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

Question

Hello I have this error with my code :

java.lang.NullPointerException: Attempt to invoke virtual method 'double fr.uge.bluetoothle.MapsActivity.calculDistance(double, double, double, double)' on a null object reference

I develop on Android Studio in Java.

It seems that I can't access to MapsActivity.java from MyLocationListener.java I think I don't instantiate well my MpasActivity class.

I have this error in the folowing code 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);
    }

And here there is my activity's name 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;

    }
}

I don't understand where is the problem I think That's the problem come from the transition between the activity and my other java file.

Reopened this question as I don't think closing it as duplicate of the canonical "how do I fix a NPE" is useful/helpful to the community or to OP. — David Wasser, Mar 14 '22 at 09:58

score -1 · Answer 1 · answered Mar 14 '22 at 09:29

Your call to mapsActivity.calculDistance() throws an exception because the variable mapsActivity is null. You haven't set the variable mapsActivity to anything.

Looking at your code, the method calculDistance() is basically a utility method and doesn't really belong to any class (it doesn't use any methods or member variables of the class). You can simply make this a static method by adding the static keyword to the method declaration, like this:

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

Now you can use this method from any other class, like this:

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

Notice that you are calling the method on the class, not on an instance of the class. In this case, you don't need to declare a variable mapsActivity, because this is no longer used.

A somewhat cleaner approach would be to move this method out of the MapsActivity class and put it in a new utility class (which should contain only public static methods and should have a private constructor so that it is not possible to actually create any instances of that utility class.

Just saying: when you give code snippets, make them fully java. As in: make sure naming conventions are met, so drop evil_underscores. (not my downvote btw) — GhostCat, Mar 14 '22 at 09:35

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

1 Answers1