设int year,month,day分别表示一个日期中的年月日，试编程求 a)对于任意三个整数，判断其是否为一个合法的日期值。 b)给定一个日期值，计算若干天后的日期值 如果有两个日期值，计算他们相距的天数。请用java写

a)判断日期是否合法的Java代码：

public static boolean isValidDate(int year, int month, int day) {
    if (month < 1 || month > 12) {
        return false;
    }
    int maxDay = 31;
    if (month == 4 || month == 6 || month == 9 || month == 11) {
        maxDay = 30;
    } else if (month == 2) {
        if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
            maxDay = 29;
        } else {
            maxDay = 28;
        }
    }
    if (day < 1 || day > maxDay) {
        return false;
    }
    return true;
}

b)计算若干天后的日期值的Java代码：

public static void calculateDateAfter(int year, int month, int day, int days) {
    Calendar calendar = Calendar.getInstance();
    calendar.set(year, month - 1, day);
    calendar.add(Calendar.DATE, days);
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日");
    String dateStr = sdf.format(calendar.getTime());
    System.out.println("计算后的日期为：" + dateStr);
}

c)计算两个日期相差的天数的Java代码：

public static int calculateDaysBetween(int year1, int month1, int day1, int year2, int month2, int day2) {
    Calendar calendar1 = Calendar.getInstance();
    calendar1.set(year1, month1 - 1, day1);
    Calendar calendar2 = Calendar.getInstance();
    calendar2.set(year2, month2 - 1, day2);
    long time1 = calendar1.getTimeInMillis();
    long time2 = calendar2.getTimeInMillis();
    long betweenDays = (time2 - time1) / (1000 * 3600 * 24);
    return (int) betweenDays;
}