java双缓冲中，为什么我调用repaint时，它直接调用paint()而不是update(),怎么改进？

Question

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(); } }
为什么我调用repaint时，它直接调用paint()而不是update(),怎么改进？

Answer 1

不是JFrame 是Frame！另外，重绘最好放在另外一个线程
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();
}
}    }

Answer 2

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。