android - 同一代码在不同的活动中提供不同的结果，同时通过AsyncTask提取JSON数据

Question

这是我从本地主机提取JSON数据的简单代码片段

public class JSONtest extends ActionBarActivity {

private ProgressDialog dialog;

String message3;
String message_short;
String[] items;
String[] short_items;
TextView tv;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_jsontest);

    String localhost = "http://10.0.2.2/example/json/notification.php";
    tv = (TextView) findViewById(R.id.textView1);
    new ReadJSONFeedTask().execute(localhost);
}

private class ReadJSONFeedTask extends AsyncTask<String, Void, String>
{
    protected void onPreExecute()
    {
        super.onPreExecute();
        dialog = new ProgressDialog(JSONtest.this);
        dialog.setMessage("Downloading Notifications. Please wait . . .");
        dialog.setIndeterminate(false);
        dialog.setCancelable(false);
        dialog.show();
    }
    protected String doInBackground(String...urls)
    {
        return readJSONFeed(urls[0]);
    }
    protected void onPostExecute(String result)
    {
        dialog.dismiss();
        try
        {
            JSONArray jsonArray = new JSONArray(result);
            items = new String[jsonArray.length()];
            short_items = new String[jsonArray.length()];
            for(int i = 0; i < jsonArray.length(); i++)
            {
                JSONObject jobj = jsonArray.getJSONObject(i);
                message3 = "PART 1 : " + jobj.getString("subject") + "\n\n" + 
                        "PART 2 : " + jobj.getString("date");
                items[i] = message3;
                message_short = message_short + "\n\n" + message3;
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }

        tv.setText(message_short);
    }
}

public String readJSONFeed(String URL)
{
    StringBuilder sb = new StringBuilder();
    HttpClient client = new DefaultHttpClient();
    HttpGet hg = new HttpGet(URL);
    try
    {
        HttpResponse response = client.execute(hg);
        StatusLine statusLine = response.getStatusLine();
        int statusCode = statusLine.getStatusCode();
        if(statusCode == 200)
        {
            HttpEntity en = response.getEntity();
            InputStream content = en.getContent();
            BufferedReader reader = new BufferedReader(new InputStreamReader(content));
            String line;
            while((line = reader.readLine()) != null)
            {
                sb.append(line);
            }
        }
        else
        {
            Log.e("JSON", "Failed to download File");
        }
    }
    catch(ClientProtocolException e)
    {
        e.printStackTrace();
    }
    catch(IOException e)
    {
        e.printStackTrace();
    }
    return sb.toString();
}
}

我在同一个应用中的两个不同的活动中使用了完全相同的代码。唯一的区别是类的名称。当一个活动完美无缺时，另一个活动会发生错误，因为没有任何反应，TextView会保持黑色。 Logcat for the Activity的日志不起作用是：

11-04 11:14:40.679: W/EGL_emulation(2925): eglSurfaceAttrib not implemented
11-04 11:14:40.749: W/System.err(2925): java.lang.NullPointerException
11-04 11:14:40.749: W/System.err(2925):     at com.onclaveinc.ioexception.HomeScreen$ReadJSONFeedTask.onPostExecute(HomeScreen.java:129)
11-04 11:14:40.749: W/System.err(2925):     at com.onclaveinc.ioexception.HomeScreen$ReadJSONFeedTask.onPostExecute(HomeScreen.java:1)
11-04 11:14:40.749: W/System.err(2925):     at android.os.AsyncTask.finish(AsyncTask.java:632)
11-04 11:14:40.749: W/System.err(2925):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
11-04 11:14:40.749: W/System.err(2925):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
11-04 11:14:40.749: W/System.err(2925):     at android.os.Handler.dispatchMessage(Handler.java:102)
11-04 11:14:40.749: W/System.err(2925):     at android.os.Looper.loop(Looper.java:136)
11-04 11:14:40.749: W/System.err(2925):     at android.app.ActivityThread.main(ActivityThread.java:5017)
11-04 11:14:40.749: W/System.err(2925):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 11:14:40.749: W/System.err(2925):     at java.lang.reflect.Method.invoke(Method.java:515)
11-04 11:14:40.749: W/System.err(2925):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
11-04 11:14:40.749: W/System.err(2925):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
11-04 11:14:40.769: W/System.err(2925):     at dalvik.system.NativeStart.main(Native Method)

有人可以帮帮我吗？我真的需要他们两个工作。