java - 调试 NullPointerException

Question

试图从 lyndas.com 讲座中找出调试我的代码中发生的空点异常，这些讲座是为 localhost 上激活的链接中的 xml 内容编写的。以下是我的 View 类和 AppMain 类的代码

View .java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.xml.parsers.*;
import org.w3c.dom.*;
import java.net.*;
import java.util.ArrayList;

public class View extends JFrame implements ActionListener {

    private static final long serialVersionUID = 12345L;
    public ArrayList<String> titles = new ArrayList<String>();
    public ArrayList<String> descriptions = new ArrayList<String>();
    public ArrayList<String> links = new ArrayList<String>();
    public ArrayList<Integer> prices = new ArrayList<Integer>();
    public ArrayList<Number> lengths = new ArrayList<Number>();
    public JList list;
    public JComboBox combo;
    public JTextArea textArea = new JTextArea();
    public JLabel priceLabel = new JLabel();
    public JLabel lengthLabel = new JLabel();
    public JScrollPane textScroller;
    public View()
    {
        super("Backpack CA");
        setLayout(new FlowLayout());
        **loadData("http://localhost:8080/using_drivers/data.jsp");**           
    }

    public void actionPerformed(ActionEvent e) {            
    }

    public void loadData(String xmlURL)
    {
        try {
            DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();                
            DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
            Document doc = dBuilder.parse(new URL(xmlURL).openStream());
            doc.getDocumentElement().normalize();

            NodeList nodes = doc.getElementsByTagName("tour");              
            for (int temp = 0; temp < nodes.getLength(); temp++) {                  
               Node n = nodes.item(temp);
               if (n.getNodeType() == Node.ELEMENT_NODE) {       
                  Element e = (Element) n;    
                  titles.add(getTagValue("tourTitle", e));
                  descriptions.add(getTagValue("description", e));
                  links.add(getTagValue("link", e).replaceAll("\\s+", ""));
                  prices.add(Integer.parseInt(getTagValue("price", e)));
                  lengths.add(Integer.parseInt(getTagValue("length", e)));
                  **System.out.println(getTagValue("tourTittle", e));**
               }
            }               

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

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

}

AppMain.java

import javax.swing.*;

public class AppMain {

    public static void main(String[] args) {
        **View testView = new View();**
        testView.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        testView.setSize(480,320);
        testView.setVisible(true);
    }    
}

Answer 1

eElement.getElementsByTagName(sTag) 或 eElement.getElementsByTagName(sTag).item(0) 返回空值。尝试拆分该指令以找出:

NodeList nl = eElement.getElementsByTagName(sTag);
Node n = nl.item(0);
nlList = n.getChildNodes();