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，使用Calendar类

好的，这是一个简单的Java GUI日历小程序，使用了下拉框和Calendar类来显示当月日历。

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

public class CalendarGUI extends JFrame implements ItemListener {
    private static final long serialVersionUID = 1L;

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

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            new CalendarGUI().setVisible(true);
        });
    }

    public CalendarGUI() {
        setTitle("Calendar");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400, 300);
        setLocationRelativeTo(null);

        Container contentPane = getContentPane();
        contentPane.setLayout(new BorderLayout());

        JPanel controlPanel = new JPanel();
        controlPanel.setLayout(new FlowLayout());

        yearComboBox = new JComboBox<>();
        yearComboBox.addItemListener(this);
        for (int i = 1900; i <= 2100; i++) {
            yearComboBox.addItem(String.valueOf(i));
        }
        controlPanel.add(yearComboBox);

        monthComboBox = new JComboBox<>();
        monthComboBox.addItemListener(this);
        for (int i = 1; i <= 12; i++) {
            monthComboBox.addItem(String.valueOf(i));
        }
        controlPanel.add(monthComboBox);

        contentPane.add(controlPanel, BorderLayout.NORTH);

        calendarPanel = new JPanel();
        calendarPanel.setLayout(new GridLayout(0, 7));

        String[] dayNames = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
        dayLabels = new JLabel[7];
        for (int i = 0; i < 7; i++) {
            dayLabels[i] = new JLabel(dayNames[i], JLabel.CENTER);
            dayLabels[i].setForeground(Color.BLUE);
            calendarPanel.add(dayLabels[i]);
        }

        contentPane.add(calendarPanel, BorderLayout.CENTER);

        Calendar calendar = Calendar.getInstance();
        int year = calendar.get(Calendar.YEAR);
        int month = calendar.get(Calendar.MONTH) + 1;
        yearComboBox.setSelectedItem(String.valueOf(year));
        monthComboBox.setSelectedItem(String.valueOf(month));
        updateCalendar(year, month);
    }

    private void updateCalendar(int year, int month) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(year, month - 1, 1);

        int firstDayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);
        int lastDayOfMonth = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);

        calendarPanel.removeAll();

        for (int i = 1; i < firstDayOfWeek; i++) {
            calendarPanel.add(new JLabel(""));
        }

        for (int i = 1; i <= lastDayOfMonth; i++) {
            JLabel dayLabel = new JLabel(String.valueOf(i), JLabel.CENTER);
            if (i == calendar.get(Calendar.DAY_OF_MONTH)) {
                dayLabel.setForeground(Color.RED);
            }
            calendarPanel.add(dayLabel);
        }

        pack();
    }

    @Override
    public void itemStateChanged(ItemEvent e) {
        if (e.getStateChange() == ItemEvent.SELECTED) {
            int year = Integer.parseInt((String) yearComboBox.getSelectedItem());
            int month = Integer.parseInt((String) monthComboBox.getSelectedItem());
            updateCalendar(year, month);
        }
    }
}

在这个程序中，我们首先创建了一个窗口和两个下拉框，其中一个用于选择年份，另一个用于选择月份。当用户选择年份或月份时，我们将调用`updateCalendar`方法来显示相应的日历。`updateCalendar`方法将使用Calendar类来计算该月份的第一天是星期几，以及该月份有多少天。然后，我们将创建一个网格布局，将星期几标签和日期标签添加到其中。如果当前日期与选定日期相同，我们将以红色显示该日期。最后，我们使用`pack`方法调整窗口大小以适应内容。