapboxMap mapboxMap) {
this.mapboxMap = mapboxMap;
mapboxMap.setStyle(getString(R.string.navigation_guidance_day), new Style.OnStyleLoaded() {
@Override
public void onStyleLoaded(@NonNull Style style) {
enableLocationComponent(style);
//Add the GPX File
VectorSource vectorSource = new VectorSource("vector-source", getString(R.string.tileset_cyclo));
style.addSource(vectorSource);
//Tracks Layer
/*CircleLayer circleLayer = new CircleLayer("circle-layer-id", "vector-source");
circleLayer.setSourceLayer("tracks");
style.addLayer(circleLayer);*/
//Waypoints Layer
CircleLayer circleLayer1 = new CircleLayer("circle-layer-id-1", "vector-source");
circleLayer1.setSourceLayer("waypoints");
style.addLayer(circleLayer1);
private void enableLocationComponent(Style loadMapStyle) {
/*Check if permissions are enabled and if not request*/
if (PermissionsManager.areLocationPermissionsGranted(this)) {
//Activate the MapBox LocationComponent to show user location
//Adding in LocationComponentOptions is also an optional parameter
locationComponent = mapboxMap.getLocationComponent();
locationComponent.activateLocationComponent(this, loadMapStyle);
if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
// TODO: Consider calling
// ActivityCompat#requestPermissions
// here to request the missing permissions, and then overriding
// public void onRequestPermissionsResult(int requestCode, String[] permissions,
// int[] grantResults)
// to handle the case where the user grants the permission. See the documentation
// for ActivityCompat#requestPermissions for more details.
ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);
return;
locationComponent.setLocationComponentEnabled(true);
//Set the component's camera mode
locationComponent.setCameraMode(CameraMode.TRACKING);
else {
permissionsManager = new PermissionsManager(this);
permissionsManager.requestLocationPermissions(this);
@Override
public void onExplanationNeeded(List<String> permissionsToExplain) {
Toast.makeText(this, R.string.user_location_permission_explanation, Toast.LENGTH_SHORT).show();
@Override
public void onPermissionResult(boolean granted) {
if (granted){
enableLocationComponent(mapboxMap.getStyle());
else {
Toast.makeText(this, R.string.user_location_permission_not_granted, Toast.LENGTH_SHORT).show();
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
permissionsManager.onRequestPermissionsResult(requestCode, permissions, grantResults);
@Override
public boolean onCreateOptionsMenu(Menu menu) {
return true;
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch(item.getItemId()) {
case android.R.id.home:
onBackPressed();
return true;
return super.onOptionsItemSelected(item);
@Override
protected void onStart() {
super.onStart();
mapView.onStart();
@Override
protected void onStop() {
super.onStop();
mapView.onStop();
@Override
protected void onPause() {
super.onPause();
mapView.onLowMemory();
@Override
protected void onDestroy() {
super.onDestroy();
mapView.onDestroy();
@Override
protected void onSaveInstanceState(Bundle outState){
super.onSaveInstanceState(outState);
mapView.onSaveInstanceState(outState);
@Override
public void onLowMemory() {
super.onLowMemory();
mapView.onLowMemory();
public Context getContext() {
return mContext;
public double calculDistance(double longitude_point, double latitude_point, double longitude_loca, double latitude_loca){
double distance;
distance = Math.acos(Math.sin(Math.toRadians(longitude_point))*Math.sin(Math.toRadians(longitude_loca))+
Math.cos(Math.toRadians(longitude_point))*Math.cos(Math.toRadians(longitude_loca))*
Math.cos(Math.toRadians(latitude_point-latitude_loca)));
distance = distance * 6371;
return distance;
我不明白问题出在哪里，我认为问题出在活动和我的其他java文件之间的转换。
1 个回答
0 人赞同
你对mapsActivity.calculDistance()的调用引发了一个异常，因为变量mapsActivity是null。你还没有把变量mapsActivity设置为任何东西。
看看你的代码，方法calculDistance()基本上是一个实用方法，并不真正属于任何一个类（它不使用任何方法或类的成员变量）。你可以通过在方法声明中添加static关键字，使其成为一个静态方法，像这样。
public static double calculDistance(double longitude_point,
double latitude_point, double longitude_loca, double latitude_loca) {
现在，你可以从任何其他类中使用这个方法，比如这样。
