无法在ActionListener中重绘

Question

我尝试使用Java创建日历，该日历在打开时显示当前月份，然后按下按钮时将显示上个月或下个月。似乎底层代码是正确的，因为我能够在命令行中看到月份发生变化，但框架中没有任何变化。我相信这是由于我的ActionListener的实现方式，但我已经尝试了其他一些方法（有些可以看出被注释掉），似乎没有什么可以做到的。

这是我的完整代码，任何帮助都将不胜感激。

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import javax.swing.JButton;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class CalendarHack extends JPanel {
  SimpleDateFormat month = new SimpleDateFormat("MMMM");
  SimpleDateFormat year = new SimpleDateFormat("yyyy");
  SimpleDateFormat day = new SimpleDateFormat("d");
  Date date = new Date();

  private static JButton back = new JButton("Back");
  private static JButton next = new JButton("Next");

  public CalendarHack()
  {

  }

  public void paintComponent(Graphics g) {
    ((Graphics2D) g).setRenderingHint(RenderingHints.KEY_ANTIALIASING,RenderingHints.VALUE_ANTIALIAS_ON);
    g.setColor(Color.BLACK);
    g.fillRect(0, 0, getWidth(), getHeight());
    g.setColor(Color.WHITE);
    g.drawString(month.format(date), 110, 36);
    g.drawString(year.format(date), 310, 36);

    Calendar today = Calendar.getInstance();
    today.setTime(date);
    Calendar cal = Calendar.getInstance();
    cal.setTime(date);
    cal.set(Calendar.DATE, 1);
    cal.add(Calendar.DATE, -cal.get(Calendar.DAY_OF_WEEK) + 1);
    String days = "Sun           Mon            Tue           Wed          Thur          Fri             Sat";
    g.drawString(days,50, 90);
    g.setFont(new Font("SansSerif", Font.PLAIN, 40));

    for (int week = 0; week < 6; week++) {
      for (int d = 0; d < 7; d++) {
        if(cal.get(Calendar.MONTH) == today.get(Calendar.MONTH))
        {
            g.drawString(day.format(cal.getTime()) , d * 30  + 50 + ((25 * d)) , week * 29 + 81 + 20 + 30 + (week * 10) );
        }
        cal.add(Calendar.DATE, +1);
      }
    }
  }

  public void paint()
  {
    JFrame frame = new JFrame();
    frame.setPreferredSize(new Dimension(500, 500));
    frame.setTitle("Event Calendar");
    CalendarHack ch = new CalendarHack();
    JPanel panel = new JPanel();
    panel.setBounds(135, 350, 200, 200);

    ActionListener monthListen = new MonthListener();
//    back.addActionListener(new ActionListener()
//    {
//        public void actionPerformed(ActionEvent e)
//        {
//          Calendar c = new GregorianCalendar();
//          c.setTime(date);
//          c.add(Calendar.MONTH, -1);
//          date = c.getTime();
//          System.out.println("Repainting...");
//          repaint();
//          System.out.println(date.toString());
//        }
//    });
    back.addActionListener(monthListen);
    next.addActionListener(monthListen);
    panel.add(back);
    panel.add(next);
    panel.setBackground(Color.BLACK);
    frame.add(panel);
    frame.getContentPane().add(ch);
    //frame.setUndecorated(true);
    frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    frame.pack();
    frame.setVisible(true);
  }

  public static void main(String[] args) 
  {
        CalendarHack cal = new CalendarHack();
        cal.paint();
  }


class MonthListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            if(e.getSource() == back)
            {
              Calendar c = new GregorianCalendar();
              c.setTime(date);
              c.add(Calendar.MONTH, -1);
              date = c.getTime();
              repaint();
              System.out.println(date.toString());
            }
            else if(e.getSource() == next)
            {
              Calendar c = new GregorianCalendar();
              c.setTime(date);
              c.add(Calendar.MONTH, 1);
              date = c.getTime();
              repaint();
              System.out.println(date.toString());
            }
            //CalendarHack.repaint();
            repaint();
        }
    }      

}