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，没有额外按钮，只通过选择下拉框显示日历

以下是一个简单的Java GUI显示日历的小程序，使用了下拉框来选择年月，并根据选择的年月动态生成当月的日历。

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

public class CalendarApp implements ActionListener {

    private JFrame frame;
    private JComboBox<String> yearComboBox, monthComboBox;
    private JLabel[] dayLabels;
    private JPanel calendarPanel;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new CalendarApp().createAndShowGUI();
            }
        });
    }

    public void createAndShowGUI() {
        // 创建主窗口
        frame = new JFrame("Calendar");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        // 创建选择年份和月份的下拉框
        yearComboBox = new JComboBox<String>();
        for (int i = 1900; i <= 2100; i++) {
            yearComboBox.addItem(Integer.toString(i));
        }
        yearComboBox.addActionListener(this);

        monthComboBox = new JComboBox<String>();
        String[] months = {"January", "February", "March", "April",
                "May", "June", "July", "August", "September", "October", "November", "December"};
        for (String month : months) {
            monthComboBox.addItem(month);
        }
        monthComboBox.addActionListener(this);

        // 创建日历面板
        calendarPanel = new JPanel(new GridLayout(7, 7));

        // 创建星期标签
        String[] days = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
        JLabel[] weekLabels = new JLabel[7];
        for (int i = 0; i < 7; i++) {
            weekLabels[i] = new JLabel(days[i], JLabel.CENTER);
            calendarPanel.add(weekLabels[i]);
        }

        // 创建日期标签
        dayLabels = new JLabel[42];
        for (int i = 0; i < 42; i++) {
            dayLabels[i] = new JLabel("", JLabel.CENTER);
            calendarPanel.add(dayLabels[i]);
        }

        // 将下拉框和日历面板添加到主窗口
        JPanel selectPanel = new JPanel();
        selectPanel.add(yearComboBox);
        selectPanel.add(monthComboBox);
        frame.add(selectPanel, BorderLayout.NORTH);
        frame.add(calendarPanel, BorderLayout.CENTER);

        // 显示主窗口
        frame.setVisible(true);

        // 初始显示当前年月的日历
        int year = Integer.parseInt(yearComboBox.getSelectedItem().toString());
        int month = monthComboBox.getSelectedIndex() + 1;
        showCalendar(year, month);
    }

    public void actionPerformed(ActionEvent e) {
        int year = Integer.parseInt(yearComboBox.getSelectedItem().toString());
        int month = monthComboBox.getSelectedIndex() + 1;
        showCalendar(year, month);
    }

    private void showCalendar(int year, int month) {
        // 计算当月第一天是星期几
        int firstDayOfWeek = getFirstDayOfWeek(year, month);

        // 计算当月有多少天
        int daysInMonth = getDaysInMonth(year, month);

        // 显示当月的日期
        for (int i = 0; i < 42; i++) {
            if (i < firstDayOfWeek || i >= firstDayOfWeek + daysInMonth) {
                dayLabels[i].setText("");
            } else {
                int day = i - firstDayOfWeek + 1;
                dayLabels[i].setText(Integer.toString(day));
            }
        }
    }

    private int getDaysInMonth(int year, int month) {
        int daysInMonth;
        switch (month) {
            case 4:
            case 6:
            case 9:
            case 11:
                daysInMonth = 30;
                break;
            case 2:
                if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) {
                    daysInMonth = 29;
                } else {
                    daysInMonth = 28;
                }
                break;
            default:
                daysInMonth = 31;
        }
        return daysInMonth;
    }

    private int getFirstDayOfWeek(int year, int month) {
        int[] daysInMonth = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        int totalDays = 0;
        for (int i = 1900; i < year; i++) {
            if ((i % 4 == 0 && i % 100 != 0) || (i % 400 == 0)) {
                totalDays += 366;
            } else {
                totalDays += 365;
            }
        }
        for (int i = 0; i < month - 1; i++) {
            totalDays += daysInMonth[i];
            if (i == 1 && ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))) {
                totalDays++;
            }
        }
        return (totalDays + 1) % 7;
    }
}

该程序使用了 `JComboBox` 来选择年月，然后根据选择的年月动态生成对应的日历。其中，`getFirstDayOfWeek` 方法用于计算当月第一天是星期几，`getDaysInMonth` 方法用于计算当月有多少天。