java - 如何获取下一个指定星期几的日期

Question

我必须根据当前日期计算下一个日期。例如，假设今天是“2014 年 11 月 5 日”，用户输入了第一个星期一。因此，根据用户输入，如果当月的第一个星期一已经过去，那么我必须找出下个月的第一个星期一。如果当月的第一个星期一还没有到来，那么我必须找出当月的日期。

我正在使用 java.util.Calendar 类。

//user inputs
int dayOfWeek = 3; // as user has provided 'Tuesday'   
int noOfWeek = 1; // as user provided 1st 

Calendar cal = Calendar.getInstance(); // getting the current instance.
int currentDayOfWeek = Calendar.getInstance().get(Calendar.DAY_OF_WEEK); // getting the cCurrentDayOfWeek in integer.
int currentNoOfWeek  = Calendar.getInstance().get(Calendar.DAY_OF_WEEK_IN_MONTH); // getting which week it is in the current month


 if(noOfWeek < currentNoOfWeek){
 // the date has gone past in the current month.
 }
 else if ((noOfWeek == currentNoOfWeek) && dayOfWeek < currentDayOfWeek){
 // though the week number is same but as the currentDayOfWeek is greater than the provided day so in this case also  date has gone past in the current month.
 }

无法继续进行。寻求您的帮助。

Answer 1

这是一个非常酷的问题，这是我想出的解决方案和我的方法:

首先是你拥有的:

public static void main(String[] args) {
    // TODO: validate user-input

    // Input by user:
    int inputDayOfWeek = 3; // Tuesday
    int inputWeekOfMonth = 2;

    if(isInNextMonth(inputDayOfWeek, inputWeekOfMonth)){
        Date outputDate = calculateNextValidDate(inputDayOfWeek, inputWeekOfMonth);

        // Do something with the outputDate
        System.out.println(outputDate.toString());
    }
}

private static boolean isInNextMonth(int inputDayOfWeek, int inputWeekOfMonth){
    // Current day:
    Calendar cal = Calendar.getInstance();
    int currentDayOfWeek = cal.get(Calendar.DAY_OF_WEEK);
    int currentWeekOfMonth = cal.get(Calendar.DAY_OF_WEEK_IN_MONTH);

    // The date has gone past in the current month
    // OR though it's the same week of the month, the day of the week is past the current day of the week
    return inputWeekOfMonth < currentWeekOfMonth || ((inputWeekOfMonth == currentWeekOfMonth) && inputDayOfWeek < currentDayOfWeek);
}

一些注意事项:我将 if 和 if-else 都放在一个 if 中，因为在这两种情况下你都想转到下个月，并且还使它成为一个单独的方法(使其成为一个单独的方法只是我自己的偏好，让事情井井有条。

我注意到的另一件事是您的 if 和 else-if 中的错误。应该是noOfWeek < currentNoOfWeek而不是 noOfWeek > currentNoOfWeek和 ((noOfWeek == currentNoOfWeek) && dayOfWeek > currentDayOfWeek)而不是 ((noOfWeek == currentNoOfWeek) && dayOfWeek < currentDayOfWeek) (< 和 > 是相反的)。

---

现在是 calculateNextValidDate 方法，这是您的问题所在。我的做法如下:

• 从下个月的第一天开始
• 转到本月的正确星期
• 然后转到本周的正确日期

这给了我以下代码:

private static Date calculateNextValidDate(int inputDayOfWeek, int inputWeekOfMonth){
    // Set the first day of the next month as starting position:
    Calendar cal = Calendar.getInstance();
    cal.add(Calendar.MONTH, 1);
    cal.set(Calendar.DAY_OF_MONTH, 1);

    // Now first go to the correct week of this month
    int weekOfNextMonth = 1;
    while(weekOfNextMonth < inputWeekOfMonth){
        // Raise by a week
        cal.add(Calendar.DAY_OF_MONTH, 7);
        weekOfNextMonth++;
    }

    // Now that we have the correct week of this month,
    // we get the correct day
    while(cal.get(Calendar.DAY_OF_WEEK) != inputDayOfWeek){
        // Raise by a day
        cal.add(Calendar.DAY_OF_MONTH, 1);
    }

    return cal.getTime();
}

此代码为我提供了以下输出(2014 年 11 月 5 日星期三 - 输入为 3 [Tuesday] 和 2):

Tue Dec 09 17:05:42 CET 2014

另请注意 // TODO:我在这篇文章的第一个代码部分的主要方法中添加了。如果用户输入无效(例如负数周或 dayOfMonth)，它可能会经过 while 循环太多次。我让您来验证用户输入。