ANDROID GPS, display ALERT message to turn on GPS to continue using Application

Question

I would like to create that user can start an activity ONLY if GPS is turn on, for API 23 I done, but for API 21 do not understand how exactly should I do.

This is GPSTracker:

public class GPSTracker extends Service implements LocationListener {

    private final Context context;

    boolean isGPSEnabled = false;
    boolean isNetworkEnabled = false;
    boolean canGetLocation = false;

    Location location;

    double latitude;
    double longitude;

    private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 10;
    private static final long MIN_TIME_BW_UPDATES = 1000 * 60 * 1;

    protected LocationManager locationManager;

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


    public Location getLocation() {

        try {


            locationManager = (LocationManager) context.getSystemService(LOCATION_SERVICE);
            isGPSEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);
            isNetworkEnabled = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);

            if (!isGPSEnabled && !isNetworkEnabled) {

            } else {
                this.canGetLocation = true;

                try {

                    if (isNetworkEnabled) {

                        locationManager.requestLocationUpdates(
                                LocationManager.NETWORK_PROVIDER,
                                MIN_TIME_BW_UPDATES,
                                MIN_DISTANCE_CHANGE_FOR_UPDATES, this);

                        if (locationManager != null) {
                            location = locationManager
                                    .getLastKnownLocation(LocationManager.NETWORK_PROVIDER);

                            if (location != null) {

                                latitude = location.getLatitude();
                                longitude = location.getLongitude();
                            }
                        }

                    }

                    if (isGPSEnabled) {
                        if (location == null) {
                            locationManager.requestLocationUpdates(
                                    LocationManager.GPS_PROVIDER,
                                    MIN_TIME_BW_UPDATES,
                                    MIN_DISTANCE_CHANGE_FOR_UPDATES, this);

                            if (locationManager != null) {
                                location = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);

                                if (location != null) {
                                    latitude = location.getLatitude();
                                    longitude = location.getLongitude();
                                }
                            }
                        }
                    }
                } catch (SecurityException e) {
                    Log.e("PERMISSION_EXCEPTION", "PERMISSION_NOT_GRANTED");
                }

            }

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

        return location;
    }


    public void stopUsingGPS() {
        if (locationManager != null) {
            try {
                locationManager.removeUpdates(GPSTracker.this);
            } catch (SecurityException e) {
                Log.e("PERMISSION_EXCEPTION", "PERMISSION_NOT_GRANTED");
            }
        }
    }

    public double getLatitude() {
        if (location != null) {
            latitude = location.getLatitude();
        }
        return latitude;
    }

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

        return longitude;
    }

    public boolean canGetLocation() {
        return this.canGetLocation;
    }

    public void showSettingsAlert() {
        AlertDialog.Builder alertDialog = new AlertDialog.Builder(context);

        alertDialog.setTitle(R.string.alertGPS1);

        alertDialog.setMessage(R.string.alertGPS2);

        alertDialog.setPositiveButton(R.string.GPSsetButt, new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                context.startActivity(intent);
            }
        });

        alertDialog.setNegativeButton(R.string.GPScancelButt, new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                dialog.cancel();
            }
        });

        alertDialog.show();
    }

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

    }

    @Override
    public void onProviderDisabled(String arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onProviderEnabled(String arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
        // TODO Auto-generated method stub

    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return null;
    }

}

What si the proper way to call showSettingsAlert()??

Currently to get location I'm using:

    gps = new GPSTracker(DVLAresult.this);

    latitudeD = gps.getLatitude();
    longitudeD = gps.getLongitude();