问题描述

java里repaint在循环里的问题

package test05;

//import java.awt.Color;
//import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.*;
import javax.swing.*;
//import javax.swing.JFrame;
//import javax.swing.JPanel;

public class Show extends JFrame {
MyPanel mp = null;

public Show() {
    mp = new MyPanel();
    this.add(mp);
    this.addKeyListener(mp);
    //
    this.setSize(900, 600);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setVisible(true);
}

public static void main(String[] args) {
    Show show = new Show();

}

}

class MyPanel extends JPanel implements KeyListener {

int x = 200;
int y = 200;

public MyPanel() {

}

public void paint(Graphics g) {
    super.paint(g);
    g.setColor(Color.BLACK);
    g.fillOval(200, 200, 10, 10);
    g.setColor(Color.red);
    g.fillOval(x, y, 5, 5);
    System.out.println("aaa");
}

@Override
public void keyTyped(KeyEvent e) {
    // TODO Auto-generated method stub

}

@Override
public void keyPressed(KeyEvent e) {
    System.out.println(this.x);
    if (e.getKeyCode() == KeyEvent.VK_J) {

        while (!(x >= 900 || x <= 0 || y >= 600 || y <= 0)) {
            this.repaint();
            System.out.println("b");
            this.x -= 5;
            System.out.println(x);
            try {
                Thread.sleep(100);
            } catch (InterruptedException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            this.repaint();
        }
        System.out.println("cccccc");

    }

}

@Override
public void keyReleased(KeyEvent e) {
    // TODO Auto-generated method stub

}

}

就是这个循环的问题
if (e.getKeyCode() == KeyEvent.VK_J) {
while (!(x >= 900 || x <= 0 || y >= 600 || y <= 0)) {
this.repaint();
this.x -= 5;
this.repaint();
}
}

repaint在出while循环后才执行，怎么回事

解决方案

根据javadoc， “If this component is a lightweight component, this method causes a call to this component's paint method as soon as possible. Otherwise, this method causes a call to this component's update method as soon as possible.” repaint不能保证调用立刻刷新。JPanel实际上是调用update，merge你的请求之后进行刷新。如果你的确要求立刻刷新，你可以调用paintImmediately