Java 代码判断日期是星期几

这是一个判断给定日期是星期几的Java代码：

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class WeekdayChecker {

    public static void main(String[] args) {
        String dateString = '2021-07-21'; // 假设要判断的日期是 '2021-07-21'
        SimpleDateFormat dateFormat = new SimpleDateFormat('yyyy-MM-dd');
        Date date = null;
        try {
            date = dateFormat.parse(dateString);
        } catch (Exception e) {
            e.printStackTrace();
        }
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        int weekday = calendar.get(Calendar.DAY_OF_WEEK);
        String weekdayString = null;
        switch (weekday) {
            case Calendar.SUNDAY:
                weekdayString = '星期日';
                break;
            case Calendar.MONDAY:
                weekdayString = '星期一';
                break;
            case Calendar.TUESDAY:
                weekdayString = '星期二';
                break;
            case Calendar.WEDNESDAY:
                weekdayString = '星期三';
                break;
            case Calendar.THURSDAY:
                weekdayString = '星期四';
                break;
            case Calendar.FRIDAY:
                weekdayString = '星期五';
                break;
            case Calendar.SATURDAY:
                weekdayString = '星期六';
                break;
            default:
                break;
        }
        System.out.println(dateString + '是' + weekdayString);
    }

}

该代码首先将给定的日期字符串解析为Date对象，然后使用Calendar类将其转换为星期几的数字，最后使用switch语句将星期几的数字转换为中文的星期几字符串。最终输出结果为：'2021-07-21是星期三'。