javaGUI通过下拉框选择日期来显示日历，不要额外加按钮

可以使用JComboBox来创建下拉框，然后根据选择的日期更新日历显示。

下面是一个简单的实现，可以根据需要进行修改和优化：

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

public class CalendarGUI extends JFrame implements ActionListener {
    private final JComboBox<String> yearBox, monthBox;
    private final JLabel[] dayLabels;
    private final DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("yyyy-M-d");
    
    public CalendarGUI() {
        setTitle("Calendar");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());
        
        JPanel topPanel = new JPanel();
        topPanel.add(new JLabel("Year:"));
        yearBox = new JComboBox<>();
        for (int year = LocalDate.now().getYear() - 10; year <= LocalDate.now().getYear() + 10; year++) {
            yearBox.addItem(String.valueOf(year));
        }
        yearBox.setSelectedItem(String.valueOf(LocalDate.now().getYear()));
        yearBox.addActionListener(this);
        topPanel.add(yearBox);
        topPanel.add(new JLabel("Month:"));
        monthBox = new JComboBox<>();
        for (int month = 1; month <= 12; month++) {
            monthBox.addItem(String.valueOf(month));
        }
        monthBox.setSelectedItem(String.valueOf(LocalDate.now().getMonthValue()));
        monthBox.addActionListener(this);
        topPanel.add(monthBox);
        add(topPanel, BorderLayout.NORTH);
        
        JPanel centerPanel = new JPanel(new GridLayout(7, 7));
        String[] weekDays = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
        for (String weekDay : weekDays) {
            centerPanel.add(new JLabel(weekDay, SwingConstants.CENTER));
        }
        dayLabels = new JLabel[42];
        for (int i = 0; i < dayLabels.length; i++) {
            dayLabels[i] = new JLabel("", SwingConstants.CENTER);
            centerPanel.add(dayLabels[i]);
        }
        updateCalendar();
        add(centerPanel, BorderLayout.CENTER);
        
        pack();
        setLocationRelativeTo(null);
        setVisible(true);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == yearBox || e.getSource() == monthBox) {
            updateCalendar();
        }
    }
    
    private void updateCalendar() {
        int year = Integer.parseInt((String) yearBox.getSelectedItem());
        int month = Integer.parseInt((String) monthBox.getSelectedItem());
        LocalDate date = LocalDate.of(year, month, 1);
        int firstDayOfWeek = date.getDayOfWeek().getValue() % 7;
        int lastDay = date.plusMonths(1).minusDays(1).getDayOfMonth();
        for (int i = 0; i < dayLabels.length; i++) {
            if (i < firstDayOfWeek || i >= firstDayOfWeek + lastDay) {
                dayLabels[i].setText("");
            } else {
                dayLabels[i].setText(String.valueOf(i - firstDayOfWeek + 1));
            }
        }
    }
    
    public static void main(String[] args) {
        SwingUtilities.invokeLater(CalendarGUI::new);
    }
}

这个GUI界面包含一个顶部面板和一个中心面板。顶部面板包含两个下拉框，用于选择年份和月份；中心面板包含一个网格布局，用于显示日历。网格布局中第一行为星期几的标签，接下来是42个用于显示日期的标签（最多有6*7=42天）。当用户选择年份或月份时，通过计算更新中心面板中的日期标签。