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

java.lang.RuntimeException: Unable to start activity ComponentInfo on a null object reference [duplicate]

提问人：Osika91 提问时间：3/11/2022 最后编辑：David WasserOsika91 更新时间：3/14/2022 访问量：1988

问：

这个问题在这里已经有答案了：

什么是 NullPointerException，如何修复它？（12 个答案）

如何从活动调用另一个活动中的方法（7 个答案）

如何从另一个类调用MainActivity中的方法？（12 个答案）

去年关闭。

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 上用 Java 进行开发。

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

我在以下代码中有这个错误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);
    }

这是我的活动名称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，我认为这就是问题来自活动和我的另一个 java 文件之间的过渡。

java android 活动 nullpointerexception 运行时异常

0赞 David Wasser 3/14/2022

重新打开这个问题，因为我认为将其作为规范“如何修复 NPE”的副本关闭它对社区或 OP 没有用/有帮助。

答：

-1赞 David Wasser 3/14/2022 #1

调用会引发异常，因为变量是。您尚未将变量设置为任何值。mapsActivity.calculDistance()mapsActivitynullmapsActivity

查看您的代码，该方法基本上是一个实用方法，实际上并不属于任何类（它不使用类的任何方法或成员变量）。您可以通过将关键字添加到方法声明中来简单地将其设置为静态方法，如下所示：calculDistance()static

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，您是在类上调用方法，而不是在类的实例上调用方法。在这种情况下，您不需要声明变量，因为不再使用它。mapsActivity

一个更简洁的方法是将此方法移出类并将其放在一个新的实用程序类中（该类应仅包含方法，并且应具有私有构造函数，以便无法实际创建该实用程序类的任何实例。MapsActivitypublic static

0赞 GhostCat 3/14/2022

只是说：当你给出代码片段时，让它们完全是 java。如：确保满足命名约定，因此请删除evil_underscores。（顺便说一句，不是我的反对票）

0赞 David Wasser 3/14/2022

@GhostCat我只是复制/粘贴了 OP 的原始代码。

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

java.lang.RuntimeException: Unable to start activity ComponentInfo on a null object reference [duplicate]

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