用Java编写一个判断指定日期为星期几的代码

以下是用Java编写的判断指定日期为星期几的代码：

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

public class DayOfWeekFinder {
    
    public static void main(String[] args) {
        String dateStr = "2021-12-31"; // 指定日期，格式为yyyy-MM-dd
        String dayOfWeek = getDayOfWeek(dateStr);
        System.out.println(dateStr + "是星期" + dayOfWeek);
    }
    
    public static String getDayOfWeek(String dateStr) {
        String dayOfWeek = "";
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
            Date date = sdf.parse(dateStr);
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date);
            int day = calendar.get(Calendar.DAY_OF_WEEK);
            switch (day) {
                case Calendar.SUNDAY:
                    dayOfWeek = "日";
                    break;
                case Calendar.MONDAY:
                    dayOfWeek = "一";
                    break;
                case Calendar.TUESDAY:
                    dayOfWeek = "二";
                    break;
                case Calendar.WEDNESDAY:
                    dayOfWeek = "三";
                    break;
                case Calendar.THURSDAY:
                    dayOfWeek = "四";
                    break;
                case Calendar.FRIDAY:
                    dayOfWeek = "五";
                    break;
                case Calendar.SATURDAY:
                    dayOfWeek = "六";
                    break;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return dayOfWeek;
    }
}

该代码通过SimpleDateFormat将指定日期字符串转换为Date对象，再通过Calendar获取该日期的星期几，最后根据星期几的值返回相应的字符串。