java - 我的 RSS 阅读器给出了 java.lang.NullPointerException

Question

我正在尝试创建 RSS 提要阅读器应用程序(按照本教程 http://www.androidbegin.com/tutorial/android-xml-parsing-dom-tutorial/ 进行操作)

并出现以下错误(不幸的是，我的应用程序在显示数据时停止工作)

@Override
protected void onPostExecute(Void args) {

    for (int temp = 0; temp < nodelist.getLength(); temp++) {
        Node nNode = nodelist.item(temp);
        if (nNode.getNodeType() == Node.ELEMENT_NODE) {
            Element eElement = (Element) nNode;
            // Set the texts into TextViews from item nodes
            // Get the title
            textview.setText(textview.getText() + "Title : "
                    + getNode("title", eElement) + "\n" + "\n");
            // Get the description
            textview.setText(textview.getText() + "Description : "
                    + getNode("description", eElement) + "\n" + "\n");
            // Get the link
            //textview.setText(textview.getText() + "Link : "
            //      + getNode("source url", eElement) + "\n" + "\n");
            // Get the date
            textview.setText(textview.getText() + "Date : "
                    + getNode("pubDate", eElement) + "\n" + "\n" + "\n"
                    + "\n");
        }
    }
    // Close progressbar
    pDialog.dismiss();
}

我必须做什么才能使其发挥作用？

这是我的完整代码

  package com.healthyhub.nadeesha.healthyhubz;

import java.net.URL;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.InputSource;
import android.app.Activity;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

public class MainActivity extends Activity {
    // Declare variables
    TextView textview;
    NodeList nodelist;
    ProgressDialog pDialog;
    // Insert image URL
    String URL = "http://www.livescience.com/home/feed/health.xml";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from activity_main.xml
        setContentView(R.layout.activity_main);
        // Locate a TextView in your activity_main.xml layout
        textview = (TextView) findViewById(R.id.text);
        // Execute DownloadXML AsyncTask
        new DownloadXML().execute(URL);
    }

    // DownloadXML AsyncTask
    private class DownloadXML extends AsyncTask<String, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Create a progressbar
            pDialog = new ProgressDialog(MainActivity.this);
            // Set progressbar title
            pDialog.setTitle("Android App");
            // Set progressbar message
            pDialog.setMessage("Loading...");
            pDialog.setIndeterminate(false);
            // Show progressbar
            pDialog.show();
        }

        @Override
        protected Void doInBackground(String... Url) {
            try {
                URL url = new URL(Url[0]);
                DocumentBuilderFactory dbf = DocumentBuilderFactory
                        .newInstance();
                DocumentBuilder db = dbf.newDocumentBuilder();
                // Download the XML file
                Document doc = db.parse(new InputSource(url.openStream()));
                doc.getDocumentElement().normalize();
                // Locate the Tag Name
                nodelist = doc.getElementsByTagName("item");

            } catch (Exception e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return null;

        }
    @Override
    protected void onPostExecute(Void args) {

        for (int temp = 0; temp < nodelist.getLength(); temp++) {
            Node nNode = nodelist.item(temp);
            if (nNode.getNodeType() == Node.ELEMENT_NODE) {
                Element eElement = (Element) nNode;
                // Set the texts into TextViews from item nodes
                // Get the title
                textview.setText(textview.getText() + "Title : "
                        + getNode("title", eElement) + "\n" + "\n");
                // Get the description
                textview.setText(textview.getText() + "Description : "
                        + getNode("description", eElement) + "\n" + "\n");
                // Get the link
                //textview.setText(textview.getText() + "Link : "
                //      + getNode("source url", eElement) + "\n" + "\n");
                // Get the date
                textview.setText(textview.getText() + "Date : "
                        + getNode("pubDate", eElement) + "\n" + "\n" + "\n"
                        + "\n");
            }
        }
        // Close progressbar
        pDialog.dismiss();
    }
}

// getNode function
private static String getNode(String sTag, Element eElement) {
    NodeList nlList = eElement.getElementsByTagName(sTag).item(0)
            .getChildNodes();
    Node nValue = (Node) nlList.item(0);
    return nValue.getNodeValue();
}
}

这就是我的 logcat 的样子

Answer 1

您是否已通过真实手机或模拟器逐步使用调试器？通常，您应该添加一个检查以确保节点列表不为空，但除此之外，在代码执行时查看实际值会非常有帮助!