Java图片显示不了

代码如下：
import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.image.*;
import java.awt.event.*;
import java.io.*;
public class AppPanel extends JPanel {
//variables and components==========================
private JPanel imageBox;
private JPanel toolPane;
private JLabel counterLabel;
private int counter;
private boolean showWholeImage;
private JButton selectButton;
private JButton breakButton;
private JButton viewImageButton;
private Image[] images;
private Image wholeImage;
private JFileChooser chooser;
//constructor=============================
public AppPanel(){
//initial variables
chooser = new JFileChooser();
images=new BufferedImage[24];
wholeImage=null;
showWholeImage = true;
counter=0;
setLayout(new BorderLayout());
//initial components
imageBox = new JPanel();
imageBox.setBackground(Color.WHITE);
toolPane = new JPanel();
counterLabel = new JLabel("操作次数："+counter);
viewImageButton = new JButton("显示原图");
selectButton = new JButton("选择图片");
selectButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event){
if(JFileChooser.APPROVE_OPTION==chooser.showOpenDialog(null)){
try{
wholeImage = ImageIO.read(chooser.getSelectedFile());
showWholeImage=true;
repaint();
}
catch(Exception e){
JOptionPane.showMessageDialog(null, "打开文件失败");
}
}
}
});
breakButton = new JButton("打乱图片");
toolPane.add(counterLabel);
toolPane.add(selectButton);
toolPane.add(breakButton);
toolPane.add(viewImageButton);
add(toolPane,BorderLayout.SOUTH);
add(imageBox);
}
//paintComponents
public void paintComponent(Graphics g){
super.paintComponent(g);
Graphics g2d = imageBox.getGraphics();
if(showWholeImage && wholeImage!=null){
g2d.drawImage(wholeImage,0,0,imageBox);
}
else{
int x =0,y=0;
for(int i=0;i<24;i++){
g2d.drawImage(images[i],x, y, null);
}
}
}
}
难道不是每次选择图片后，调用repaint直接将图片绘制到imageBox上吗，为什么总是显示不了？