java随机验证码的生成

import java.awt.Color;
import java.awt.Font;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Panel;
import java.util.Random;
public class Test{
	public static void main(String []aa){
		final Frame frame = new Frame("验证码");
		final Panel panel = new MyPanel();
		frame.add(panel);
		frame.setSize(200,100);
		frame.setLocationRelativeTo(null);
		frame.setVisible(true);
		
	}
}
class MyPanel extends Panel{
	public void paint(Graphics g){
		int width = 160;
		int height = 40;
		g.setColor(Color.LIGHT_GRAY);
		g.fillRect(0, 0, width, height);
		g.setColor(Color.BLACK);
		g.drawRect(0, 0, width-1, height-1);
		
		Random r = new Random();
		for(int i=0;i<100;i++){
			int x=r.nextInt(width)-2;
			int y=r.nextInt(height)-2;
			g.drawOval(x, y, 2, 2);
		}
		g.setFont(new Font("黑体",Font.BOLD,30));
		g.setColor(Color.BLUE);
		
		char [] chars = ("0123456789abcdefghijklmnopqrstuvwxxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
		+"HIJKLMNOQRSTUVWXYZ").toCharArray();
		
		StringBuilder sb = new StringBuilder();
		for(int i=0; i<4; i++){
			int pos = r.nextInt(chars.length);
			char c = chars[pos];
			sb.append(c+" ");
		}
		g.drawString(sb.toString(), 30, 30);
	}
}