Question

My Android App needs some basic data to run. This data is downloaded from a server using JSON. In Xcode I simply used the sendsynchronous request but I noticed that Eclipse gives me a error when i do networking on the main ui.
Found a lot of stuff on asynctask but i want my app to wait till the required data is downloaded (synchronous?).

I tried using asynctask .execute().get() and setting the variables in onPostExecute but when I return the variable I get a NullPointerException. Does someone know how to make this work? I really need this data before the app can run so I want my app to wait till the data is downloaded.

MainActivity calls this:

SingletonClass appIDSingleton = SingletonClass.getInstance();
this.ID = appIDSingleton.getAppID();

Singleton Class:

public String getAppID() {
           try {
            new DownloadAppID().execute(APP_ID_URL).get(5000, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (TimeoutException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
           return AppID; //AppID is still NULL (because the download isnt finished yet?)
       }

private class DownloadAppID extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... urls) {
            String response = "";
            for (String url : urls) {
                DefaultHttpClient client = new DefaultHttpClient();
                HttpGet httpGet = new HttpGet(url);
                try {
                    HttpResponse execute = client.execute(httpGet);
                    InputStream content = execute.getEntity().getContent();

                    BufferedReader buffer = new BufferedReader(new InputStreamReader(content));
                    String s = "";
                    while ((s = buffer.readLine()) != null) {
                        response += s;
                    }

                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            return response;
        }

        @Override
        protected void onPostExecute(String result) {

            System.out.println(result);
            AppID = result;
            }
       }

Answer 1

You need to understand that your getAppID method can’t return a result that is going to be computed asynchronously.

You could for instance provide a listener to your async task in order to notify when app ID is available:

    SingletonClass appIDSingleton = SingletonClass.getInstance();
    appIDSingleton.getAppID(new AppIdDownloadListener() {

        @Override
        public void appIDAvailable(String appId) {
            this.ID = appId; 
        }
    });


public void getAppID(AppIdDownloadListener listener) {
    try {
        new DownloadAppID(listener).execute(APP_ID_URL).get(5000, TimeUnit.MILLISECONDS);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (ExecutionException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (TimeoutException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

public interface AppIdDownloadListener {
    public void appIDAvailable(String appId);
}

private class DownloadAppID extends AsyncTask<String, Void, String> {

    private AppIdDownloadListener listener;

    public DownloadAppID(AppIdDownloadListener listener) {
        this.listener = listener;
    }

    @Override
    protected String doInBackground(String... params) {
        /* Your stuff here */
    }

    @Override
    protected void onPostExecute(String result) {
        System.out.println(result);
        listener.appIDAvailable(result);
    }
}