Android AsynTask PostExecute

Question

I have AsyncTask.class in here i parse json and return List, and in postExecute i need add this List to my Adapter and notify to change, but LogCat give Error, how i can fix this?

My Adapter have public state, why i cant add to him results?

 FATAL EXCEPTION: main
 java.lang.NullPointerException
    at com.gazetaimage.LoadTask.onPostExecute(LoadTask.java:84)
    at com.gazetaimage.LoadTask.onPostExecute(LoadTask.java:1)
    at android.os.AsyncTask.finish(AsyncTask.java:631)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5191)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
    at dalvik.system.NativeStart.main(Native Method)

LoadTask.java

public class LoadTask extends AsyncTask<Void, Void, List<String>> {

    MainActivity main = new MainActivity();

    private int page = 1;

    public int addPage(int i){
        return page = i;
    }

    @Override
    protected List<String> doInBackground(Void... params) {

        InputStream ips = null; 
        JSONObject jsonObj = null;
        String json = "";       
        try {
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpResponse response = httpClient.execute(new HttpPost("http://iapp.gazeta.uz/fotolenta?page="+page)); 
            ips = response.getEntity().getContent();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            BufferedReader bufff = new BufferedReader(new InputStreamReader(ips, "UTF-8"));
            StringBuilder sb = new StringBuilder();
            String line = null;

            while ((line = bufff.readLine()) != null){
                sb.append(line + "\n");                 
            }
            ips.close();
            json = sb.toString();

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

        try {
            jsonObj = new JSONObject(json);
        } catch (JSONException e) {
            e.printStackTrace();
        }

        if(jsonObj != null){

        JSONArray array = jsonObj.optJSONArray("img_list"); 
            for (int i = 0; i < array.length(); i++) {
                main.thumbs.add(array.optJSONObject(i).optString("thumbnail"));
            }
        }
        else{
            Log.d("Log", "Nullll");
        }

        return main.thumbs;
    }

    @Override
    protected void onPostExecute(List<String> result) {
        if (result != null){
            main.adapter.add(result);// HERE I HAVE Error 84
        }

    }
}

Answer 1

You cannot create an instance of the activity inside the task, and expect it will be the same activity running..

Instead, You can create interface, pass it to AsyncTask (in constructor), and then call method in onPostExecute

For example:

Your interface:

public interface OnTaskCompleted{
    void onTaskCompleted(values);
}

Your Activity:

public YourActivity implements OnTaskCompleted{
    //your Activity
    YourTask  task = new YourTask(this); // here is the initalization code for your asyncTask
}

And your AsyncTask:

public YourTask extends AsyncTask<Object,Object,Object>{ //change Object to required type
    private OnTaskCompleted listener;

    public YourTask(OnTaskCompleted listener){
        this.listener=listener;
    }

    //required methods

    protected void onPostExecute(Object o){
        //your stuff
        listener.onTaskCompleted(values);
    }
}

taken from https://stackoverflow.com/a/9963705/1434631