Why the function getLastKnownLocation return Null on Android SDK?

Question

I use android sdk on eclipse and i want to get location from device so i wrote an program in other class(diffrent from main) and i call mContext function inside this class from main class:

package com.example.deomanapp;

import java.io.IOException;
import java.util.List;
import java.util.Locale;

import com.example.deomanapp.MainActivity.PlaceholderFragment;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.location.Address;
import android.location.Geocoder;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class Mhelper {

    public void mContext(Context context)
    {
        LocationManager lm;
        lm   = (LocationManager)context.getSystemService(context.LOCATION_SERVICE); 
        Location location = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);

        double longitude = location.getLongitude();
        double latitude = location.getLatitude();
        String slongitude = String.valueOf((long) longitude);
        String slatitude = String.valueOf((long) latitude);

        Toast.makeText(context, slongitude, Toast.LENGTH_SHORT).show();

    }

}

The problem is getLongitude or getLatitude return null,so the program crash on this line with this log:

04-20 04:30:30.410: E/AndroidRuntime(5151): java.lang.NullPointerException
04-20 04:30:30.410: E/AndroidRuntime(5151):     at com.example.deomanapp.Mhelper.mContext(Mhelper.java:29)

What is wrong with this code ?

PS: I read other question with the same title and non of theme help (non of them have actual answer) because:

1-I test this program on the real device(not emulator) with GPS ON and working , but this program can't able to get location although the Device get its location before and it must show LastKnownLocation .

2-I gave the program ACCESS_FINE_LOCATION and ACCESS_COARSE_LOCATION permission.

3-I even use an function to see if the GPS is on, when i turn it off program alert me.

Answer 1

问这是一个愚蠢的问题，因为它为NULL，所以为null，我通过对主要问题的评论来理解它，我的解决方案是在sdk管理器中下载android api图像 ，运行一次Google地图 ，然后通过DDMS将GPS位置发送给它运行程序。

Answer 2

I have tried to modify some of my code, hopefully it works for your needs.

You will find the implementation of my LocationsCoreModule code in the bottom of the answer:

LocationsCoreModule locationService;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    LocationRequest mLocationRequest = LocationRequest.create();
    // Use high accuracy
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    // Set the update interval to 5 seconds
    mLocationRequest.setInterval(5000);
    // Set the fastest update interval to 1 second
    mLocationRequest.setFastestInterval(1000);

    locationService = new LocationsCoreModule(this, mLocationRequest);
    locationService.setLocationsListener(new LocationsCoreModuleCallback() {

        @Override
        public void locationClientConnected() {
              Location location = locationService.getLastLocation();
              double longitude = location.getLongitude();
              double latitude = location.getLatitude();
              String slongitude = String.valueOf((long) longitude);
              String slatitude = String.valueOf((long) latitude);

              Toast.makeText(getApplicationContext(), slongitude, Toast.LENGTH_SHORT).show();
        }
    });
}

If you want the application to start listening for new GPS locations right away:

    @Override
protected void onStart() {
    super.onStart();

    locationService.start(new LocationListener() {

        @Override
        public void onLocationChanged(Location arg0) {
            // TODO Auto-generated method stub

        }
    }, new OnConnectionFailedListener() {

        @Override
        public void onConnectionFailed(ConnectionResult arg0) {
            // TODO Auto-generated method stub

        }
    });
}

@Override
protected void onStop() {
    super.onStop();

    locationService.stop();
}

And finally, the LocationsCoreModule:

public class LocationsCoreModule implements
        GooglePlayServicesClient.ConnectionCallbacks {

    private static final String TAG = "LocationsCoreModule";

    public interface LocationsCoreModuleCallback {
        public void locationClientConnected();
    }

    private LocationsCoreModuleCallback locationsCoreModuleCallback;

    private com.google.android.gms.location.LocationListener locationListener;

    private Location lastLocation;

    private LocationClient mLocationClient;

    private final LocationRequest mLocationRequest;
    private final Context context;


    @Inject 
    public LocationsCoreModule(Context context, LocationRequest locationRequest) {
        this.context = context;
        this.mLocationRequest = locationRequest;


    }

    public void setLastLocation(Location lastLocation) {
        this.lastLocation = lastLocation;
    }

    public void setLocationsListener(
            LocationsCoreModuleCallback locationsCoreModuleCallback) {
        this.locationsCoreModuleCallback = locationsCoreModuleCallback;
    }

    public void start(
            com.google.android.gms.location.LocationListener locationListener,
            GooglePlayServicesClient.OnConnectionFailedListener connectionFailedListener) {
        this.locationListener = locationListener;
        mLocationClient = new LocationClient(context, this,
                connectionFailedListener);
        mLocationClient.connect();
    }

    public void stop() {
        if (mLocationClient != null) {
            // If the client is connected
            if (mLocationClient.isConnected() && locationListener != null) {
                /*
                 * Remove location updates for a listener. The current Activity
                 * is the listener, so the argument is "this".
                 */
                mLocationClient.removeLocationUpdates(locationListener);
            }

            // Disconnecting the client invalidates it.
            mLocationClient.disconnect();
        }
    }


    public boolean isConnected() {
        if (mLocationClient == null) return false;
        return mLocationClient.isConnected();
    }

    public Location getLastLocation() {

        if (lastLocation != null) {
            return lastLocation;
        }

        if (mLocationClient != null) {
            if (mLocationClient.isConnected()) {
                return lastLocation = mLocationClient.getLastLocation();
            }

            if (!mLocationClient.isConnecting())
                mLocationClient.connect();
        }

        return null;

    }

    @Override
    public void onConnected(Bundle connectionHint) {
        Log.d(TAG, "GooglePlayServices connected!");

        Location lastLocation = mLocationClient.getLastLocation();

        if (lastLocation == null)
            Log.e(TAG, "Lastlocation is null even after connected!!!!");

        if (locationsCoreModuleCallback != null) {
            locationsCoreModuleCallback.locationClientConnected();
            locationsCoreModuleCallback = null; // single shot
        }

    }

    public void requestLocationUpdates() {
        if (mLocationClient != null && mLocationClient.isConnected()) {
            Log.d(TAG, "Requesting location updates");
            mLocationClient.requestLocationUpdates(mLocationRequest,
                    locationListener);
        }
    }

    public void stopLoactionUpdates() {
        if (mLocationClient != null && mLocationClient.isConnected()) {
            mLocationClient.removeLocationUpdates(locationListener);
        }
    }

    @Override
    public void onDisconnected() {
        Log.d(TAG, "GooglePlayServices disconnected!");
    }

}