android - RSS 提要空指针异常

Question

基本上我创建了一个单独的 RSS，它可以工作。我为 ListView 外观使用了 DOMParsing 和自定义适配器。但现在我决定将该列表放在一个选项卡下，该选项卡表示 rss 提要的特定类别(即突发新闻、教育新闻等)。这是我的代码。

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 方法中得到一个 NullPointer 异常，该方法用于更新检索到的数据列表(标题、ulr、链接)。显然它们不是，但我现在想不出如何修复它。

    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)

预先感谢您，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 android 包中的 ID“list”，而不是您自己的资源。这就是为什么它在您的代码中无法识别的原因。此外，您尝试访问 ID 为 list_item 的元素，它甚至不存在于您的布局中，因此您得到一个 NullPointerException 因为 listView 是 空。

因此，在获取 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);