b) Make the clock show the current time. For this you should use the Calendar class from java.util with: ● Calendar.getInstance() - create Calendar object for current date/time ● get(field) - return integer field from Calendar object, where field may be Calendar.HOUR, Calendar.MINUTE, Calendar.SECOND, Calendar.AM_PM, etc. ● the value returned from Calendar.AM_PM may be compared with the values Calendar.AM and Calendar.PM to determine if it is before or after mid-day Create a function that runs continuously and updates the display to always show the current time.怎么做

你可以使用以下代码实现这个任务：

import java.awt.Font;
import java.awt.GridLayout;
import java.util.Calendar;

import javax.swing.JFrame;
import javax.swing.JLabel;

public class Clock extends JFrame {
    private static final long serialVersionUID = 1L;
    private JLabel timeLabel;

    public Clock() {
        super("Current Time");
        timeLabel = new JLabel();
        timeLabel.setFont(new Font("Arial", Font.PLAIN, 48));
        setLayout(new GridLayout(1, 1));
        add(timeLabel);
        pack();
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
    }

    public void updateTime() {
        Calendar calendar = Calendar.getInstance();
        int hour = calendar.get(Calendar.HOUR);
        int minute = calendar.get(Calendar.MINUTE);
        int second = calendar.get(Calendar.SECOND);
        int amPm = calendar.get(Calendar.AM_PM);
        String amPmStr = (amPm == Calendar.AM) ? "AM" : "PM";
        String time = String.format("%02d:%02d:%02d %s", hour, minute, second, amPmStr);
        timeLabel.setText(time);
    }

    public static void main(String[] args) {
        Clock clock = new Clock();
        while (true) {
            clock.updateTime();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

这个代码定义了一个 `Clock` 类，它继承了 `JFrame` 类，用于显示当前时间。在 `Clock` 类中，我们创建了一个标签 `timeLabel`，用于显示当前时间。然后我们使用 `GridLayout` 将标签放置在 `JFrame` 中。在 `updateTime()` 方法中，我们使用 `Calendar` 类获取当前时间，并将其格式化为字符串。最后我们将时间字符串设置到标签中。

在 `main()` 方法中，我们创建了一个 `Clock` 对象，并在一个无限循环中不断更新时间，并在每次更新后等待 1 秒钟。这样就可以保证时间的实时更新。