java.lang.NullPointerException about gps api?

as the title i have a question about the gps api follow is my code

 LocationManager loctionManager; 
                 String contextService=Context.LOCATION_SERVICE;
                 loctionManager=(LocationManager) getSystemService(contextService); 
                 Criteria criteria = new Criteria(); 
                    criteria.setAccuracy(Criteria.ACCURACY_FINE);
                    criteria.setAltitudeRequired(false);
                    criteria.setBearingRequired(false); 
                    criteria.setCostAllowed(true); 
                    criteria.setPowerRequirement(Criteria.POWER开发者_JAVA技巧_LOW);
                    String provider = loctionManager.getBestProvider(criteria, true); 
                    Location location = loctionManager.getLastKnownLocation(provider);


                     double a=location.getLatitude();
                    double b=location.getLongitude();
Log.d(""+a,""+b);

then the error is java.lang.NullPointerException anyone can help me?thx

getLastKnownLocation() can return null you need to ensure it does not.

In order to get a Location you need at first to set permission in your Manifest.

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

Then in your code you need to listen for location avalability. You do that by registering a LocationListener. The activity will display, and when Android will have a location for you it will call you back.

private final LocationListener locationListener = new LocationListener() {
public void onLocationChanged(Location location) {
    latitude = location.getLatitude();
    longitude = location.getLongitude();
    //Do what you want with your coordinates
}
loctionManager.requestLocationUpdates(LocationManager.GPS, 100, 1, locationListener);

If you are using the emulator and not a real device you need to force a location, you can use DDMS inside Eclipse to send a fake location: