依照书上敲了个简单实现WebBrowser的代码，运行结果，网站总是在变化

package ex30;import java.awt.*;import java.awt.event.*;import javax.swing.*;import java.net.URL;import javax.swing.event.*;import java.io.*;public class WebBrowser extends JApplet{    // JEditor pane to view HTML files    private JEditorPane jep = new JEditorPane();        // Label for URL    private JLabel jlblURL = new JLabel("URL");        // Text field for entering URL    private JTextField jtfURL = new JTextField();        /** Initialize the applet */    public void init(){        // Create a panel jpURL to hold the label and text field        JPanel jpURL = new JPanel();        jpURL.setLayout(new BorderLayout());        jpURL.add(jlblURL, BorderLayout.WEST);        jpURL.add(jtfURL, BorderLayout.CENTER);                // Place jpURL and jspViewer in the applet        add(new JScrollPane(jep), BorderLayout.CENTER);        add(jpURL, BorderLayout.NORTH);                // Set jep noneditable        jep.setEditable(false);                // Register listener        jep.addHyperlinkListener(new HyperlinkListener(){            public void hyperlinkUpdate(HyperlinkEvent e){                try{                    jep.setPage(e.getURL());                }                catch(IOException ex){                    System.out.println(ex);                }            }        });        jtfURL.addActionListener(new ActionListener(){            public void actionPerformed(ActionEvent e){                try{                    // Get the URL from text field                    URL url = new URL(jtfURL.getText().trim());                                        // Display the HTML file                    jep.setPage(url);                }                catch(IOException ex){                    System.out.println(ex);                }            }        });    }}