Swing绘制一个4位的验证码

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 = ("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ").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(),20,30);}