java双缓冲中，为什么小弟我调用repaint时，它直接调用paint()而不是update(),如何改进

import java.awt.Graphics;import java.awt.Image;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import javax.swing.ImageIcon;import javax.swing.JFrame;public class pic extends JFrame{     Image image=null;     Image offscreen;     int hight=400;     int width=400;        public pic(){        this.setSize(hight,width);        this.addWindowListener(new WindowAdapter() {            public void windowClosing(WindowEvent e) {           setVisible(false);                System.exit(0);            }        });        this.setResizable(false);        this.setVisible(true);                   }    public void paint(Graphics g){                    System.out.println("paint...");            }    public void update(Graphics g){        image=new ImageIcon("C:\\Users\\Administrator\\Desktop\\z.jpg").getImage();//路径改变        this.offscreen=this.createImage(hight,width);        Graphics goff=this.offscreen.getGraphics();        goff.drawImage(image,0,0,null);        System.out.println("update.....");        paint(goff);        g.drawImage(offscreen,0,0,null);    }        public static void main(String[] args) {        // TODO Auto-generated method stub        new pic().repaint();    }    }

import java.awt.Frame;import java.awt.Graphics;import java.awt.Image;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import javax.swing.ImageIcon;public class pic extends Frame implements Runnable{     Image image=null;     Image offscreen;     int hight=400;     int width=400;        public pic(){        this.setSize(hight,width);        this.addWindowListener(new WindowAdapter() {            public void windowClosing(WindowEvent e) {           setVisible(false);                System.exit(0);            }        });        this.setResizable(false);        this.setVisible(true);                   }    public void paint(Graphics g){        System.out.println("paint...");            }    public void update(Graphics g){        image=new ImageIcon("C:\\Users\\Administrator\\Desktop\\z.jpg").getImage();//路径改变        this.offscreen=this.createImage(hight,width);        Graphics goff=this.offscreen.getGraphics();        goff.drawImage(image,0,0,null);        System.out.println("update.....");        paint(goff);        g.drawImage(offscreen,0,0,null);    }        public static void main(String[] args) {        // TODO Auto-generated method stub       new Thread( new pic()).start();    }    @Override    public void run() {        while(true){            try {                Thread.sleep(50);            } catch (InterruptedException e) {                // TODO Auto-generated catch block                e.printStackTrace();            }            repaint();        }    }    }
------解决方案--------------------
JFrame的update方法只是调用 paint(g)。重写此方法，以防止不必要的调用清除背景。

JFrame的repaint方法是由类 java.awt.Component 继承的方法，如果此组件是轻量级组件，则repaint方法会尽快调用此组件的 paint 方法。否则此方法会尽快调用此组件的 update 方法。

双缓冲绘图一般都不直接在JFrame上绘制，一般是用JFrame绘制组件吧。

JDK API doc 上面有你的问题的答案。