App crashes with thread

Question

I'm very new to Java programming and I'm having troubles with while loops in Threads. I post here an example where I use the while loop.

public class MainActivity extends AppCompatActivity {

private boolean running = true;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView text = (TextView)findViewById(R.id.textView);

    new Thread(new Runnable() {
        @Override
        public void run() {
            while (running) {

                try{Thread.sleep(1000);} 
                catch(InterruptedException e){}

                running = false;
                text.setText("false!");

                running = true;

                try{Thread.sleep(1000);}
                catch(InterruptedException e){}

                text.setText("True!");
            }
        }
    }).start();
}
}

When running is set to false, the Thread doesn't run and the app does not show anything. When running is set to true, the app crashes after text.setText("false!");

Thank you all in advance!

Read the exception message ( and post it in your question too). It should tell you, you can't update UI outside of the UI thread — AxelH, May 08 '17 at 11:54
This [answer](http://stackoverflow.com/a/5185140/4391450) is the simplest to use — AxelH, May 08 '17 at 11:57
Android is the wrong platform to learn Java threads, IMO. Try a desktop app — OneCricketeer, May 08 '17 at 12:00
thread sleeps your UI, meaning you cannot update UI during that time. either use Handler or Looper or an AsyncTask to update UI after your background action @Jens — Yashika, May 08 '17 at 12:02
you can't update UI in background Thread. You need to update UI in Main thread. For this you can use: runOnUiThread(new Runnable() { @Override public void run() { //your code goes here } }); — Gurvinder Rajpal, May 08 '17 at 12:09
Thank you all very much! I just didn't know this thing! sorry, i'm just a beginner :) — Jacopo Trapani, May 08 '17 at 16:29

Reflection · Answer 1 · 2017-05-08T12:03:33.060

0

you cannot access textView from thread, you can only change view objects from main thread

edited May 08 '17 at 12:03

answered May 08 '17 at 12:00

Reflection

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2
11

The object is final. It can be accessed – OneCricketeer May 08 '17 at 12:02
it does not matter, you cannot change gui objects from threads rather than main thread. – Reflection May 08 '17 at 12:06
@cricket_007 Even if TextView is final, we can not access it on background or other thread. UI must be updated on mainThread. Here we can use Activity.runOnUIThread instead. or an Handler – Yashika May 08 '17 at 12:07
I understand that. Maybe you can re-write the answer instead of saying "access" to "update" – OneCricketeer May 08 '17 at 12:11
plz read this post, it completely answered to your question http://stackoverflow.com/questions/5185015/updating-android-ui-using-threads – Reflection May 08 '17 at 12:18

App crashes with thread

1 Answers1