Android JAVA - NullPointerException while using and Interface Field

Question

I'm coding an Android App. At a certain point, the App is asked to retrieve data from a Web Service that I have on Heroku. The class that contains the methods that retreive data and the class with the methods that use those data are different. I'm trying to pass the data from an AsyncTask's onPostExecute() to my other class using an interface; however I'm getting an annoying NullPointerException and I'm not understanding the reason of this!

My "main" class:

 public class Details extends AppCompatActivity implements ResponseFromWebService.AsyncResponse {
 @Override
    public void processFinish(String output){
        System.out.println(output);
    }
//Other Stuff
}

My ResponseFromWebService class:

public class ResponseFromWebService {

public void getData (String Name) {
    new JSONTask().execute("censored-URL");
}

public interface AsyncResponse {
    void processFinish(String output);
}

public class JSONTask extends AsyncTask<String, String, String> {

    AsyncResponse delegate = null;

    @Override
    protected String doInBackground(String... params) {
        HttpURLConnection connection = null;
        BufferedReader reader = null;
        StringBuffer buffer = new StringBuffer();

        try {
            URL url = new URL(params[0]);
            connection = (HttpURLConnection) url.openConnection();
            connection.connect();

            InputStream stream = connection.getInputStream();

            reader = new BufferedReader(new InputStreamReader(stream));

            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            String finalJson = buffer.toString();

            JSONObject finalObject = new JSONObject(finalJson);

            String Desc = finalObject.getString("desc");

            return Desc;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (connection != null)
                connection.disconnect();
            try {
                if (reader != null)
                    reader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
        delegate.processFinish(s);
    }
}
}

The NPE occurs at the last line, when there is a call to delegate.processFinish(s);

Really need help!

score 4 · Accepted Answer · answered Oct 17 '16 at 14:40

delegate is never set.

Set it with a constructor for instance

public class JSONTask extends AsyncTask<String, String, String> {

    public JSONTask(AsyncResponse delegate) {
        this.delegate = delegate
    }

    AsyncResponse delegate = null;

    @Override
    protected String doInBackground(String... params) {
        // [...]

And then in ResponseFromWebService

public void getData (String Name, AsyncResponse ar) {
    new JSONTask(ar).execute("censored-URL");
}

THANKS A LOT!!! This solved my problem! – Ale TheFe Oct 17 '16 at 14:49 — Ale TheFe, Oct 17 '16 at 14:49

score 0 · Answer 2 · answered Oct 17 '16 at 14:38

Should you not assign a reference to the delegate variable? You assign null to this variable at the beginning, and nothing else is assigned until it is called in the final statement, it's still null, so you get a NullPointerException.

AsyncResponse delegate = null;

Android JAVA - NullPointerException while using and Interface Field

2 Answers2