书上的程序、编译的时候报错，是什么原因？解决思路

书上的程序、编译的时候报错，是什么原因？

Java code

/* * Core Java  P407  **///   本程序是《Java核心技术 卷一：基础知识》（第八版）（中文版 机械工业出版社出版）第407页的源程序//   我在编译的时候出了错误，但是却找不出来，望高人指点package first;import java.awt.*;import java.awt.event.*;import java.beans.*;import java.io.File;import javax.swing.*;import javax.swing.filechooser.*;import javax.swing.filechooser.FileFilter;/*  这里书上还导入了另一个包：    import java.util.*;    但是我在eclipse上编译的时候编译器提示说这个包从来没有用到过*/public class FileChooserTest {    public static void main(String[] args)    {        EventQueue.invokeLater(new Runnable()        {            public void run()            {                ImageViewerFrame frame = new ImageViewerFrame();             //    这里报错                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);                frame.setTitle("FileChooserTest");                frame.setVisible(true);            }        });    }}/* *  A frame that has a menu for loading an image and a display area for the loaded image * */class ImageViewerFrame extends DrawFrame          // 这里的DrawFrame是我自己定义的一个类，主要是设置Frame                                                  // 的宽和高{    /**     *      */    private static final long serialVersionUID = 1L;    private JLabel label ;    private JFileChooser chooser ;        public ImageViewerFrame()    {        // set a menu bar        JMenuBar mBar = new JMenuBar();        setJMenuBar(mBar);                JMenu menu = new JMenu("File");        mBar.add(menu);                JMenuItem openItem = new JMenuItem("Open");        menu.add(openItem);        openItem.addActionListener(new FileOpenListener());                JMenuItem exitItem = new JMenuItem("Exit");        menu.add(exitItem);        exitItem.addActionListener(new ActionListener()        {            public void actionPerformed(ActionEvent event)            {                System.exit(0);            }        });                // use a label to display the image         label = new JLabel();        add(label);                // set up file chooser        chooser = new JFileChooser();                // accept all image files ending with .jpg .jpeg .gif        /*          final ExtensionFileFilter filter = new ExtensionFileFilter();          filter.addExtension("jpg");          filter.addExtension("jpeg");          filter.addExtension("gif");          filter.setDescription("Image files");        */        FileNameExtensionFilter filter = new FileNameExtensionFilter("Image files", "jpg", "jpeg", "gif");        chooser.setFileFilter(filter);        chooser.setAccessory(new ImagePreviewer(chooser));      //这里报错        chooser.setFileView(new FileIconView(filter, new ImageIcon("palette.gif")));    }        private class FileOpenListener implements ActionListener    {        public void actionPerformed(ActionEvent event)        {            chooser.setCurrentDirectory(new File("."));                        // shoe file chooser dialog            int result = chooser.showOpenDialog(ImageViewerFrame.this);                        // if image file accepted , set it as icon of the label            if(result == JFileChooser.APPROVE_OPTION)            {                String name = chooser.getSelectedFile().getPath();                label.setIcon(new ImageIcon(name));            }        }    }}/* * A file view that display an icon for all files that match a file filter  * */class FileIconView extends FileView{    private FileFilter filter ;    private Icon icon ;        // constructs a FileIconView    public FileIconView(FileFilter aFilter, Icon anIcon)    {        filter = aFilter;        icon = anIcon;    }        public Icon getIcon(File f)    {        if(!f.isDirectory() && filter.accept(f))            return icon;        else             return null;    }}/* *  A file chooser accessory that previews images * */class ImagePreviewer extends JLabel{    /**     *      */    private static final long serialVersionUID = 1L;    // constructs an ImagePreviewer    public ImagePreviewer(JFileChooser chooser)    {        setPreferredSize(new Dimension(100, 100));         setBorder(BorderFactory.createEtchedBorder());                chooser.addPropertyChangeListener(new PropertyChangeListener()        {            public void propertyChange(PropertyChangeEvent event)            {                if(event.getPropertyName() == JFileChooser.SELECTED_FILE_CHANGED_PROPERTY);                {                    // the user has selected a new file                    File f = (File) event.getNewValue();      //这里报错                    if(f == null)                    {                        setIcon(null);                        return;                    }                                        // read the image into an icon                    ImageIcon icon = new ImageIcon(f.getPath());                                        // if the icon is too large to fit ,scale it                    if(icon.getIconWidth() &gt; getWidth())                        icon = new ImageIcon(icon.getImage().getScaledInstance(getWidth(), -1, Image.SCALE_DEFAULT));                    setIcon(icon);                }            }        });    }}

编译的时候报错是这样的

Java code

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: first.ImagePreviewer cannot be cast to java.io.File    at first.ImagePreviewer$1.propertyChange(FileChooserTest.java:153)    at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:339)    at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:276)    at java.awt.Component.firePropertyChange(Component.java:8132)    at javax.swing.JFileChooser.setAccessory(JFileChooser.java:1235)    at first.ImageViewerFrame.&lt;init&gt;(FileChooserTest.java:83)    at first.FileChooserTest$1.run(FileChooserTest.java:21)    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)    at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

[解决办法]
first.ImagePreviewer 不能转换成 java.io.File
[解决办法]
刚才试了下代码，确实
event.getNewValue()返回一个object，而且这个是chooser的事件，为啥会运行时变成ImagePreviewer 的对象了
mark下
[解决办法]
File f = (File) event.getNewValue();

event.getNewValue()得到的是一个ImagePreviewer，强制转换为File，肯定会有问题啊
[解决办法]
首先，你这不叫编译错误，叫运行时错误
错误的原因在于

Java code

public void propertyChange(PropertyChangeEvent event)            {                if(event.getPropertyName() == JFileChooser.SELECTED_FILE_CHANGED_PROPERTY);                {                    // the user has selected a new file                    File f = (File) event.getNewValue();      //这里报错
[解决办法]
be careful!!
[解决办法]
因为很多地方都会导致propertyChange事件发生，不一定每次发生都是因为选择了文件，比如你调用
chooser.setFileFilter(filter);
chooser.setAccessory(new ImagePreviewer(chooser));
chooser.setFileView(new FileIconView(filter, new ImageIcon("palette.gif")));
这些都会导致chooser的属性发生改变而发生事件，所以你修改一下你的事件里的代码，判断一下属性改变的是否是文件，是再继续强行转换，就可以了，比如

Java code    public ImagePreviewer(JFileChooser chooser)    {        setPreferredSize(new Dimension(100, 100));         setBorder(BorderFactory.createEtchedBorder());                chooser.addPropertyChangeListener(new PropertyChangeListener()        {            public void propertyChange(PropertyChangeEvent event)            {                if(event.getPropertyName() == JFileChooser.SELECTED_FILE_CHANGED_PROPERTY);                {                    // the user has selected a new file                    if (event.getNewValue() instanceof File) { //追加一个判断                        File f = (File) event.getNewValue();      //这里报错                            if(f == null)                        {                            setIcon(null);                            return;                         }                                            // read the image into an icon                        ImageIcon icon = new ImageIcon(f.getPath());                                            // if the icon is too large to fit ,scale it                        if(icon.getIconWidth() &gt; getWidth())                            icon = new ImageIcon(icon.getImage().getScaledInstance(getWidth(), -1, Image.SCALE_DEFAULT));                        setIcon(icon);                    }                }            }        });    } 
 
[解决办法]
7L好眼力
if(event.getPropertyName() == JFileChooser.SELECTED_FILE_CHANGED_PROPERTY);
这个本身就是对选择文件造成propertyg改变发生事件作的判断，只是因为后面多了个; 所以if相当于后面接的是空代码，然后后面的{}内的代买相当无条件执行。

书上的程序、编译的时候报错是什么原

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