android null pointer exceptions in AsyncTask [closed]

Question

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.

Closed 8 years ago.

I'm just starting to write under the android and ran into the problem that I do not know how to solve

PostData.java

import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import android.app.Activity;

import android.os.AsyncTask;
import android.text.format.Time;
import android.util.Log;
import android.widget.EditText;
import android.widget.Toast;

class PostData extends AsyncTask<Void, Void, Void> {
    // GPSTracker class
        GPSTracker gps;
    //определяем переменную главного активити
        MainActivity ma;
        Teleport_user_profile_activity UP;

        @Override
        protected void onPreExecute() {
            // TODO Auto-generated method stub
            // do stuff before posting data
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Void... params) {
            // TODO Auto-generated method stub
            postData();
            return null;
        }


        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            // do stuff after posting data
            super.onPostExecute(result);
        }



        public void postData() {
            // TODO Auto-generated method stub
            // Create a new HttpClient and Post Header
            //переводим значение double в стринг
            double latitudep = gps.getLatitude();
            double longitudep = gps.getLongitude();
        double totalLatitude = latitudep;
            String stotalLatitude = String.valueOf(totalLatitude);
            double totalLongitude = longitudep;
            String stotalLongitude = String.valueOf(totalLongitude);
            // временная переменная для определения времени устройства
            Time nowTime = new Time();
            nowTime.setToNow();
            String snowTime = String.valueOf(nowTime);
            //берем информацию о юзере
                UP.LoadUserInfoFromFile();
                UP.LoadUserInfopassFromFile();
                String UserInfo = Teleport_user_profile_activity.UserLoginFile;
                String UserPass = Teleport_user_profile_activity.UserPassFile;
            //посылка данных на сервер
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://myheart.pp.ua/Android_in.php");

        try {
            // Add your data
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(4);

//          nameValuePairs.add(new BasicNameValuePair("latitude", stotalLatitude));
//          nameValuePairs.add(new BasicNameValuePair("longitude", stotalLongitude));
            nameValuePairs.add(new BasicNameValuePair("Android_device_time", snowTime));
//          nameValuePairs.add(new BasicNameValuePair("user_info", UserInfo));
//          nameValuePairs.add(new BasicNameValuePair("user_pass", UserPass));
            nameValuePairs.add(new BasicNameValuePair("separator", "______________________________________"));
            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            httpclient.execute(httppost);

            // Execute HTTP Post Request
            HttpResponse response = httpclient.execute(httppost);

        } catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
        } catch (IOException e) {
            // TODO Auto-generated catch block
        } catch (NullPointerException e) {
            e.printStackTrace();
        } 

}
}

MainActivity.java

 R.id.menu_send_location_manual: {

             GPSdetermination();
             new PostData().execute();


            // postData(); - вызывает ошибку на реальном устройстве
             Toast.makeText(getApplicationContext(), R.string.location_send_manuall_toast, Toast.LENGTH_LONG).show();
        break;
    }

 public void GPSdetermination() {       
     // create class object
     gps = new GPSTracker(MainActivity.this);

     // check if GPS enabled        
     if(gps.canGetLocation()){

         double latitude = gps.getLatitude();
         double longitude = gps.getLongitude();

         // \n is for new line
         Toast.makeText(getApplicationContext(), "Your Location is - \nLat: " + latitude + "\nLong: " + longitude, Toast.LENGTH_LONG).show();   
     }else{
         // can't get location
         // GPS or Network is not enabled
         // Ask user to enable GPS/network in settings
         gps.showSettingsAlert();
     }

 } 

GPSTracker.java

    public class GPSTracker extends Service implements LocationListener {

private final Context mContext;

//определяем переменную главного активити
    MainActivity ma;

// flag for GPS status
boolean isGPSEnabled = false;

// flag for network status
boolean isNetworkEnabled = false;

// flag for GPS status
boolean canGetLocation = false;

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

// The minimum distance to change Updates in meters
private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 10; // 10 meters

// The minimum time between updates in milliseconds
private static final long MIN_TIME_BW_UPDATES = 1000 * 60 * 1; // 1 minute

// Declaring a Location Manager
protected LocationManager locationManager;

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

// Функция для определения местоположения
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;
            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();
                    }
                }
            }
            // if GPS Enabled get lat/long using GPS Services
            if (isGPSEnabled) {
                if (location == null) {
                    locationManager.requestLocationUpdates(
                            LocationManager.GPS_PROVIDER,
                            MIN_TIME_BW_UPDATES,
                            MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
                    Log.d("GPS Enabled", "GPS Enabled");
                    if (locationManager != null) {
                        location = locationManager
                                .getLastKnownLocation(LocationManager.GPS_PROVIDER);
                        if (location != null) {
                            latitude = location.getLatitude();
                            longitude = location.getLongitude();
                        }
                    }
                }
            }
        }

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

    return location;
}
/**
 * Stop using GPS listener
 * Calling this function will stop using GPS in your app
 * */
public void stopUsingGPS(){
    if(locationManager != null){
        locationManager.removeUpdates(GPSTracker.this);
    }       
}

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

    // return latitude
    return latitude;
}

/**
 * Function to get longitude
 * */
public double getLongitude(){
    if(location != null){
        longitude = location.getLongitude();
    }

    // return longitude
    return longitude;
}

/**
 * Function to check GPS/wifi enabled
 * @return boolean
 * */
public boolean canGetLocation() {
    return this.canGetLocation;
}

/**
 * Function to show settings alert dialog
 * On pressing Settings button will lauch Settings Options
 * */
public void showSettingsAlert(){
    AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);

    // Setting Dialog Title
    alertDialog.setTitle("GPS is settings");

    // Setting Dialog Message
    alertDialog.setMessage("GPS is not enabled. Do you want to go to settings menu?");

    // On pressing Settings button
    alertDialog.setPositiveButton("Settings", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog,int which) {
            Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
            mContext.startActivity(intent);
        }
    });

    // on pressing cancel button
    alertDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int which) {
        dialog.cancel();
        }
    });

    // Showing Alert Message
    alertDialog.show();
}


//события которые происходят если позиция поменялась
@Override
public void onLocationChanged(Location location) {
}

@Override
public void onProviderDisabled(String provider) {
}

@Override
public void onProviderEnabled(String provider) {
}

@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
}

@Override
public IBinder onBind(Intent arg0) {
    return null;
}

When I try to call PostData I get an error java.lang.NullPointerException in this code

double latitudep = gps.getLatitude();
double longitudep = gps.getLongitude();

can not seem to figure out how to fix it.

Answer 1

You have declared GPSTracker gps; inside your AsyncTask, it never seems to get initialised. In other words, it's null. Thus, NullPointerException.

I guess you need a constructor for your AsyncTask to set it:

public PostData (GPSTracker gps) {
    this.gps = gps;
}

Then in MainActivity

new PostData(gps).execute();

AsyncTask is just a class like any other, with member variables, that need setting, if you want to use them.

Answer 2

gps is null because it isn't instantiated. In doInBackground() you can instantiate it

gps = new GPSTracker();

But since your GPSTracker constructor needs a Context you will have to add a constructor to your AsyncTas that accepts a Context

Async

pubic class PostData extends AsyncTask<Void, Void, Void> {
   Context mContext;

    MainActivity ma;
    Teleport_user_profile_activity UP;
    public PostData(Context c)
   {
         mContext = c;
   }

Then in doInBackground()

@Override
    protected Void doInBackground(Void... params) {
        // TODO Auto-generated method stub
        gps = new GPSTracker(mContext);
        postData();
        return null;
    }

And call your AsyncTask like this in MainActivity

 R.id.menu_send_location_manual: {

         GPSdetermination();
         PostData task = new PostData(MainActivity.this);
         task.execute();
         new PostData().execute();

Answer 3

Your problem is that GPS can only be listened to within a regular thread, like the UI thread and not the doInBackground() method of an AsyncTask. This has to do with the fact that it requires a Looper to function properly. Here's a good post that's very similar to your problem.

Get your GPS data first in onCreate then you can transfer that data to your AsyncTask as varargs and use it in doInBackground that way.