Asynctask和传递给它的ArrayList给我错误

Question

我有一个简单的程序，从Google Images API获取JSON数据。它将该JSON中的所有URL存储在ArrayList中。我创建了一个AsyncTask来处理这个问题。当我运行程序时，我收到以下错误：

java.lang.RuntimeException: An error occured while executing doInBackground() 
      at android.os.AsyncTask$3.done(AsyncTask.java:299) 
      at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273) 
      at java.util.concurrent.FutureTask.setException(FutureTask.java:124) 
      at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307) 
      at java.util.concurrent.FutureTask.run(FutureTask.java:137) 
      at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) 
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076) 
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569) 
      at java.lang.Thread.run(Thread.java:856) 
    Caused by: java.lang.NullPointerException 
      at com.example.bab.googleimagesapi.MainActivity$myTask.doInBackground(MainActivity.java:84) 
      at com.example.bab.googleimagesapi.MainActivity$myTask.doInBackground(MainActivity.java:58) 
      at android.os.AsyncTask$2.call(AsyncTask.java:287) 
      at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305) 
            at java.util.concurrent.FutureTask.run(FutureTask.java:137) 
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) 
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076) 
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569) 
            at java.lang.Thread.run(Thread.java:856) 

它说，有一个NullPointerException在线路58和84 这是我的代码的AsyncTask，我评论过线58和84，所以你可以找到他们。

private class myTask extends AsyncTask<Void, Void, ArrayList<String> > { **//LINE 58** 

     @Override 
     protected ArrayList<String> doInBackground(Void... params) { 
      String imageUrl = "x"; 
      String query = editText.getText().toString();   //get users search query 
      query = query.replaceAll(" ", "%20"); 
      try { 
       URL url = new URL("https://ajax.googleapis.com/ajax/services/search/imagesv=1.0&q=" + query + "&userip=INSERT-USER-IP"); 
       URLConnection connection = url.openConnection(); 
       connection.addRequestProperty("referer", "www.github.com"); 
       String line; 
       StringBuilder builder = new StringBuilder(); 

       BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream())); 
       while ((line = reader.readLine()) != null) { 
        builder.append(line); 
       } 

       try { 
        JSONObject jsonObject = new JSONObject(builder.toString()); 
        JSONObject sub = jsonObject.getJSONObject("responseData"); 
        JSONArray results = sub.getJSONArray("results"); 
        for(int x = 0; x < results.length(); x++) { 
         JSONObject jsonImage = results.getJSONObject(x); 
         imageUrl = jsonImage.getString("url"); 
         urlList.add(imageUrl);  **//LINE 84** 
        } 
        return urlList; 
       } catch (JSONException e) { 
        e.printStackTrace(); 
       } 

      } catch (MalformedURLException e) { 
       e.printStackTrace(); 
       // textResult = e.toString(); 
      } catch (IOException e) { 
       e.printStackTrace(); 
       //textResult = e.toString(); 
      } 
      return null; 
     } 


     protected void onPostExecute(ArrayList<String> list) { 

      for(int x = 0; x < list.size(); x++){ 
       textView.append(list.get(x)); 
      } 
     } 
    } 

我是新来的一般使用AsyncTask和Android，所以我感谢任何帮助！

Answer 1

你需要使用它之前初始化urlList：

urlList = new ArrayList<>(); 

Answer 2

需要初始化你urlList里面你myTask

urlList = new ArrayList<>(); 

或者其声明为final。

在附注上，我不建议您在Android上使用AsyncTask。阅读更多关于它在这里： http://simonvt.net/2014/04/17/asynctask-is-bad-and-you-should-feel-bad/