Question

我的.java文件中出现Null指针异常，我不知道我需要做些什么来修复它们。

这是我的logCat，显示错误发生的位置。

09-05 11:17:19.569  12588-12588/au.gov.nsw.shellharbour.saferroadsshellharbour E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
        at android.content.ContextWrapper.getSystemService(ContextWrapper.java:370)
        at com.android.internal.app.AlertController$AlertParams.<init>(AlertController.java:801)
        at android.app.AlertDialog$Builder.<init>(AlertDialog.java:273)
        at au.gov.nsw.shellharbour.saferroadsshellharbour.GPSTracker.<init>(GPSTracker.java:116)
        at au.gov.nsw.shellharbour.saferroadsshellharbour.HomeScreen$1.onClick(HomeScreen.java:30)
        at android.view.View.performClick(View.java:2494)
        at android.view.View$PerformClick.run(View.java:9122)
        at android.os.Handler.handleCallback(Handler.java:587)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:130)
        at android.app.ActivityThread.main(ActivityThread.java:3806)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:507)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
        at dalvik.system.NativeStart.main(Native Method)

我应该指出，我已经开始在Android中，在iOS中做了相当多的工作。

这是我的.java文件

HomeScreen.java：

public class HomeScreen extends ActionBarActivity {
Button btnShowLocation;

//GPSTracker Class
GPSTracker gps;

@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home_screen);

    btnShowLocation = (Button) findViewById(R.id.btnShowLocation);

    //show location button click event
    btnShowLocation .setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //create class object
            gps = new GPSTracker(HomeScreen.this); //being thrown here (line30)

            if (gps.canGetLocation()){
                double latitude = gps.getLatitude();
                double longitude = gps.getLongitude();

                // \n indicates new line
                Toast.makeText(getApplicationContext(), "your Lcoation is - \nLat: " + latitude + "\nLon: " + longitude, Toast.LENGTH_LONG).show();

            }else{
                //cant get location
                gps.alertDialog.show();
            }
        }
    });

}

GPSTracker.java

public class GPSTracker extends Service implements LocationListener {


private final Context mContext;

//flag for GPS status
boolean isGPSEnabled = false;
//flag for network status
boolean isNetworkEnabled = false;

boolean canGetLocation = false;

Location location;
double latitude; //latitude variable
double longitude; //longitude variable

//The minimum distance to change updates in meters

private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 1;
private static final long MIN_TIME_BW_UPDATES = 1000;

//declare location manager
protected LocationManager locationManager;


public GPSTracker(Context context){
    this.mContext = context;
    getLocation();
}

//function to get latitude
public double getLatitude(){
    if (location != null){
        latitude = location.getLatitude();

    }
    //must have a return (as its a function)
    return latitude;
}

public double  getLongitude(){
    if (location !=null){
        longitude = location.getLongitude();
    }

    return longitude;
}





public Location getLocation() {
    try {
        locationManager = (LocationManager) mContext.getSystemService(LOCATION_SERVICE);

        //getting GPS Status
        isGPSEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);

        //getting Network Status
        isNetworkEnabled = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);

        if (!isGPSEnabled && !isNetworkEnabled) {
            //No Network provider is enabled
        }else{
            this.canGetLocation=true;
            //first get location from Network Provider
            if(isNetworkEnabled){
                locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER,MIN_TIME_BW_UPDATES,MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
                Log.d("Network", "Network");
                if (locationManager !=null){
                    location=locationManager.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
                    if(location !=null){
                        latitude = location.getLatitude();
                        longitude = location.getLongitude();

                    }
                }
            }

        }
    }
    catch (Exception e){
        e.printStackTrace();

    }
    return location;
}

//check if this is the best network provider
public boolean canGetLocation(){
    return this.canGetLocation;
}

//show GPs settings in alert box
AlertDialog.Builder alertDialogBuilder = //also being thrown here (line 116)
        new AlertDialog.Builder(this)
                .setTitle("GPS is Settings")
                .setMessage("GPS is not Enabled. Do you want to go to settings menu?")
                .setPositiveButton("Settings", new DialogInterface.OnClickListener() {
                    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
                    public void onClick(DialogInterface dialog, int which) {
                        Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                        mContext.startActivities(new Intent[]{intent});
                    }
                })
                .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.cancel();
                    }
                });

// Show the AlertDialog.
AlertDialog alertDialog = alertDialogBuilder.show();

如果有人能够指出我做错了什么，以及如何解决它，我们将不胜感激。

Answer 1

我认为您需要将mContext作为AlertDialog.Builder()的参数作为

new AlertDialog.Builder(mContext)

2 .java文件中的NullPointerException

1 个答案: