How to get a string back from AsyncTask?

Question

I have the following class:

public class getURLData extends AsyncTask<String, Integer, String>{  @Override protected String doInBackground(String... params) {     String line;     try {           DefaultHttpClient httpClient = new DefaultHttpClient();         HttpPost httpPost = new HttpPost(params[0]);          HttpResponse httpResponse = httpClient.execute(httpPost);         HttpEntity httpEntity = httpResponse.getEntity();         line = EntityUtils.toString(httpEntity);      } catch (UnsupportedEncodingException e) {         line = "<results status=\"error\"><msg>Can't connect to server</msg></results>";     } catch (MalformedURLException e) {         line = "<results status=\"error\"><msg>Can't connect to server</msg></results>";     } catch (IOException e) {         line = "<results status=\"error\"><msg>Can't connect to server</msg></results>";     }     return line; }  @Override protected void onPostExecute(String result) {     super.onPostExecute(result); }  } 

And I am trying to call it like this:

String output = null; output = new getURLData().execute("http://www.domain.com/call.php?locationSearched=" + locationSearched); 

But the output variable isn't getting data, instead I am getting an error:

Type mismatch: cannot convert from AsyncTask<String,Integer,String> to String 

Answer 1

The method execute returns the AynscTask itself, you need to call get:

output =     new getURLData()         .execute("http://www.example.com/call.php?locationSearched=" + locationSearched)         .get(); 

This will start a new thread (via execute) while blocking the current thread (via get) until the work from the new thread has been finished and the result has been returned.

If you do this, you just turned your async task into a sync one.

However, the problem with using get is that because it blocks, it needs to be called on a worker thread. However, AsyncTask.execute() needs to be called on the main thread. So although this code could work, you may get some undesired results. I also suspect that get() is under-tested by Google, and it is possible that they introduced a bug somewhere along the line.

Reference: AsyncTask.get

Answer 2

I'd rather create callback than block UI thread. In your class create method which will be invoked when data arrive. For example:

private void setData(String data){     mTextView.setText(data); } 

Then in AsyncTask implement onPostExecute:

@Override protected void onPostExecute(String result) {     setData(result); } 

And then somewhere in code just execute task:

new getURLData().execute(... 

When task finishes setData is invoked and mTextView is filled.

AsyncTask.get() will blok your UI, so there is no reason to use AsyncTask.