使用 java.time 查找一年中最后一周的第一天

Question

我需要使用 Java 8 日期和时间 API (java.time) 找到一年中最后一周的第一天，最后得出了这个解决方案：

LocalDate date = LocalDate.of(2016, 2, 17);
LocalDate lastWeekOfYear = LocalDate.of(date.getYear() + 1, 1, 7)
    .with(WeekFields.ISO.weekOfYear(), 1)
    .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY)).minusDays(7);

此解决方案查找下一年的第一周，如有必要，将星期几调整为星期一，并向后移动 7 天。有没有更聪明的方法来达到同样的效果？

Answer 1

按照建议，我将自己回答这个问题（通过 cmets 的小幅改进），因为似乎没有更简单的解决方案。

LocalDate date = LocalDate.of(2016, 2, 17);
LocalDate lastWeekOfYear = LocalDate.of(date.getYear() + 1, 1, 7)
    .with(WeekFields.ISO.weekOfWeekBasedYear(), 1)
    .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY)).minusWeeks(1);

Answer 2

根据您的评论更新：

您可以从您的日期开始，获取一年中的最后一天，然后返回到星期一。获取该结果的 WeekBasedYear，如果 weekBasedYear 与 day 年份相同则接受，否则返回上周一接受结果：

LocalDate day = LocalDate.of(2012, 2, 17);
LocalDate result =  day.with(TemporalAdjusters.lastDayOfYear())
                            .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));      

//if day and result weekBasedYear are in the same weekYear then result else go back to previous Mondays
result = result.get(WeekFields.ISO.weekBasedYear())== day.getYear()? result :
                result.with(TemporalAdjusters.previous(DayOfWeek.MONDAY));

System.out.println(result);

输出：

2012-12-24

希望这能有所帮助。

Answer 3

我认为 OP 找到的最后一个解决方案仍然不正确，因为我们是在 ISO 周日期而不是普通日历日期的背景下移动。所以表达式LocalDate.of(date.getYear() + 1, 1, 7) 在年份边界附近通常是错误的（参见示例方法lastWeekOfYearOld(...)）。相反，我们必须添加一个基于周的年份，而不是日历年（绑定到 1 月至 12 月），它现在已在第二种方法 lastWeekOfYearCorrect(...) 中使用。我还根据周字段的上下文相关值范围展示了另一个正确的替代方案（参见方法lastWeekOfYearAlternative(...)。

public static void main(String[] args) {
    System.out.println(
              lastWeekOfYearOld(LocalDate.of(2015, 12, 31))); // 2015-12-28 (OK)
    System.out.println(
              lastWeekOfYearOld(LocalDate.of(2016, 1, 1))); // 2016-12-26 (Wrong)

    System.out.println(
              lastWeekOfYearCorrect(LocalDate.of(2015, 12, 31))); // 2015-12-28 (OK)
    System.out.println(
              lastWeekOfYearCorrect(LocalDate.of(2016, 1, 1))); // 2015-12-28 (OK)

    System.out.println(
              lastWeekOfYearAlternative(LocalDate.of(2015, 12, 31))); // 2015-12-28 (OK)
    System.out.println(
              lastWeekOfYearAlternative(LocalDate.of(2016, 1, 1))); // 2015-12-28 (OK)
}

private static LocalDate lastWeekOfYearOld(LocalDate date) {
    return LocalDate.of(date.getYear() + 1, 1, 7)
         .with(WeekFields.ISO.weekOfWeekBasedYear(), 1)
         .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY)).minusWeeks(1);
}

private static LocalDate lastWeekOfYearCorrect(LocalDate date) {
    date =
      date.plus(1, IsoFields.WEEK_BASED_YEARS)
          .with(DayOfWeek.MONDAY)
          .with(WeekFields.ISO.weekOfWeekBasedYear(), 1);
    return date.minusWeeks(1);
}

private static LocalDate lastWeekOfYearAlternative(LocalDate date) {
    TemporalField woyField = WeekFields.ISO.weekOfWeekBasedYear();
    date = date.with(woyField, woyField.rangeRefinedBy(date).getMaximum());
    return date.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
}

Answer 4

tl;博士

org.threeten.extra.YearWeek.from(  // Represents an entire week at a time, using standard ISO 8601 definition of “week”. 
    LocalDate.of( 2016 , Month.FEBRUARY , 17 )
)                                  // Returns a `YearWeek` object to represent the week containing that date. 
.plusYears( 1 )                    // Move to following year.
.withWeek( 1 )                     // Move to the first week of that following year.
.minusWeeks( 1 )                   // Move to the prior week, which would be the last week of our desired week-based-year.
.atDay(                            // Determine the date of a particular day-of-week within that week.
    DayOfWeek.MONDAY               // Use the handy `java.time.DayOfWeek` enum predefining seven objects, one for each day of the week.
)                                  // Returns a `LocalDate` object.
.toString()                        // Generate a String representing this `LocalDate` value in standard ISO 8601 format.

2016-12-26

YearWeek

ThreeTen-Extra 库提供了额外的日期时间类来补充 java.time 中的那些。

该库中的 YearWeek 类适合您的目的。此类使用标准 ISO 8601 定义表示基于一周的一年中的整周。

获取当前的年-周。

YearWeek yw = YearWeek.now( ZoneId.of( "Africa/Tunis" ) ) ;  // Get the current year-week as seen in the wall-clock time of people inhabiting a particular region (a time zone). 

或者获取特定日期的YearWeek。

LocalDate ld = LocalDate.of( 2016 , Month.FEBRUARY , 17 ) ;  // Feb 2, 2016.
YearWeek yw = YearWeek.from( ld ) ;

确定今年的最后一周数，52 或 53。这里我们使用三元运算符而不是较长的 if-else 语句。

int w = yw.is53WeekYear​() ? 53 : 52 ;  // Determine last week number, either 52 or 53.

移至本年度的最后一周。

YearWeek ywLastOfYear = yw.withWeek( w ) ; // Produce a new `YearWeek` object based on the original’s values but for some adjustment - here we use the same year but use a different week number.

最后，得到该周第一天的日期，第一天是星期一。

LocalDate firstDayOfLastYearWeek = ywLastOfYear.atDay( DayOfWeek.MONDAY ) ; 

这是一种方法。有关另一种方法，请参阅上面的 tl;dr 部分。