Using AsyncTask

It would be something like this

   public class TalkToServer extends AsyncTask<Void, Void, Void> {

   @Override
   protected void onPreExecute() {
        /*
         *    do things before doInBackground() code runs
         *    such as preparing and showing a Dialog or ProgressBar
        */
   }

   @Override
   protected void onProgressUpdate(Void... values) {
        /*
         *    updating data
         *    such a Dialog or ProgressBar
        */

   }

   @Override
   protected Void doInBackground(Void... params) {
      //do your work here
      return null;
   }

   @Override
   protected void onPostExecute(Void result) {
        /*
         *    do something with data here
         *    display it or send to mainactivity
         *    close any dialogs/ProgressBars/etc...
        */
   }
}

Then you can execute it with

TalkToServer myTask = new MyTask(); // can add params for a constructor if needed
myTask.execute(); // here is where you would pass data to doInBackground()

You may not need onProgressUpdate() or onPreExecute() if you aren’t using them to show a Dialog, progress, or other tasks before or during doInBackground().

onPreExecute() can be used to initialize and show a ProgressDialog. Then it could be dismissed in onPostExecute()

After a task finishes

onPostExecute() will be called. If the class is an inner-class of your Activity then you can update UI elements there or call a function to run code once the task finishes. If it is a separate file than the Activity then you can use an interface and create a callBack to the Activity and run code there once the task finishes.

This answer talks about using an interface for a callback if you need

Make sure any UI updating is done in any method besides doInBackground() or sent back to the Activity in any function besides doInBackground(). Heavy-lifting such as network operations should be done in doInBackground().

Also, be sure to read through the AsyncTask Docs completely. Especially the Threading Rules