GPS android studio

locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);
Вот код : 
package com.example.root.gps12;
 
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.content.Context;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.widget.TextView;
 
import android.util.Log;
public class MainActivity extends AppCompatActivity implements LocationListener{
protected LocationManager locationManager;
protected LocationListener locationListener;
protected Context context;
TextView txtLat;
String lat;
String provider;
protected String latitude,longitude;
protected boolean gps_enabled,network_enabled;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
txtLat = (TextView) findViewById(R.id.textView2);
locationManager = (LocationManager)getSystemService(Context.LOCATION_SERVICE);
gps_enabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);
if(gps_enabled) {
locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);
}
}
 
@Override
public void onLocationChanged(Location location) {
txtLat = (TextView) findViewById(R.id.textView2);
txtLat.setText("Latitude:" + location.getLatitude() + ", Longitude:" + location.getLongitude());
}
 
@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
Log.d("Latitude","status");
}
 
@Override
public void onProviderEnabled(String provider) {
Log.d("Latitude","enable");
}
 
@Override
public void onProviderDisabled(String provider) {
Log.d("Latitude","disable");
}
}