import java.applet.*;
import java.awt.*;
imprt java.awt.image. * ;
import javax.swing.*;
import java.event.*;
public class Example7_6 extends Applet{
  Image myPicture;
  /*init()方法中，先定义一个Image对象，并赋予createImage()方法的返回值，接着创建Graphics对象并赋予其图形环境。最后，让Graphics对象调用drawImage()方法显示图像。
由于这里的Graphics对象offScreenGc是非屏幕对象是，小程序窗口不会有图像显示*/
  public void init(){
    myPicture = getImage(getCodeBase(), "myPic.JPG");
    Image offScreenImage = createImage(size().width, size().height);
    Graphics offScreenGc = offScreenImage.getGraphics();
    new BufferedDemo(myPicture);
  }
  /*drawImage()方法的第四个参数是实现ImageObserver接口，在init()方法中，调用drawImage()方法的参数是this，所以小程序要定义imageUpdate()方法*/
  public boolean imageUpdate(Image img, int infoFlg, int x, int y, int w, int h){
    if (infoFlg = ALLBITS){ // 表示图像已全部装入内存
      repaint();
      return false;// 防止线程再次调用imageUpdate()方法
    }
    else
      return true;
  }
}
/*程序的执行过程是，当小程序调用drawImage()方法时，drawImage()方法将创建一个调用 imageUpdate()方法的线程，在imageUpdate()方法中，测定图像是否已在部分调入内存。创建的线程不断调用imageUpdate()方法，直到该方法返回false为止。参数infoFlg使小程序能知道图像装入内存的情况。当infoFlg等于ALLBITS时，表示图像已全部装入内存。当该方法发现图像已全部装入内存后，置imageLoaded为真，并调用repaint()方法重画小程序窗口。方法返回false防止线程再次调用imageUpdate()方法。*/
class BufferedDemo extends JFrame{
  public BufferedDemo(Image img){
    this.getContentPane().add(new PicPanel(img));
    setTile("双缓技术演示");
    setSize(300, 300);
    setVisible(true);
  }
}
class PicPane extends JPanel implements MouseListener, MouseMotionListener{
  int x = 0, y = 0, dx = 0, cy = 0;
  BufferedImage bimg1, bimg2;
  boolean upstate = true;
  public picPanel(Image img){
    this.setBackground(Color.white);
    this.addMouseListener(this);
    this.addMouseMotionListener(this);
    bimg1 = new BufferedImage(img.getWidth(this), img.getHeight(this),
    BufferedImage.TYPE_INT_ARGB);
    bimg2 = new BufferedImage(img.getWidth(this), img.getHeight(this),
    BufferedImage.TYPE_INT_ARGB);
    Graphics2D g2D1 = bimg1.createGraphics();
    Graphics2D g2D2 = bimg2.createGraphics();
    g2D1.drawImage(img, 0, 0, this);
    g2D2.drawImage(img, 0, 0, this);
    g2D2.drawRect(1, 1, img.getWidth(this) - 3, img.getHeight(this) - 3);
  }
  public void paintComponent(Graphics g){
    super.painComponent(g);
    Graphics2D g2D = (Graphics2D)g;
    if (upState)
      g2D.drawImage(bimg1, x, y, this);
    else
      g2D.drawImage(bimg2.x, y, this);
  }
  public void mousePress(MouseEvent e){
    if (e.getX() >= x && e.getX() < x + bimg1.getWidth(this) && e.getY() >= y&& e.getY() < y + bimg1.getHeight(this)){
      upstate = false;
      setCursor(Cursor.getPredefinedCursor(Coursor.HAND_CURSOR));
      dx = e.getX() - x;
      dy = e.getY() - y;
      repain();
    }
  }
  public void mouseExited(MouseEvent e){}
  public void mouseClicked(MouseEvent e){}
  public void mouseEntered(MouseEvent e){}
  public void MouseReleased(MouseEvent e){
    this.setCursor(Cursor.getpredefinedCursor(Cursor.DEFAULT_CURSOR));
    upState = true;
    repaint();
  }
  public void mouseMoved(MouseEvent e){}
  public void mouseDragged(MouseEvent e){
    if (!upState){
      x = e.getX() - dx;
      y = e.getY() - dy;
      repaint();
    }
  }
}

  BufferedImage bimage = (BufferedImage)this.createImage(this.getWidth(),this.getHeight());

  BufferedImage(int width,int heigh, int imageType);

  Graphics2D g2d = bimge.createGraphics();