Android : Android Studio AsyncTask progressRingDialog not working properly

While trying to impliment How to show ProgressDialog across launching a new Activity? via @Slartibartfast's answer, I tried unsuccessfully to get it to work in my editor. Here, I am trying to display a ring progressDialog while the program fetches some contacts' information. Then, later on, in the OnCreate, it puts it in a ListView. My problem is that no progressDialog ever appears. My code is as follows:

Declaration

private ProgressDialog ringProgressDialog = null;

Same as suggested in his example

AsyncTask - sets off and ends the ring progressDialog

private class load_contact_list extends AsyncTask<String, Void, Integer> {

    @Override
    protected Integer doInBackground(String... url) {

       ...
    }

    @Override
    protected void onPostExecute(Integer list_length) {
        ringProgressDialog.dismiss();
        setContentView(R.layout.activity_main);
    }
}

OnCreate

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

    ringProgressDialog = ProgressDialog.show(MainActivity.this, "Loading contacts", "Please Wait", true);

    new load_contact_list().execute(...);
    }
  ...

My best efforts to make my code like his has proved in vein, I don't know why it isn't working. Thanks in advance.