我试着用JavaSwing开发一个日历应用程序，我有两个JComboBox来选择年份和月份，还有一个jbutton。当按下jbutton时，我的日历应该出现并显示所选年份的所选月份。当选择其他年份和月份时，按下按钮，我的日历应该显示新选择的月份。但我的日历只有在第一次点击按钮时才起作用，之后我的日历数据不会改变。下面是calendartable类的代码和一个简单的示例。有人能帮我找到问题吗？
日历表类：

import java.util.Calendar;
import java.util.Date;
import javax.swing.table.AbstractTableModel;
public class Calendartable extends AbstractTableModel{
     public static String[] COLUMN_NAMES = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday","Sunday"};
     private int tablerows = 0;
     private Date startOfWeek;
     private Date startDate;
     private Date endDate;
     public Calendartable(Date start, Date end) {
         this.startDate=start;
         this.endDate=end;
         Calendar cal = Calendar.getInstance();
         cal.setTime(startDate);
         int i=0;
         while (cal.getTime().before(endDate)) {
             cal.add(Calendar.DATE, 7);
             tablerows++;
         }
         tablerows++;
         if (tablerows > 0) {
             cal.setTime(startDate);
             while (cal.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY) {
                 cal.add(Calendar.DATE, -1);
             }
             startOfWeek = cal.getTime();
         }
     }
    @Override
    public int getRowCount() {
        return tablerows;
    }
    @Override
    public int getColumnCount() {
        return 7;
    }
     @Override
     public Class<?> getColumnClass(int columnIndex) {
         return Date.class;
     }
     @Override
     public String getColumnName(int column) {
         return COLUMN_NAMES[column];
     }
    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
         Date date = null;
         if (startOfWeek != null) {
             Calendar cal = Calendar.getInstance();
             cal.setTime(startOfWeek);
             int day = (rowIndex * 7) + columnIndex;            
             cal.add(Calendar.DATE, day);
             if (cal.getTime().before(startDate) || cal.getTime().after(endDate)) {
                 date = null;
             } else {
                 date = cal.getTime();           
             }
         }
         return date;
    }
}

再举个例子：

import java.awt.CardLayout;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Calendar2 {
    JFrame frame = new JFrame("Calendar");
    JPanel panelCont = new JPanel();
    JPanel panelFirst = new JPanel(new GridBagLayout());
    JTable table;
    JButton showbuttonmonth=new JButton("Show");
    JComboBox jcbyear;
    JComboBox<String> jcbmonth;
    CardLayout cl = new CardLayout();
    Object month[]=new Object[12];{
        month[0]="January";
        month[1]="February";
        month[2]="March";
        month[3]="April";
        month[4]="May";
        month[5]="June";
        month[6]="July";
        month[7]="August";
        month[8]="September";
        month[9]="October";
        month[10]="November";
        month[11]="December";
    }
    Object year[]=new Object[30];{
        for(int i=2010;i<=2039;i++)
            year[i-2010]=i;
    }
    public Calendar2() {
        panelCont.setLayout(cl);
        GridBagConstraints gbc=new GridBagConstraints();
        gbc.insets=new Insets(5,5,5,5);
jcbyear=new JComboBox(year);
        gbc.insets=new Insets(15,0,0,5);
        gbc.gridx=0;
        gbc.gridy=2;
        panelFirst.add(new JLabel("Year:"),gbc);
        gbc.insets=new Insets(0,0,5,5);
        gbc.gridx=0;
        gbc.gridy=3;
        panelFirst.add(jcbyear, gbc);
        gbc.insets=new Insets(15,0,0,5);
        gbc.gridx=1;
        gbc.gridy=2;
        panelFirst.add(new JLabel("Month:"),gbc);
        jcbmonth=new JComboBox(month);
        gbc.insets=new Insets(0,0,5,5);
        gbc.gridx=1;
        gbc.gridy=3;
        panelFirst.add(jcbmonth, gbc);
        gbc.gridx=2;
        gbc.gridy=3;
        panelFirst.add(showbuttonmonth, gbc);
        gbc.gridx=1;
        gbc.gridy=4;
        SimpleDateFormat sdf = new SimpleDateFormat("MM.dd.yyyy");
         Date startDate = null;
        try {
            startDate = sdf.parse("1.1.2010");
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
         Date endDate = null;
        try {
            endDate = sdf.parse("1.31.2010");
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
         table = new JTable(new Calendartable(startDate, endDate));
         panelFirst.add(new JScrollPane(table), gbc);
         panelCont.add(panelFirst, "1");
        cl.show(panelCont, "1");
        showbuttonmonth.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                int year = Integer.parseInt(String.valueOf(jcbyear.getSelectedItem()));
                int month=jcbmonth.getSelectedIndex()+1;
                String start=month+".1."+year;
                String end=month+".31."+year;
                gbc.gridx=1;
                gbc.gridy=4;
                SimpleDateFormat sdf = new SimpleDateFormat("MM.dd.yyyy");
                 Date startDate = null;
                try {
                    startDate = sdf.parse(start);
                } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                 Date endDate = null;
                try {
                    endDate = sdf.parse(end);
                } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                 table = new JTable(new Calendartable(startDate, endDate));
                 panelFirst.add(new JScrollPane(table), gbc);
            }
        });
    }
    public static void main(String[] args) {
        Calendar2 cal=new Calendar2();
        }
}