RSS源Null指针异常

Question

基本上我创建了一个单独的RSS，它可以工作。我使用了DOMParsing和一个用于列表视图外观的自定义适配器。但现在我决定把这个列表放在一个选项卡下，表示rss提要的特定类别（即突发新闻、教育新消息等）。这是我的密码。

public class FragmentA extends ListFragment {
ListView listView;
ArrayList<RowItem> items;
@Override
public void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    return inflater.inflate(R.layout.fragment_a, container, false);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onActivityCreated(savedInstanceState);
                    listView = (ListView)getView().findViewById(R.id.list_item);
                    //listView.setOnItemClickListener(this);
                    // create the RowItem list - used for storing one news feed
                    items = new ArrayList<RowItem>();
                    // start the background task to get the news feed
                    //new DownloadXMLTask(items).execute("http://feeds.bbci.co.uk/news/rss.xml");
    new DownloadXMLTask(items).execute("http://www.nasa.gov/rss/dyn/breaking_news.rss");
}

private class RowItem {
    String title = null;
    String description = null;
    String url = null;
    public RowItem(String title,String url,String description) {
        this.title = title;
        this.description = description;
        this.url = url;
    }


}
private class DownloadXMLTask extends AsyncTask<String, Void, String> {
    ArrayList<RowItem> items = null;
    public DownloadXMLTask(ArrayList<RowItem> items) {
        this.items = items;
    }
    // local helper method
    private String getNodeValue(Element entry, String tag) {
        Element tagElement = (Element) entry.getElementsByTagName(tag).item(0);
        return tagElement.getFirstChild().getNodeValue();
    }
    private String downloadAndParseXML(String url) {
        try {
            InputStream in = new java.net.URL(url).openStream();
            // build the XML parser
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            DocumentBuilder db = dbf.newDocumentBuilder();
            // parse and get XML elements
            Document dom = db.parse(in);
            Element documentElement = dom.getDocumentElement();
            // we want all XML children called 'item'
            NodeList nodes = documentElement.getElementsByTagName("item");
            // for each 'item' do the following
            for (int i = 0; i < nodes.getLength(); i++) {
                Element entry = (Element) nodes.item(i);
                // get the nodes from 'item' that you need
                String title = getNodeValue(entry, "title");
                //String description = getNodeValue(entry, "description");
                String link = getNodeValue(entry, "link");
                String description = getNodeValue(entry, "description");
                // add them to your list
                items.add(new RowItem(title,link,description));
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
    protected String doInBackground(String... urls) {
        if (urls.length <= 0)
            return null;
        return downloadAndParseXML(urls[0]);
    }
    protected void onPostExecute(String result) {
        updateList(items);
    }
}
public void updateList(ArrayList<RowItem> items) {
    CustomArrayAdapter adapter = new CustomArrayAdapter(getActivity(),
            R.layout.row, items);
    listView.setAdapter(adapter);
}
// class used to have custom view for the list item
private class CustomArrayAdapter extends ArrayAdapter<RowItem> {
    Context context;
    List<RowItem> items;
    private class ViewHolder {
        public TextView title;
        public TextView description;
    }
    public CustomArrayAdapter(Context context, int resource,
            List<RowItem> items) {
        super(context, resource, items);
        this.context = context;
        this.items = items;
    }
    @Override
    public View getView(int position, View rowView, ViewGroup parent) {
        // reuse the rowView if possible - for efficiency and less memory consumption
        if (rowView == null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            rowView = inflater.inflate(R.layout.row, null);
            // configure view holder
            ViewHolder viewHolder = new ViewHolder();
            viewHolder.title = (TextView) rowView.findViewById(R.id.title);
            viewHolder.description = (TextView) rowView.findViewById(R.id.desc);
            rowView.setTag(viewHolder);
        }
        // set the view
        ViewHolder holder = (ViewHolder) rowView.getTag();
        RowItem item = items.get(position);
        holder.title.setText(item.title);
        holder.description.setText(item.description);

        return rowView;
    }
 }
}   

我在updatelist方法中得到了一个NullPointer异常，该方法用于更新检索到的数据的列表（title、ulr、link）。。显然他们不是，但我现在想不出该怎么解决。

    09-21 16:07:41.310: E/AndroidRuntime(3678): FATAL EXCEPTION: main
    09-21 16:07:41.310: E/AndroidRuntime(3678): java.lang.NullPointerException
    09-21 16:07:41.310: E/AndroidRuntime(3678): at
    com.example.tabstesting.FragmentA.updateList(FragmentA.java:159)
    09-21 16:07:41.310: E/AndroidRuntime(3678):at    
    com.example.tabstesting.FragmentA$DownloadXMLTask.onPostExecute(FragmentA.java:152)
    09-21 16:07:41.310: E/AndroidRuntime(3678):     at  
    com.example.tabstesting.FragmentA$DownloadXMLTask.onPostExecute(FragmentA.java:1)
    09-21 16:07:41.310: E/AndroidRuntime(3678):     at  
    android.os.AsyncTask.finish(AsyncTask.java:631)
    09-21 16:07:41.310: E/AndroidRuntime(3678):     at  
    android.os.AsyncTask.access$600(AsyncTask.java:177)

提前谢谢你，很抱歉代码太长。

Answer 1

好的，所以我想我知道你的问题是什么。

在fragment_a.xml中，您将ListView声明为：

<ListView 
    android:id="@android:id/list"
    // ... rest of properties
</ListView>

第一个问题是你在说：

android:id="@android:id/list" 

这将使用来自android包的ID"列表"，而不是来自您自己的资源。这就是为什么在代码中无法识别它的原因。此外，您尝试访问id为list_item的元素，该元素甚至不存在于您的布局中，因此您得到了NullPointerException，因为listView是null。

因此，在您获得ListView的代码中，您应该更改：

listView = (ListView)getView().findViewById(R.id.list_item);

收件人：

listView = (ListView)getView().findViewById(android.R.id.list);