大家帮忙看下以下代码  
   单击按钮后,字符能放大或缩小,
   为什么就是画布不能刷新...?  
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/*
*编写一个Applet程序显示一行字符串，Applet包含两个按钮“放大”和“缩小”
*当用户单击放大时，显示的字符串字体放大一号，单击缩小时，显示的字符串字体缩小一号
*/

import java.awt.*;
import javax.swing.*;
import java.applet.*;
import java.awt.event.*;

public class Bian extends JApplet
{
    int zihao = 10;
    public void init()
    {
        
        setLayout(new BorderLayout());         //设置布局
        JButton b1 = new JButton("增大字体");    //声明组件 
        JButton b2 = new JButton("减小字体");
        JPanel p =new JPanel();              //声明面板
        p.setLayout(new FlowLayout());
        p.add(b1);        //添加组件                    
        p.add(b2);
        add(p,BorderLayout.SOUTH);
        setBounds(300,300,800,200);
        setVisible(true);
        

        b1.addActionListener(new ActionListener()    //b1事件处理 
           {
                 public void actionPerformed(ActionEvent e)
                 {
                      zihao = zihao + 2;
                      repaint();
                 }
           }
                            );
        b2.addActionListener(new ActionListener()    //b2事件处理
           {
                public void actionPerformed(ActionEvent e)
                {
                     zihao = zihao - 2;
                     repaint();
                }
           }
                            );
    }
  
    public void paint(Graphics g)
    {
        Font f = new Font("DialogInput",Font.BOLD,zihao);
        g.setFont(f);
        g.drawString("点击按钮改变大小",175,150);
    }
}