JAVA NullPointerExceptionエラー（学校プロジェクト）

Question

import android.app.ProgressDialog; 
import android.content.Context; 
import android.os.AsyncTask; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.util.Log; 

import org.w3c.dom.Document; 
import org.w3c.dom.Element; 
import org.w3c.dom.Node; 
import org.w3c.dom.NodeList; 

import java.io.InputStream; 
import java.lang.reflect.Array; 
import java.net.HttpURLConnection; 
import java.net.MalformedURLException; 
import java.net.URL; 
import java.util.ArrayList; 

import javax.xml.parsers.DocumentBuilder; 
import javax.xml.parsers.DocumentBuilderFactory; 

/** 
* Created by Tim on 17-8-2016. 
*/ 
public class ReadRss extends AsyncTask<Void, Void, Void> { 

    Context context; 

    //The URL where the app needs to get the information from 
    String address = "http://www.sciencemag.org/rss/news_current.xml"; 
    ProgressDialog progressDialog; 
    ArrayList<FeedItem>feedItems; 
    RecyclerView recyclerView; 
    URL url; 

    //The loading (Loading feed...) message 
    public ReadRss(Context context, RecyclerView recyclerView){ 
     this.recyclerView = recyclerView; 
     this.context = context; 
     progressDialog = new ProgressDialog(context); 
     progressDialog.setMessage("Loading feed..."); 
    } 
    @Override 

    //Shows the user that the page is loading (Loading feed...) 
    protected void onPreExecute() { 
     progressDialog.show(); 
     super.onPreExecute(); 
    } 

    @Override 
    protected void onPostExecute(Void aVoid) { 
     super.onPostExecute(aVoid); 
     progressDialog.dismiss(); 
     MyAdapter adapter = new MyAdapter(context, feedItems); 
     recyclerView.setLayoutManager(new LinearLayoutManager(context)); 
     recyclerView.addItemDecoration(new VerticalSpace(50)); 
     recyclerView.setAdapter(adapter); 
    } 

    @Override 
    protected Void doInBackground(Void... voids) { 
     processXml(Getdata()); 

     return null; 
    } 

    //!!!!!This part ensures all the data is collected from the feed and it is made ready 
    //so it can be used by recycler view 
    private void processXml(Document data) { 
     if (data!=null) { 
      //Add all feedItems to an ArrayList 
      feedItems = new ArrayList<>(); 
      //Object that will store root elements 
      Element root = data.getDocumentElement(); 
      //Node object that will show channel 
      Node channel = root.getChildNodes().item(1); 
      //Store all child of channel element 
      NodeList items = channel.getChildNodes(); 
      //Loop trough each child element of items 
      for (int i=0; i<items.getLength(); i++){ 
       Node currentchild = items.item(i); 
       //Check that node is item node by using an if statement 
       if (currentchild.getNodeName().equalsIgnoreCase("item")){ 
        FeedItem item = new FeedItem(); 
        //Store childs in NodeList of current item 
        NodeList itemchilds = currentchild.getChildNodes(); 
        //For loop to loop through all childs of item tag 
        for (int j=0; j<itemchilds.getLength(); j++){ 
         //Store current node 
         Node current = itemchilds.item(j); 
         //Check node is title, description, pubDate, link or thumbnail node by if else conditions 
         if (current.getNodeName().equalsIgnoreCase("title")){ 
          //If node is title, description, pubDate, link or thumbnail then set textcontent of our current node 
          item.setTitle(current.getTextContent()); 
         }else if (current.getNodeName().equalsIgnoreCase("description")){ 
          item.setDescription(current.getTextContent()); 
         }else if (current.getNodeName().equalsIgnoreCase("pubDate")){ 
          item.setPubDate(current.getTextContent()); 
         }else if (current.getNodeName().equalsIgnoreCase("link")){ 
          item.setLink(current.getTextContent()); 
         }else if (current.getNodeName().equalsIgnoreCase("media:thumbnail")){ 
          //This will return a thumbnail url 
          String url = current.getAttributes().item(0).getTextContent(); 
          item.setThumbnailUrl(url); 
         } 
        } 
        //feedItems to be added to the ArrayList<FeedItem> 
        feedItems.add(item); 

       } 
      } 
     } 
    } 

    public Document Getdata() { 
     try { 
      url = new URL(address); 
      //Open connection by using Http url connection object 
      HttpURLConnection connection = (HttpURLConnection) url.openConnection(); 
      //Sets request method as GET 
      connection.setRequestMethod("GET"); 
      InputStream inputStream = connection.getInputStream(); 
      DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance(); 
      DocumentBuilder builder = builderFactory.newDocumentBuilder(); 
      Document xmlDoc = builder.parse(inputStream); 
      return xmlDoc; 

     } catch (Exception e) { 
      e.printStackTrace(); 
      return null; 
     } 
    } 
} 

我々はこのエラーを取得している（学校の）最終的なプロジェクトにRSSリーダーのために、このコードを追加：

08-21 16:53:27.583 2734-2734/com.example.codru.stendensocial E/AndroidRuntime: FATAL EXCEPTION: main 
                       Process: com.example.codru.stendensocial, PID: 2734 
                       java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference 
                        at com.example.codru.stendensocial.ReadRss.onPostExecute(ReadRss.java:61) 
                        at com.example.codru.stendensocial.ReadRss.onPostExecute(ReadRss.java:28) 
                        at android.os.AsyncTask.finish(AsyncTask.java:651) 
                        at android.os.AsyncTask.-wrap1(AsyncTask.java) 
                        at android.os.AsyncTask$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) 

我々は修正する方法がわかりませんこの。 setLayoutManagerの前にfinalを追加する方法については読んでいますが、これを行う方法を理解できませんでした。

ご協力いただければ幸いです。

〜ティム

Answer 1

recyclerViewの値がコンストラクタに割り当てられ、あなたがnull値を送信しているものと思われます。

このオブジェクトをどのように呼び出すことができますか？