用java设计一个简单画图程序遇到的问题，求指教！

Question

设计的一个画图程序，想在拖动鼠标的时候可以即时显示矩形的大小，所以在mouseDragged()中使用了repaint()方法，在mouseReleased()中使用了paint()方法，但是这样只能画一个，再画时，之前的就全部清屏了，有什么别的方法可以达到我的目的吗？代码如下import java.awt.*;
import java.awt.event.*;
 
public class MyCanvas extends Canvas implements MouseMotionListener,MouseListener,ActionListener
{
 static int a,b,c,d; 
static MyCanvas mc;
 static Button bu[]=new Button[4]; public static void main(String args[])
 {
 mc=new MyCanvas();
 mc.addMouseListener(new MyCanvas());
 mc.addMouseMotionListener(new MyCanvas()); Frame f=new Frame("Test17"); Panel p1=new Panel(); bu[0]=new Button("铅笔");
bu[1]=new Button("直线");
bu[2]=new Button("矩形");
bu[3]=new Button("圆形");
for(int i=0;i<bu.length;i++)
 {
 bu[i].setEnabled(true);
 bu[i].addActionListener(new MyCanvas()); 
p1.add(bu[i]);
 } f.add(p1,BorderLayout.WEST);
 f.add(mc,BorderLayout.CENTER);
 f.pack();
 f.setVisible(true);
 }
 
public MyCanvas()
 {
 setSize(800,600);
 } public void paint(Graphics g)
 {
 if(!bu[0].isEnabled())
 g.drawLine(c,d,c,d);
 else if(!bu[1].isEnabled())
 g.drawLine(a,b,c,d);
 else if(!bu[2].isEnabled())
 g.drawRect(a,b,+(c-a),+(d-b)); 
}
 
public void mousePressed(MouseEvent e)
 {
 a=e.getX();
 b=e.getY();
 } public void mouseEntered(MouseEvent e){} public void mouseClicked(MouseEvent e){}
 
public void mouseExited(MouseEvent e){} public void mouseReleased(MouseEvent e)
 { 
c=e.getX();
 d=e.getY();
 Graphics g=mc.getGraphics();
 mc.paint(g);
 }
 
public void mouseDragged(MouseEvent e)
 {
 c=e.getX();
 d=e.getY();
 Graphics g=mc.getGraphics();
 mc.repaint();
 } public void mouseMoved(MouseEvent e){} public void actionPerformed(ActionEvent e)
 {
 for(int i=0;i<bu.length;i++)
 {
 bu[i].setEnabled(true);
 } Button but=(Button)e.getSource();
 but.setEnabled(false);
 }
}

Answer 1

import java.awt.*;import java.awt.datatransfer.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.*;import javax.swing.*;
import javax.swing.filechooser.FileFilter;import com.sun.image.codec.jpeg.*;@SuppressWarnings("serial")
class ImagePanel extends JPanel
{
    private BufferedImage image;    public ImagePanel(BufferedImage image)
    {
        this.image = image;
        prefSize();
    }    public void setImage(BufferedImage image)
    {
        this.image = image;
        prefSize();
        repaint();
    }    /**
     * 设置面板大小
     */
    private void prefSize()
    {
        this.setSize(image.getWidth(), image.getHeight());
    }    @Override
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.drawImage(image, 0, 0, null);
    }
}/**
 * 文件类型
 */
class ImageFilter extends FileFilter
{    public boolean accept(File f)
    {
        if (f.isDirectory())
        {
            return true;
        }        String extension = getExtension(f);
        if (extension != null)
        {
            if (extension.equals("jpeg") || extension.equals("jpg")
                    || extension.equals("bmp"))
            {
                return true;
            } else
            {
                return false;
            }
        }        return false;
    }    public String getExtension(File f)
    {
        String ext = null;
        String s = f.getName();
        int i = s.lastIndexOf('.');        if (i > 0 && i < s.length() - 1)
        {
            ext = s.substring(i + 1).toLowerCase();
        }
        return ext;
    }    public String getDescription()
    {
        return "图片类型";
    }
}
 
public class CutScreen extends JFrame
{
    private int x, y, w, h;
    private BufferedImage bufImage;
    private ImagePanel imagePanel;
    private Graphics g;
    private static Dimension screenDims;
    private GraphicsDevice gd = GraphicsEnvironment
            .getLocalGraphicsEnvironment().getDefaultScreenDevice();
    private JPopupMenu popup = new JPopupMenu();
    private JMenuItem[] popupm =
    { new JMenuItem("Copy"), new JMenuItem("Save"), new JMenuItem("Reset"),
            new JMenuItem("Exit") };
    private JFileChooser fileChooser = new JFileChooser();    public CutScreen()
    {        this.setLayout(new BorderLayout());
        this.add(buildControls());        // 设置标题
        this.setTitle("截图程序");
        // 去掉窗口的装饰
        this.setUndecorated(true);
        // 采用指定的窗口装饰风格
        this.getRootPane().setWindowDecorationStyle(JRootPane.NONE);
        // 设置全屏
        gd.setFullScreenWindow(this);
        // 窗体始终靠前
        this.setAlwaysOnTop(true);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    }    private JComponent buildControls()
    {
        // 获取屏幕图像
        bufImage = getScreenImage();
        imagePanel = new ImagePanel(bufImage);        // 注册事件监听
        imagePanel.addMouseListener(new MouseAdapter()
        {            // 当鼠标按下时
            public void mousePressed(MouseEvent e)
            {
                // 获取按下时的鼠标坐标
                x = e.getX();
                y = e.getY();
            }            public void mouseReleased(MouseEvent e)
            {
                if (SwingUtilities.isLeftMouseButton(e))
                {
                    // 设置截取图像的宽和高
                    w = Math.abs(x - e.getX());
                    h = Math.abs(y - e.getY());
                    if (w > 0 && h > 0)
                    {
                        g.drawLine(x, y, e.getX(), y);
                        g.drawLine(x, y, x, e.getY());
                        g.drawLine(x, e.getY(), e.getX(), e.getY());
                        g.drawLine(e.getX(), y, e.getX(), e.getY());
                        g.dispose();
                        popup.show(e.getComponent(), e.getX(), e.getY());
                    }
                }
            }            public void mouseClicked(MouseEvent e){}
            public void mouseEntered(MouseEvent e){}
            public void mouseExited(MouseEvent e){}
        });
        imagePanel.addMouseMotionListener(new MouseMotionListener()
        {            // 当鼠标拖动时
            public void mouseDragged(MouseEvent e)
            {
                if (SwingUtilities.isLeftMouseButton(e))
                {
                    g = imagePanel.getGraphics();
                    g.setColor(Color.blue);
                    imagePanel.setImage(bufImage);                    g.drawLine(x, y, e.getX(), y);
                    g.drawLine(x, y, x, e.getY());
                    g.drawLine(x, e.getY(), e.getX(), e.getY());
                    g.drawLine(e.getX(), y, e.getX(), e.getY());
                }
            }            public void mouseMoved(MouseEvent e)
            {
            }        });        popupm[0].addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            { // 复制到剪贴板
                setClipboardImage(getAreaImage());
                System.exit(0);
            }
        });
        popupm[1].addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                save();
                System.exit(0);
            }
        });
        popupm[2].addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                imagePanel.setImage(bufImage);
            }
        });
        popupm[3].addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                System.exit(0);
            }
        });        this.addKeyListener(new KeyListener()
        {
            // 按Esc键退出
            public void keyPressed(KeyEvent e)
            {
                if (e.getKeyCode() == KeyEvent.VK_ESCAPE)
                {
                    System.exit(0);
                }
            }            public void keyReleased(KeyEvent e){}
            public void keyTyped(KeyEvent e){}
        });        for (int i = 0; i < popupm.length; i++)
        {
            popup.add(popupm[i]);
        }        fileChooser.addChoosableFileFilter(new ImageFilter());
        fileChooser.removeChoosableFileFilter(fileChooser
                .getAcceptAllFileFilter());        return imagePanel;
    }    /**
     * 获取屏幕图像
     *
     * @return:返回BufferedImage对象
     */
    private BufferedImage getScreenImage()
    {
        Robot robot;
        try
        {
            robot = new Robot();
        } catch (Exception e)
        {
            throw new RuntimeException("unable to construct Robot");
        }
        // 获取屏幕尺寸
        screenDims = Toolkit.getDefaultToolkit().getScreenSize();        BufferedImage screen = robot.createScreenCapture(new Rectangle(0, 0,
                screenDims.width, screenDims.height));        return screen;
    }    /**
     * 获取截图区域
     *
     * @return:返回BufferedImage对象
     */
    private BufferedImage getAreaImage()
    {
        return bufImage.getSubimage(x, y, w, h);
    }    private void save()
    {
        if (fileChooser.showSaveDialog(null) == JFileChooser.APPROVE_OPTION)
        {
            save(fileChooser.getSelectedFile());
        }
    }    /**
     * 保存文件
     *
     * @param file:文件路径
     */
    private void save(File file)
    {
        BufferedOutputStream imageOut;
        try
        {
            imageOut = new BufferedOutputStream(new FileOutputStream(file));
            JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(imageOut);
            encoder.encode(getAreaImage());
        } catch (FileNotFoundException e)
        {
            e.printStackTrace();
        } catch (ImageFormatException e)
        {
            e.printStackTrace();
        } catch (IOException e)
        {
            e.printStackTrace();
        }
    }    /**
     * 复制到剪贴板
     *
     * @param image:BufferedImage对象
     */
    private void setClipboardImage(final BufferedImage image)
    {
        Transferable trans = new Transferable()
        {
            public DataFlavor[] getTransferDataFlavors()
            {
                return new DataFlavor[]
                { DataFlavor.imageFlavor };
            }            public boolean isDataFlavorSupported(DataFlavor flavor)
            {
                return DataFlavor.imageFlavor.equals(flavor);
            }            public Object getTransferData(DataFlavor flavor)
                    throws UnsupportedFlavorException, IOException
            {
                if (isDataFlavorSupported(flavor))
                    return image;
                throw new UnsupportedFlavorException(flavor);
            }        };
        Toolkit.getDefaultToolkit().getSystemClipboard().setContents(trans,
                null);
    }
    public static void main(String[] args)
    {        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                new CutScreen();
            }
        });
    }
}研究研究这个

Answer 2

画图程序，首要的是能保存已画的东西，并在每次paint时按照先后顺序（下面的先画）重新repaint