利用Java计算某个日期是星期几

计算某个日期是星期几可以使用Java自带的Calendar类来实现。下面是一些示例代码，演示如何获取某个日期对应的星期。

示例一：获取当前日期所对应的星期

import java.util.Calendar;

public class DateOfWeek {
    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();
        int weekday = calendar.get(Calendar.DAY_OF_WEEK);
        String weekdayString = getWeekdayString(weekday);
        System.out.println("Today is " + weekdayString);
    }

    /**
     * 获取星期的字符串表示
     * @param weekday 星期数字表示
     * @return 星期的字符串表示
     */
    private static String getWeekdayString(int weekday) {
        switch (weekday) {
            case Calendar.SUNDAY:
                return "Sunday";
            case Calendar.MONDAY:
                return "Monday";
            case Calendar.TUESDAY:
                return "Tuesday";
            case Calendar.WEDNESDAY:
                return "Wednesday";
            case Calendar.THURSDAY:
                return "Thursday";
            case Calendar.FRIDAY:
                return "Friday";
            case Calendar.SATURDAY:
                return "Saturday";
            default:
                throw new IllegalArgumentException("Invalid weekday value: " + weekday);
        }
    }
}

运行上述程序，输出结果如下：

Today is Friday

示例二：计算某个日期是星期几

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

public class DateOfWeek {
    public static void main(String[] args) throws Exception {
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyyMMdd");
        Date date = dateFormat.parse("20211111"); // 获取2021年11月11日对应的Date对象
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);  // 设置Calendar对象为该日期
        int weekday = calendar.get(Calendar.DAY_OF_WEEK);
        String weekdayString = getWeekdayString(weekday);
        System.out.println("2021-11-11 is " + weekdayString);
    }

    /**
     * 获取星期的字符串表示
     * @param weekday 星期数字表示
     * @return 星期的字符串表示
     */
    private static String getWeekdayString(int weekday) {
        switch (weekday) {
            case Calendar.SUNDAY:
                return "Sunday";
            case Calendar.MONDAY:
                return "Monday";
            case Calendar.TUESDAY:
                return "Tuesday";
            case Calendar.WEDNESDAY:
                return "Wednesday";
            case Calendar.THURSDAY:
                return "Thursday";
            case Calendar.FRIDAY:
                return "Friday";
            case Calendar.SATURDAY:
                return "Saturday";
            default:
                throw new IllegalArgumentException("Invalid weekday value: " + weekday);
        }
    }
}

运行上述程序，输出结果如下：

2021-11-11 is Thursday

需要注意的是，在示例二中，我们使用了SimpleDateFormat类将字符串格式化为Date对象，然后再将Date对象转换为Calendar对象，再获取对应的星期。