java.lang.NullPointerException：ヌルオブジェクトリファレンスでインタフェースメソッド 'int java.util.List.size（）'を呼び出そうとしています4

Question

JSONデータをListViewに表示しようとしていますが、私は、アプリケーションを実行しようとすると、次のエラーが発生します。

java.lang.NullPointerExceptionが：ここでヌルオブジェクト参照にインターフェース方法 を起動しようと 'INT java.util.List.size（）'

スタックトレースです。

05-14 22:06:30.169 14034-14034/com.example.jsonparsing_v2 E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: com.example.jsonparsing_v2, PID: 14034 
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 
    at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:337) 
    at android.widget.ListView.setAdapter(ListView.java:491) 
    at com.example.jsonparsing_v2.MainActivity$JSONTask.onPostExecute(MainActivity.java:131) 
    at com.example.jsonparsing_v2.MainActivity$JSONTask.onPostExecute(MainActivity.java:49) 
    at android.os.AsyncTask.finish(AsyncTask.java:651) 
    at android.os.AsyncTask.-wrap1(AsyncTask.java) 
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

マイコード。

package com.example.jsonparsing_v2; 

import android.content.Context; 
import android.os.AsyncTask; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ArrayAdapter; 
import android.widget.Button; 
import android.widget.ImageView; 
import android.widget.ListView; 
import android.widget.TextView; 

import com.example.jsonparsing_v2.models.ArticleModel; 

import org.json.JSONArray; 
import org.json.JSONException; 
import org.json.JSONObject; 

import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStream; 
import java.io.InputStreamReader; 
import java.net.HttpURLConnection; 
import java.net.MalformedURLException; 
import java.net.URL; 
import java.util.ArrayList; 
import java.util.List; 

public class MainActivity extends AppCompatActivity { 

    private ListView lvArticles; 

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

     lvArticles = (ListView)findViewById(R.id.lvArticles); 
    } 

    public class JSONTask extends AsyncTask<String,String,List<ArticleModel>>{ 
     @Override 
     protected List<ArticleModel> doInBackground(String... params) { 
      HttpURLConnection connection = null; 
      BufferedReader reader = null; 

      try { 
       URL url = new URL(params[0]); 
       connection =(HttpURLConnection) url.openConnection(); 
       connection.connect(); 

       InputStream stream = connection.getInputStream(); 
       reader = new BufferedReader(new InputStreamReader(stream)); 
       StringBuffer buffer = new StringBuffer(); 

       String line = ""; 
       while ((line = reader.readLine())!= null){ 
        buffer.append(line); 
       } 

       String finalJson = buffer.toString(); 

       JSONObject parentObject = new JSONObject(finalJson); 
       JSONArray parentArray = parentObject.getJSONArray(""); 

       List<ArticleModel> articleModelList = new ArrayList<>(); 

       for (int i=0;i<parentArray.length(); i++) { 
        JSONObject finalObject = parentArray.getJSONObject(i); 
        ArticleModel articleModel = new ArticleModel(); 
        articleModel.setId(finalObject.getString("id")); 
        articleModel.setSectionId(finalObject.getString("sectionId")); 
        articleModel.setWebPublicationDate(finalObject.getInt("webPublicationDate")); 
        articleModel.setWebTitle(finalObject.getString("webTitle")); 
        articleModel.setWebUrl(finalObject.getString("webUrl")); 
        articleModel.setApiUrl(finalObject.getString("apiUrl")); 
        articleModel.setSectionName(finalObject.getString("sectionName")); 
        articleModel.setHeadline(finalObject.getString("headline")); 
        articleModel.setThumbnail(finalObject.getString("thumbnail")); 

        // adding the final object in the list 
        articleModelList.add(articleModel); 
       } 
       return articleModelList; 
      } 
      catch(MalformedURLException e) { 
       e.printStackTrace(); 
      } 
      catch (IOException e){ 
       e.printStackTrace(); 
      } catch (JSONException e) { 
       e.printStackTrace(); 
      } finally{ 
       if (connection != null){ 
        connection.disconnect(); 
       } 
       try { 
        if (reader != null){ 
         reader.close(); 
        } 
       } 
       catch (IOException e){ 
        e.printStackTrace(); 
       } 
      } 
      return null; 
     } 

     @Override 
     protected void onPostExecute(List<ArticleModel> result) { 
      super.onPostExecute(result); 
      ArticleAdapter adapter = new ArticleAdapter(getApplicationContext(), R.layout.row, result); 
      lvArticles.setAdapter(adapter); 
     } 
    } 

    public class ArticleAdapter extends ArrayAdapter{ 
     private List<ArticleModel> articleModelList; 
     private int resource; 
     private LayoutInflater inflater; 

     public ArticleAdapter(Context context, int resource, List<ArticleModel> objects) { 
      super(context, resource, objects); 
      articleModelList = objects; 
      this.resource = resource; 
      inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE); 
     } 

     @Override 
     public View getView(int position, View convertView, ViewGroup parent) { 

      if(convertView == null){ 
       convertView = inflater.inflate(resource, null); 
      } 

      ImageView ivArticleIcon; 
      TextView tvDate; 
      TextView tvSummary; 

      ivArticleIcon = (ImageView) convertView.findViewById(R.id.ivIcon); 
      tvDate = (TextView) convertView.findViewById(R.id.tvDate); 
      tvSummary = (TextView) convertView.findViewById(R.id.tvSummary); 

      tvDate.setText(articleModelList.get(position).getWebPublicationDate()+":"); 
      tvSummary.setText(articleModelList.get(position).getHeadline()); 

      return convertView; 
     } 
    } 

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

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_refresh) { 
      new JSONTask().execute("http://public.ocito.com/m/test/guardian.html"); 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 
} 

Answer 1

あなたは `）

List<ArticleModel> articleModelList = new ArrayList<ArticleModel>(); 

が、私はそれはあなたが最も可能性が高い` doInBackground（中tryブロック内のエラーを取得しているあなた