將JSONArray顯示爲自定義ListView異常錯誤Android AsyncTask線程

Question

我正在從服務器獲取JSONArray，然後將其插入到自定義ListView中。我的代碼執行，但我不知道爲什麼程序崩潰之前顯示任何東西。我把一些System.out.println(x)看到控制檯在崩潰之前得到了多少，但它貫穿了我放置的所有它們？

在此先感謝您的幫助！

public class MainActivity extends Activity 
{ 
    private ListView list_view; 
    private TrendingAdapter ta = null; 

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

     //Create a ListView, defined in the activity_main.xml file 
     this.list_view = (ListView) findViewById(R.id.list_view); 
     System.out.println("1"); 

     //Display trending results 
     try 
     { 
      get_trending(); 
      System.out.println("16"); 
     } 
     catch (Exception e) 
     { 
      e.printStackTrace(); 
     }  
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) 
    { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    //Async class... 
    private class AsyncOp extends AsyncTask<String, Void, String> 
    { 
     @Override 
     protected String doInBackground(String... strings) 
     { 
      System.out.println("5"); 
      JSONObject response_json = new JSONObject(); 
      JSONObject request_json = new JSONObject(); 

      //Take passed String[0] and turn into JSON 
      try 
      { 
       request_json = new JSONObject(strings[0]); 
      } 
      catch (Exception e) 
      { 
       e.printStackTrace(); 
      } 

      //Send the JSON to the server and grab response 
      try 
      { 
       System.out.println("6"); 
       HttpClient client = new DefaultHttpClient(); 
       String request_url = "http://SOME_IP/api/api.php?package=" + Uri.encode(request_json.toString(), null); 
       HttpGet get_request = new HttpGet(request_url); 
       HttpResponse get_response = client.execute(get_request); 
       HttpEntity get_entity = get_response.getEntity(); 
       System.out.println("7"); 

       //Ensure a response before doing anything foolish 
       if (get_entity != null) 
       { 
        System.out.println("8"); 
        String response = EntityUtils.toString(get_entity); 
        response_json = new JSONObject(response); 
        return response_json.toString(); 
       }    
      } 
      catch (Exception e) 
      { 
       e.printStackTrace(); 
      } 

      //If we're here, the above response didn't work for some reason... 
      return "crazy shit happened"; 
     } 

     //What to do after the request is done... 
     @Override 
     protected void onPostExecute(String result) 
     { 
      System.out.println("9"); 
      //Call the resppnse parser function... 
      try 
      { 
       server_response(result); 
       System.out.println("14"); 
      } 
      catch (Exception e) 
      { 
       e.printStackTrace(); 
      } 
     }  
    } 

    private class TrendingAdapter extends ArrayAdapter<TrendingTopic> 
    { 
     private Context context; 
     private int resource_id; 
     private ArrayList<TrendingTopic> trending_list = new ArrayList<TrendingTopic>(); 

     public TrendingAdapter(Context context, int resource_id, ArrayList<TrendingTopic> trending_list) 
     { 
      super(context, resource_id, trending_list); 
      this.resource_id = resource_id; 
      this.context = context; 
      this.trending_list = trending_list; 
     } 

     @Override 
     public View getView(int position, View convertView, ViewGroup parent) 
     { 
      ViewHolder holder = new ViewHolder(convertView); 
      holder = (ViewHolder) convertView.getTag(); 
      holder.populateFrom(trending_list.get(position)); 

      return (convertView); 
     }  
    } 

    private class ViewHolder 
    { 
     public TextView topic_rank = null; 
     public TextView topic_title = null; 
     public TextView num_comments = null; 

     public ViewHolder(View row) 
     { 
      this.topic_rank = (TextView) row.findViewById(R.id.topic_rank); 
      this.topic_title = (TextView) row.findViewById(R.id.topic_title); 
      this.num_comments = (TextView) row.findViewById(R.id.num_comments); 
     } 

     public void populateFrom(TrendingTopic topic) 
     { 
      this.topic_rank.setText(topic.rank); 
      this.topic_title.setText(topic.body.substring(0, Math.min(topic.body.length(), 30))); 
      this.num_comments.setText(topic.comments); 
     } 
    } 

    public void get_trending() throws JSONException 
    { 
     System.out.println("2"); 
     JSONObject type = new JSONObject(); 
     JSONObject request = new JSONObject(); 

     type.put("type", "trending"); 
     request.put("key", "SOME_KEY"); 
     request.put("request", "info"); 
     request.put("info", type); 

     System.out.println("3"); 
     AsyncOp operation = new AsyncOp(); 
     System.out.println("4"); 
     operation.execute(new String[] {request.toString()}); 
     System.out.println("15"); 
    } 

    public void server_response(String json_string) throws JSONException 
    { 
     System.out.println("10"); 
     //Get JSON info into ArrayList 
     JSONObject json = new JSONObject(json_string); 
     JSONArray trending = json.getJSONArray("trending"); 
     ArrayList<TrendingTopic> trending_list = new ArrayList<TrendingTopic>(); 

     int x = 0; 
     int end_of_list = trending.length(); 
     TrendingTopic topic = new TrendingTopic(); 
     while (x < end_of_list) 
     { 
      topic = new TrendingTopic(trending.getJSONObject(x)); 
      trending_list.add(topic); 
      x++; 
     }  

     System.out.println("11"); 
     //Display response into main ListView 
     this.ta = new TrendingAdapter(this, android.R.layout.simple_list_item_1, trending_list); 
     System.out.println("12"); 
     this.list_view.setAdapter(ta); 
     System.out.println("13"); 
    } 
} 

而這就是在logcat的

05-14 03:11:16.410: I/System.out(1706): 1 
05-14 03:11:16.410: I/System.out(1706): 2 
05-14 03:11:16.410: I/System.out(1706): 3 
05-14 03:11:16.410: I/System.out(1706): 4 
05-14 03:11:16.420: I/System.out(1706): 15 
05-14 03:11:16.420: I/System.out(1706): 16 
05-14 03:11:16.430: I/System.out(1706): 5 
05-14 03:11:16.430: I/System.out(1706): 6 
05-14 03:11:16.561: D/gralloc_goldfish(1706): Emulator without GPU emulation detected. 
05-14 03:11:17.771: I/System.out(1706): 7 
05-14 03:11:17.771: I/System.out(1706): 8 
05-14 03:11:17.840: D/dalvikvm(1706): GC_CONCURRENT freed 844K, 24% free 3177K/4164K, paused 5ms+9ms, total 36ms 
05-14 03:11:18.320: I/System.out(1706): 9 
05-14 03:11:18.320: I/System.out(1706): 10 
05-14 03:11:18.470: D/dalvikvm(1706): GC_CONCURRENT freed 370K, 22% free 3278K/4164K, paused 12ms+21ms, total 101ms 
05-14 03:11:18.491: I/System.out(1706): 11 
05-14 03:11:18.500: I/System.out(1706): 12 
05-14 03:11:18.500: I/System.out(1706): 13 
05-14 03:11:18.500: I/System.out(1706): 14 
05-14 03:11:18.510: D/AndroidRuntime(1706): Shutting down VM 
05-14 03:11:18.510: W/dalvikvm(1706): threadid=1: thread exiting with uncaught exception (group=0x40a71930) 
05-14 03:11:18.542: E/AndroidRuntime(1706): FATAL EXCEPTION: main 
05-14 03:11:18.542: E/AndroidRuntime(1706): java.lang.NullPointerException 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at com.example.actionbartutorial.MainActivity$ViewHolder.<init>(MainActivity.java:160) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at com.example.actionbartutorial.MainActivity$TrendingAdapter.getView(MainActivity.java:144) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.AbsListView.obtainView(AbsListView.java:2159) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.ListView.measureHeightOfChildren(ListView.java:1246) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.ListView.onMeasure(ListView.java:1158) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.View.measure(View.java:15518) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.RelativeLayout.measureChild(RelativeLayout.java:666) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:477) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.View.measure(View.java:15518) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.View.measure(View.java:15518) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.LinearLayout.measureVertical(LinearLayout.java:847) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:588) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.View.measure(View.java:15518) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2176) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.View.measure(View.java:15518) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1874) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1089) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1265) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.Choreographer.doCallbacks(Choreographer.java:562) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.Choreographer.doFrame(Choreographer.java:532) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.os.Handler.handleCallback(Handler.java:725) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.os.Handler.dispatchMessage(Handler.java:92) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.os.Looper.loop(Looper.java:137) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at android.app.ActivityThread.main(ActivityThread.java:5041) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at java.lang.reflect.Method.invokeNative(Native Method) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at java.lang.reflect.Method.invoke(Method.java:511) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
05-14 03:11:18.542: E/AndroidRuntime(1706):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

你還沒有佔到事實convertView只是一個有效的對象，一旦觀點開始得到回收利用生產。最初，當爲列表創建第一個視圖時，convertView爲空，並且您的ViewHolder構造函數不會非常優雅地處理空參數。您的ViewHolder圖案有點不對。當convertView爲空時，您必須膨脹並返回要使用的視圖的新實例，並且僅在該實例中創建新的持有者。你應該有更多的東西一樣：

ViewHolder holder; 
if (convertView == null) { 
    convertView = getLayoutInflater().inflate(resource_id, parent, false); 
    holder = new ViewHolder(convertView); 
    convertView.setTag(holder); 
} else { 
    holder = (ViewHolder) convertView.getTag(); 
} 

holder.populateFrom(trending_list.get(position)); 

return convertView; 

Answer 2

錯誤是使用android.R.layout.simple_list_item_1，而不是利用我所作的自定義XML列。雖然，我懷疑它會走得更遠沒有@Devunwired響應的幫助，所以我選擇了他的答案...