请问我的程序哪里出错了?为什么按了“<<”后会没有反应，并且当窗口缩小再还原时，日历的数值都清空了？
（因为我下面的程序是测试用的，所以只做了一个功能）
import java.util.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class test extends JFrame
{
        private JPanel JP1=new JPanel();
        private JPanel JP2=new JPanel();
        private JPanel JP3=new JPanel();
        private int day,week,month,year;
        private JLabel left,center,right;
        Listener L;
        Calendar C;
        
        test()
        {
              C=Calendar.getInstance();
        month=C.get(Calendar.MONTH);
        year=C.get(Calendar.YEAR);
        
              this.setLayout(null);
              JP1.setLayout(new GridLayout(1,7));
                JP1.add(new JLabel("日 "));
                JP1.add(new JLabel("一 "));
                JP1.add(new JLabel("二 "));
                JP1.add(new JLabel("三 "));
                JP1.add(new JLabel("四 "));
                JP1.add(new JLabel("五 "));
                JP1.add(new JLabel("六"));
                this.add(JP1);
        JP1.setBounds(5,0,200,25);
        
        JP2.setLayout(new GridLayout(6,7));
        JP2.setBackground(Color.white);
        GregorianCalendar G1=new GregorianCalendar(year,month,1);   
        week=G1.get(Calendar.DAY_OF_WEEK);
                for(int i=1;i<=week-1;i++)
        {
                JP2.add(new JLabel(""));
        }
        for(int i=1;i<=31;i++)
        {
                JP2.add(new JLabel(String.valueOf(i),JLabel.CENTER));
        }
        this.add(JP2);
        JP2.setBounds(0,25,195,100);
        
        JP3.setLayout(new BorderLayout());
        GregorianCalendar G2=new GregorianCalendar();
        day=G2.get(Calendar.DAY_OF_MONTH);
        left=new JLabel("<<");
        right=new JLabel(">>");
        center=new JLabel("今天："+String.valueOf(year)+"年"+String.valueOf(month+1)+"月"+String.valueOf(day)+"日",JLabel.CENTER);
        JP3.add(left,BorderLayout.WEST);
        JP3.add(center,BorderLayout.CENTER);
        JP3.add(right,BorderLayout.EAST);
        this.add(JP3);
        JP3.setBounds(5,125,185,50);
        
        left.addMouseListener(new Listener());
    }
      void update()
    {
            JP2.removeAll();
            JP3.removeAll();
              GregorianCalendar G3=new GregorianCalendar(year,month,1);   
              week=G3.get(Calendar.DAY_OF_WEEK);
                for(int i=1;i<=week-1;i++)
              {
                      JP2.add(new JLabel(""));
              }
               for(int i=1;i<=29;i++)
              {
                      JP2.add(new JLabel(String.valueOf(i),JLabel.CENTER));
              }        
              center=new JLabel("今天："+String.valueOf(year)+"年"+String.valueOf(month)+"月"+String.valueOf(day)+"日");
          JP3.add(left,BorderLayout.WEST);
          JP3.add(center,BorderLayout.CENTER);
          JP3.add(right,BorderLayout.EAST);
          add(JP3);
          JP3.setBounds(5,125,185,50);
      }
        class Listener extends MouseAdapter
    {      
                public void mousePressed(MouseEvent e)
                {
                        month=month-1;
                  update();
                }
    }
    public static void main(String[] args)
        {
              test JF=new test();
                JF.setSize(200,200);
                JF.setLocation(250,100);
                JF.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                JF.setResizable(false);
                JF.setVisible(true);
        }
}