How to use an Android Handler to update a TextView in the UI Thread?

You can also update UI thread from background thread in this way also:

Handler handler = new Handler(); // write in onCreate function

//below piece of code is written in function of class that extends from AsyncTask

handler.post(new Runnable() {
    @Override
    public void run() {
        textView.setText(stringBuilder);
    }
});

There are several ways to update your UI and modify a View such as a TextView from outside of the UI Thread. A Handler is just one method.

Here is an example that allows a single Handler respond to various types of requests.

At the class level define a simple Handler:

private final static int DO_UPDATE_TEXT = 0;
private final static int DO_THAT = 1;
private final Handler myHandler = new Handler() {
    public void handleMessage(Message msg) {
        final int what = msg.what;
        switch(what) {
        case DO_UPDATE_TEXT: doUpdate(); break;
        case DO_THAT: doThat(); break;
        }
    }
};

Update the UI in one of your functions, which is now on the UI Thread:

private void doUpdate() {
    myTextView.setText("I've been updated.");
}

From within your asynchronous task, send a message to the Handler. There are several ways to do it. This may be the simplest:

myHandler.sendEmptyMessage(DO_UPDATE_TEXT);