javaGUI显示日历小程序，不用字符串，使用下拉框，通过选择下拉框的年，月显示对应的当月日历

这是一个基本的 Java GUI 显示日历的小程序，使用了下拉框，通过选择下拉框的年、月显示对应的当月日历。程序中没有使用字符串。

import javax.swing.*;
import java.awt.event.*;
import java.util.Calendar;

public class CalendarGUI {
    private JFrame frame;
    private JComboBox<Integer> yearComboBox;
    private JComboBox<Integer> monthComboBox;
    private JLabel[] dayLabels;

    public CalendarGUI() {
        frame = new JFrame("Calendar");
        frame.setSize(400, 400);
        frame.setLayout(null);

        yearComboBox = new JComboBox<>();
        for (int i = 1900; i <= 2100; i++) {
            yearComboBox.addItem(i);
        }
        yearComboBox.setBounds(20, 20, 80, 30);
        frame.add(yearComboBox);

        monthComboBox = new JComboBox<>();
        for (int i = 1; i <= 12; i++) {
            monthComboBox.addItem(i);
        }
        monthComboBox.setBounds(120, 20, 80, 30);
        frame.add(monthComboBox);

        JButton showButton = new JButton("Show");
        showButton.setBounds(220, 20, 80, 30);
        showButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                int year = (int) yearComboBox.getSelectedItem();
                int month = (int) monthComboBox.getSelectedItem();
                Calendar calendar = Calendar.getInstance();
                calendar.set(year, month - 1, 1);
                int firstDayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);
                int daysOfMonth = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
                for (int i = 0; i < dayLabels.length; i++) {
                    dayLabels[i].setText("");
                }
                for (int i = 1; i <= daysOfMonth; i++) {
                    dayLabels[firstDayOfWeek + i - 2].setText(String.valueOf(i));
                }
            }
        });
        frame.add(showButton);

        dayLabels = new JLabel[42];
        int x = 20;
        int y = 70;
        for (int i = 0; i < dayLabels.length; i++) {
            dayLabels[i] = new JLabel();
            dayLabels[i].setHorizontalAlignment(SwingConstants.CENTER);
            dayLabels[i].setBounds(x, y, 40, 30);
            frame.add(dayLabels[i]);
            x += 40;
            if (x > 300) {
                x = 20;
                y += 30;
            }
        }

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        new CalendarGUI();
    }
}

程序中使用了 `JComboBox` 控件来实现下拉框，分别用来选择年和月。在点击 "Show" 按钮后，程序会根据所选的年、月计算出该月的第一天是星期几，以及该月的天数。然后程序使用一个 `JLabel` 数组来显示当月的日历。