How to display a Toast in Android AsyncTask?

This is another way which is not mentioned here as follows:

Step 1: Define Handler as global

Handler handler;

Step 2: Initialise handler in doInBackground() method as follows:

@Override
protected Void doInBackground(Void... params) {
    Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        if (msg.what == 1) {
           //your code
        }
    }
  };
}

Step 3: And now you can call that handler by anywhere in code by calling

if(handler != null){
  handler.sendEmptyMessage(1);
}

What more you can do is you can send data through handler as follows:

Message message = new Message();
Bundle bundle = new Bundle();
bundle.putInt("KEY", value);
message.setData(bundle);
handler.sendMessage(message);

And handle data in you handler as below

handler = new Handler(){ 
    @Override
    public void handleMessage(Message message) {
        Bundle bundle = message.getData();
        Integer value = bundle.getInt("KEY");

    }
};

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

Create a handler object and execute all your Toast messages using that.

@Override
protected Void doInBackground(Void... params) {
    Handler handler =  new Handler(context.getMainLooper());
    handler.post( new Runnable(){
        public void run(){
            Toast.makeText(context, "Created a server socket",Toast.LENGTH_LONG).show(); 
        }
    });
  }