java - 日历返回错误的一年中的第几周

Question

我想将本周的第一天设置为星期日后，将 2018 年 9 月 10 日这一天作为一年中的第 42 周。我仍然从下面的代码片段中得到输出 41

 Calendar c = Calendar.getInstance();
 c.setFirstDayOfWeek(Calendar.SUNDAY);
 System.out.println( c.get(Calendar.WEEK_OF_YEAR)  );

我在这里遗漏了什么吗？

背景:我们的一周开始时间可能会有所不同。我们的配置能够让用户决定一周的第一天。

Answer 1

抱歉，我太执着了。我仍然认为您应该将计算留给自定义 WeekFields 对象。如果我正确理解你的评论，你想要:

    final int daysPerWeek = DayOfWeek.values().length; // A wordy way of writing 7 :-)
    WeekFields customWeekFields = WeekFields.of(firstDayOfWeek, daysPerWeek);
    int customWeekNumber = date.get(customWeekFields.weekOfWeekBasedYear());

为了测试这是否与您已经在做的事情一致，我编写了以下方法:

static void printWeekNumber(DayOfWeek firstDayOfWeek, LocalDate date) {
    DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("EEE uuuu-MM-dd", Locale.ENGLISH);
    final int daysPerWeek = DayOfWeek.values().length;

    // Week number according to your comment: “using the Temporal adjusters
    // I am getting the date on the last day of the week.
    // After that dividing the Day of the Year from the API by 7”
    DayOfWeek lastDayOfWeek = firstDayOfWeek.minus(1);
    int askersCommentWeekNumber = date
            .with(TemporalAdjusters.nextOrSame(lastDayOfWeek))
            .getDayOfYear()
                    / daysPerWeek;

    // My suggested way of calculating the same week number
    WeekFields customWeekFields = WeekFields.of(firstDayOfWeek, daysPerWeek);
    int customWeekNumber = date.get(customWeekFields.weekOfWeekBasedYear());

    System.out.format(Locale.ENGLISH, "Week begins on %-8s Date is %s. Week %2d or %2d, agree? %s%n", 
            firstDayOfWeek, date.format(dateFormatter), 
            askersCommentWeekNumber, customWeekNumber, 
            askersCommentWeekNumber == customWeekNumber);
}

为了方便手动检查计算结果，我选择了不同年份的一月份的日期:

    printWeekNumber(DayOfWeek.SUNDAY, LocalDate.of(2017, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.MONDAY, LocalDate.of(2017, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.THURSDAY, LocalDate.of(2017, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.SUNDAY, LocalDate.of(2018, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.MONDAY, LocalDate.of(2018, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.SUNDAY, LocalDate.of(2019, Month.JANUARY, 9));
    printWeekNumber(DayOfWeek.MONDAY, LocalDate.of(2019, Month.JANUARY, 9));

输出:

Week begins on SUNDAY   Date is Mon 2017-01-09. Week  2 or  2, agree? true
Week begins on MONDAY   Date is Mon 2017-01-09. Week  2 or  2, agree? true
Week begins on THURSDAY Date is Mon 2017-01-09. Week  1 or  1, agree? true
Week begins on SUNDAY   Date is Tue 2018-01-09. Week  1 or  1, agree? true
Week begins on MONDAY   Date is Tue 2018-01-09. Week  2 or  2, agree? true
Week begins on SUNDAY   Date is Wed 2019-01-09. Week  1 or  1, agree? true
Week begins on MONDAY   Date is Wed 2019-01-09. Week  1 or  1, agree? true

不过，请检查结果是否如您所愿。对于您问题中的示例，结果既不是 41 也不是 42:

    printWeekNumber(DayOfWeek.SUNDAY, LocalDate.of(2018, Month.OCTOBER, 9));

Week begins on SUNDAY   Date is Tue 2018-10-09. Week 40 or 40, agree? true

编辑:如果您需要 ISO 8601 周数，请使用 date.get(WeekFields.ISO.dayOfWeek())。如果您想要自定义一周的第一天和第一周的 4 天(如 ISO 8601 中所示)，请使用:

    WeekFields customWeekFields = WeekFields.of(firstDayOfWeek, 4);