返回空列表導致NullPointerException

Question

我試圖製作一個簡單的源XML閱讀器，並且我從Android Developers站點跟着Parsing XML Data。

java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 

的List在ParseXML.java類實現的，我不知道這是錯的：
我轉換List到Array時得到一個錯誤。
我在下面留下我的代碼，謝謝大家！

NewsFeedFragment.java

public void downloadWebpage(String link) { 

    if (checkInternetConnection()) { 
     // CALL AND GET LIST 
     new DownloadFile(this).execute(link); 
    } 
    else { 
     Toast.makeText(getActivity(), "Disconnected!", Toast.LENGTH_SHORT).show(); 
    } 
} 

public boolean checkInternetConnection() { 
    ConnectivityManager connectivityManager = 
      (ConnectivityManager) getContext().getSystemService(Context.CONNECTIVITY_SERVICE); 
    NetworkInfo activeNetwork = connectivityManager.getActiveNetworkInfo(); 

    return activeNetwork != null && activeNetwork.isConnectedOrConnecting(); 
} 

@Override 
public void onTaskCompleted(List<PostData> data) { 
    PostData[] array = data.toArray(new PostData[data.size()]); 
    PostItemAdapter adapter = new PostItemAdapter(getActivity(), R.layout.postitem, array); 
    ListView listView = (ListView) getActivity().findViewById(R.id.news_feed_listView); 
    listView.setAdapter(adapter); 
} 

DownloadFile.java

private OnTaskCompleted listener; 

public DownloadFile(OnTaskCompleted listener) { 
    this.listener = listener; 
} 

@Override 
protected List doInBackground(String... urls) { 
    try { 
     return loadXmlFromNetwork(urls[0]); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } catch (XmlPullParserException e) { 
     e.printStackTrace(); 
    } 

    return null; 
} 

@Override 
protected void onPostExecute(List result) { 
    listener.onTaskCompleted(result); 
} 


private List loadXmlFromNetwork(String link) throws XmlPullParserException, IOException { 
    List<PostData> postData = null; 
    InputStream inputStream = null; 
    ParseXML parseXML = new ParseXML(); 
    //String stringHtml = null; 

    try { 
     inputStream = downloadUrl(link); 
     //stringHtml = convertToString(inputStream);    
     postData = parseXML.parse(inputStream); 
    } finally { 
     if (inputStream != null) { 
      inputStream.close(); 
     } 
    } 

    return postData; 
} 

private InputStream downloadUrl(String urlString) throws IOException { 
    URL url = new URL(urlString); 
    HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
    conn.setReadTimeout(10000); 
    conn.setConnectTimeout(15000); 
    conn.setRequestMethod("GET"); 
    conn.setDoInput(true); 

    conn.connect(); 
    return conn.getInputStream(); 
} 

ParseXML.java

public class ParseXML extends DefaultHandler { 

private static final String ns = null; 

// Requires InputStream and create a XmlPullParser with a IS input 
public List parse(InputStream in) throws XmlPullParserException, IOException { 
    try { 
     XmlPullParser parser = Xml.newPullParser(); 
     parser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false); 
     parser.setInput(in, null); 
     parser.nextTag(); 
     return readFeed(parser); 
    } finally { 
     in.close(); 
    } 
} 

private List readFeed(XmlPullParser parser) throws XmlPullParserException, IOException { 
    List entries = new ArrayList(); 

    parser.require(XmlPullParser.START_TAG, ns, "channel"); 
    while (parser.next() != XmlPullParser.END_TAG) { 
     if (parser.getEventType() != XmlPullParser.START_TAG) { 
      continue; 
     } 
     String name = parser.getName(); 
     if (name.equals("item")) { 
      entries.add(readItem(parser)); 
     } 
     else{ 
      skip(parser); 
     } 
    } 
    return entries; 
} 

private PostData readItem(XmlPullParser parser) throws XmlPullParserException, IOException { 
    parser.require(XmlPullParser.START_TAG, ns, "entry"); 
    String title = null; 
    String description = null; 
    String link = null; 

    while (parser.next() != XmlPullParser.END_TAG) { 
     if (parser.getEventType() != XmlPullParser.START_TAG) { 
      continue; 
     } 
     String name = parser.getName(); 
     if (name.equals("title")) { 
      title = readTitle(parser); 
     } 
     else if (name.equals("link")) { 
      link = readLink(parser); 
     } 
     else if (name.equals("description")) { 
      description = readDescription(parser); 
     } 
    } 
    return new PostData(title, link, description); 
} 

private String readTitle(XmlPullParser parser) throws XmlPullParserException, IOException { 
    parser.require(XmlPullParser.START_TAG, ns, "title"); 
    String title = readText(parser); 
    parser.require(XmlPullParser.END_TAG, ns, "title"); 
    return title; 
} 

private String readLink(XmlPullParser parser) throws XmlPullParserException, IOException { 
    parser.require(XmlPullParser.START_TAG, ns, "link"); 
    String link = readText(parser); 
    parser.require(XmlPullParser.END_TAG, ns, "link"); 
    return link; 
} 

private String readDescription(XmlPullParser parser) throws XmlPullParserException, IOException { 
    parser.require(XmlPullParser.START_TAG, ns, "description"); 
    String description = readText(parser); 
    parser.require(XmlPullParser.END_TAG, ns, "description"); 
    return description; 
} 

private String readText(XmlPullParser parser) throws XmlPullParserException, IOException { 
    String result = ""; 
    if (parser.next() == XmlPullParser.TEXT) { 
     result = parser.getText(); 
     parser.nextTag(); 
    } 
    return result; 
} 

private void skip(XmlPullParser parser) throws XmlPullParserException, IOException { 
    if (parser.getEventType() != XmlPullParser.START_TAG) { 
     throw new IllegalStateException(); 
    } 
    int depth = 1; 
    while (depth != 0) { 
     switch (parser.next()) { 
      case XmlPullParser.END_TAG: 
       depth--; 
       break; 
      case XmlPullParser.START_TAG: 
       depth++; 
       break; 
     } 
    } 
} 
} 

Answer 1

@Override 
protected List doInBackground(String... urls) { 
    try { 
     return loadXmlFromNetwork(urls[0]); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } catch (XmlPullParserException e) { 
     e.printStackTrace(); 
    } 

    return null; 
} 

您正在返回null例外。改爲返回Collections.emptyList()。

也將此默認值更改爲空列表。

private List loadXmlFromNetwork(String link) throws XmlPullParserException, IOException { 
    List<PostData> postData = Collections.<PostData>emptyList();