Cannot update UI from AsyncTask

Question

I know that this is most likely going to be marked a duplicate, but this is extremely frustrating me. I cannot, for the life of me, figure out a way to modify a textview from an AsyncTask. This is my function that is in the MainActivity class:

public void updateTextView(String data){
    TextView textview = (TextView)findViewById(R.id.TextView1);
    textview.setText(data);
}

and then in my AsyncTask i have

@Override
protected void onPostExecute(String result) {
    MainActivity ma = new MainActivity();
    ma.updateTextView(result);
}

However, after running this code and any modifications that involve attempting to modify the TextView from the AsyncTask, I get:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference

However when calling the same exact function from the main thread, it runs fine.

Any help would be greatly appreciated.

`MainActivity ma = new MainActivity()`??? You are trying to create a new activity? — azizbekian, Mar 12 '17 at 16:00
Possible duplicate of [How to get the result of OnPostExecute() to main activity because AsyncTask is a separate class?](http://stackoverflow.com/questions/12575068/how-to-get-the-result-of-onpostexecute-to-main-activity-because-asynctask-is-a) — OneCricketeer, Mar 12 '17 at 16:01
Do not create a new Instance of your MainActivity! Instead pass the instance as parameter when creating your AsyncTask and use it to call the method. — Schokokuchen Bäcker, Mar 12 '17 at 16:01
Never ever call `new` on any Activity class. You're just asking for problems with that. — OneCricketeer, Mar 12 '17 at 16:01

Ali Gholami · Accepted Answer · 2017-03-12T19:46:50.463

2

you connot make a new object from an activity and work with it. try to get a reference to the activity

add this code to your AsyncTask class

MainActivity activity;

public void setContext(MainActivity activity){
    this.activity = activity;
  }

also edit this

@Override
protected void onPostExecute(String result) {
     activity.updateTextView(result);
   }

call this method when you create your AsyncTask object in your activity

//at is the object you created from your AsyncTask Class
at.setContext(this);

edited Mar 12 '17 at 19:46

answered Mar 12 '17 at 16:15

Ali Gholami

36
3

Works great! Thanks! – Jake Mar 12 '17 at 16:27
You don't have to use things like runOnUiThread. OnPostExecute in AsyncTask is called on the UI thread already. – pablo432 Mar 12 '17 at 19:13

Cannot update UI from AsyncTask

1 Answers1