Android : Get Notification from Asynctask process

I need to send notification from underground process. in this project i tried to send a simple notification 10 seconds after 10seconds. it runs but don't send me notification!

public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Long previousDate = new Long(System.currentTimeMillis());
     NotificationManager manager = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
    new GetMethodEx1().execute(previousDate);


    }



    //this is the method that send notifications


}




class GetMethodEx1 extends AsyncTask<Long,Void,Void>{

    private Context mContext;
    private int NOTIFICATION_ID = 1;
    private Notification mNotification;
    private NotificationManager mNotificationManager;





     @Override
        protected Void doInBackground(Long... params){
         Long previousDate = params[0];
         Long Intervall = new Long(System.currentTimeMillis());
            Boolean a = false;
            while ( a = false){
            if ((Intervall - previousDate) > 10000 ){


                String c = "ciao";
                String b = "ciai";
            //sendNotification()
                createNotification( c, b);


            previousDate = Intervall;



            }


     }
            return null;

}





    private void createNotification(String contentTitle, String contentText) {

        //Build the notification using Notification.Builder
        Notification.Builder builder = new Notification.Builder(mContext)
        .setSmallIcon(android.R.drawable.stat_sys_download)
        .setAutoCancel(true)
        .setContentTitle(contentTitle)
        .setContentText(contentText);

        //Get current notification
        mNotification = builder.getNotification();

        //Show the notification
        mNotificationManager.notify(NOTIFICATION_ID, mNotification);
    }

it don't crash but don't work!