How to display a Toast in Android AsyncTask?

I am trying to display Toast in my initial_background class extended with AsyncTask<URL, Integer, Long>. I am receiving this error in logcat.

public class InitialBackgroundTask extends AsyncTask<URL, Integer, Long> {

    @Override
    protected Long doInBackground(URL... params) {
        // TODO Auto-generated method stub
        show a = new show();
        a.loop();
        return null;
    }

public class show {

    void loop()
    {
        for(int i=0; i<10; i++)
        {
            Toast.makeText(MainActivity.me, "test", Toast.LENGTH_LONG).show();
        }
    }
}

This is the exception:

05-30 12:08:12.641: E/AndroidRuntime(30840): FATAL EXCEPTION: AsyncTask #1
05-30 12:08:12.641: E/AndroidRuntime(30840): java.lang.RuntimeException: An error occured while executing doInBackground()
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.os.AsyncTask$3.done(AsyncTask.java:278)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.FutureTask.run(FutureTask.java:137)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:208)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.lang.Thread.run(Thread.java:856)
05-30 12:08:12.641: E/AndroidRuntime(30840): Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.os.Handler.<init>(Handler.java:121)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.widget.Toast$TN.<init>(Toast.java:317)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.widget.Toast.<init>(Toast.java:91)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.widget.Toast.makeText(Toast.java:233)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at   com.example.toast.show.loop(show.java:11)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at com.example.toast.InitialBackgroundTask.doInBackground(InitialBackgroundTask.java:13)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at com.example.toast.InitialBackgroundTask.doInBackground(InitialBackgroundTask.java:1)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at android.os.AsyncTask$2.call(AsyncTask.java:264)
05-30 12:08:12.641: E/AndroidRuntime(30840):    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
05-30 12:08:12.641: E/AndroidRuntime(30840):    ... 5 more

The above code is showing the whole story. Actually, I want to show toast in doInBackground method

Solution

You cannot update UI on background thread. doInBackground() is invoked on the background thread. You should update UI on the UI thread.

      runOnUiThread(new Runnable(){

          @Override
          public void run(){
            //update ui here
            // display toast here 
          }
       });

onPreExecute(), onPostExecute(Result), are invoked on the UI thread. So you can display toast here.

onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...) can be used to animate a progress bar or show logs in a text field.

The result of doInBackground() computation is a parameter to onPostExecute(Result) so return the result in doinBackground() and show your toast in onPostExecute(Result)

You can also use a handler as suggested by @Stine Pike

For clarity, check the link below under the topic: The 4 steps.

http://developer.android.com/reference/android/os/AsyncTask.html