java.lang.RuntimeException：无法在空对象引用上启动活动 ComponentInfo

Question

您好，我的代码出现此错误：

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 上使用 Java 进行开发。

似乎我无法从 MyLocationListener.java 访问 MapsActivity.java 我认为我没有很好地实例化我的 MpasActivity 类。

我在以下代码中遇到此错误 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);
    }

这里有我的活动名称 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，我认为这就是问题所在来自活动和我的其他 java 文件之间的转换。

Answer 1

对 mapsActivity.calculDistance() 的调用会引发异常，因为变量 mapsActivity 为 null。您尚未将变量 mapsActivity 设置为任何内容。

查看您的代码，方法 calculDistance() 基本上是一个实用方法，并不真正属于任何类（它不使用该类的任何方法或成员变量）。您可以简单地通过将 static 关键字添加到方法声明中来使其成为静态方法，如下所示：

public static double calculDistance(double longitude_point,
     double latitude_point, double longitude_loca, double latitude_loca) {

现在您可以从任何其他类中使用此方法，如下所示：

distance = MapsActivity.calculDistance(2.58622,48.84008,
             location.getLongitude(),location.getLatitude());

请注意，您正在调用该方法类，而不是类的实例。在这种情况下，您不需要声明变量 mapsActivity，因为不再使用它。

一种更简洁的方法是将此方法移出 MapsActivity 类并将其放入新的实用程序类中（该类应仅包含公共静态方法，并且应具有私有方法）构造函数，因此不可能实际创建该实用程序类的任何实例。