How to get the result of OnPostExecute() to main activity because AsyncTask is a separate class?

Easy:

1. Create interface class, where String output is optional, or can be whatever variables you want to return.

    public interface AsyncResponse {
        void processFinish(String output);
    }
   

2. Go to your AsyncTask class, and declare interface AsyncResponse as a field :

    public class MyAsyncTask extends AsyncTask<Void, Void, String> {
      public AsyncResponse delegate = null;
   
        @Override
        protected void onPostExecute(String result) {
          delegate.processFinish(result);
        }
     }
   

3. In your main Activity you need to implements interface AsyncResponse.

    public class MainActivity implements AsyncResponse{
      MyAsyncTask asyncTask =new MyAsyncTask();
   
      @Override
      public void onCreate(Bundle savedInstanceState) {
   
         //this to set delegate/listener back to this class
         asyncTask.delegate = this;
   
         //execute the async task 
         asyncTask.execute();
      }
   
      //this override the implemented method from asyncTask
      @Override
      void processFinish(String output){
         //Here you will receive the result fired from async class 
         //of onPostExecute(result) method.
       }
     }
   

---

UPDATE

I didn’t know this is such a favourite to many of you. So here’s the simple and convenience way to use interface.

still using same interface. FYI, you may combine this into AsyncTask class.

in AsyncTask class :

public class MyAsyncTask extends AsyncTask<Void, Void, String> {

  // you may separate this or combined to caller class.
  public interface AsyncResponse {
        void processFinish(String output);
  }

  public AsyncResponse delegate = null;

    public MyAsyncTask(AsyncResponse delegate){
        this.delegate = delegate;
    }

    @Override
    protected void onPostExecute(String result) {
      delegate.processFinish(result);
    }
}

do this in your Activity class

public class MainActivity extends Activity {
  
   MyAsyncTask asyncTask = new MyAsyncTask(new AsyncResponse(){
    
     @Override
     void processFinish(String output){
     //Here you will receive the result fired from async class 
     //of onPostExecute(result) method.
     }
  }).execute();

 }

Or, implementing the interface on the Activity again

public class MainActivity extends Activity 
    implements AsyncResponse{
      
    @Override
    public void onCreate(Bundle savedInstanceState) {

        //execute the async task 
        new MyAsyncTask(this).execute();
    }
      
    //this override the implemented method from AsyncResponse
    @Override
    void processFinish(String output){
        //Here you will receive the result fired from async class 
        //of onPostExecute(result) method.
    }
}

As you can see 2 solutions above, the first and third one, it needs to create method processFinish, the other one, the method is inside the caller parameter. The third is more neat because there is no nested anonymous class.

Tip: Change String output, String response, and String result to different matching types in order to get different objects.