Question

我有3个可滑动的空视图。第三视图是“ViewAll”。第一个视图工作正常，但是当我向第二个视图滑动时，它会显示此错误。

我试图在AsyncTask中显示我使用JSON加载的数据，但是存在此NullPointerException错误而且我不知道如何解决它......

11-12 10:00:23.046: E/InputEventReceiver(12007): Exception dispatching input event.
11-12 10:00:23.046: E/MessageQueue-JNI(12007): Exception in MessageQueue callback: handleReceiveCallback
11-12 10:00:23.056: E/MessageQueue-JNI(12007): java.lang.NullPointerException
11-12 10:00:23.056: E/MessageQueue-JNI(12007):  at com.example.rumorz.ViewAll$PostTask.onPreExecute(ViewAll.java:52)
11-12 10:00:23.056: E/MessageQueue-JNI(12007):  at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)

这是我的代码：

public class ViewAll extends Fragment {

    //URL to get JSON Array
    private static String url = ""; //URL to my site with JSON-code

    //JSON Node Names 
    private static final String TAG_USER = "user";
    private static final String TAG_message= "message";

    TextView tvuser ;
    TextView tvmessage ;

    JSONArray user = null;


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    new PostTask().execute();
    View rootView = inflater.inflate(R.layout.viewall, container, false);

    return rootView; 
}


private class PostTask extends AsyncTask<String, String, JSONObject> {
       @Override
       protected void onPreExecute() {
          super.onPreExecute();

             tvuser = (TextView)getView().findViewById(R.id.tvUser);
         tvmessage = (TextView)getView().findViewById(R.id.tvmessage);
       }

       @Override
       protected JSONObject doInBackground(String... args) {
           JSONParser jParser = new JSONParser();

           // Getting JSON from URL
           JSONObject json = jParser.getJSONFromUrl(url);
           return json;
       }




       @Override
       protected void onPostExecute(JSONObject json) {
        try {
               user = json.getJSONArray(TAG_USER);
                JSONObject c = user.getJSONObject(0);

                // Storing  JSON item in a Variable
                 String users = c.getString(TAG_USER);
                 String message = c.getString(TAG_message);


                tvuser.setText(users);
                tvmessage.setText(message);

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

}

Answer 1

onPreExecute（），在执行任务后立即在UI线程上调用。初始化

tvuser = (TextView)rootView.findViewById(R.id.tvUser);
    tvmessage = (TextView)rootview.findViewById(R.id.tvmessage);

onCreateView

中的

Answer 2

在onCreateView初始化它。使用膨胀的视图对象初始化textview。或使用getView中的onActivityCreated来初始化textview。

由于textview被声明为类成员，因此您可以在onPostEecute中使用它来将文本设置为相同。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.viewall, container, false);
    tvuser = (TextView)rootView.findViewById(R.id.tvUser);
    tvmessage = (TextView)rootview.findViewById(R.id.tvmessage);

    new PostTask().execute();
    return rootView; 
}

在Fragment中的AsyncTask中加载JSON时遇到问题

2 个答案: