LocalDate.plus错误答案

Question

Java的LocalDate API似乎在plus(...)用long 调用时给出了错误的答案Period，在这种情况下我遇到一个错误。我在这里做错什么了吗？

import java.time.LocalDate;
import java.time.Month;
import java.time.Period;
import java.time.temporal.ChronoUnit;

public class Main
{
    public static void main(String[] args)
    {
        // Long Period
        LocalDate birthA = LocalDate.of(1965, Month.SEPTEMBER, 27);
        LocalDate eventA = LocalDate.of(1992, Month.MAY, 9);
        LocalDate halfA = eventA.plus(Period.between(birthA, eventA));
        System.out.println(halfA); // 2018-12-21 ????
        System.out.println(ChronoUnit.DAYS.between(birthA, eventA)); // 9721
        System.out.println(ChronoUnit.DAYS.between(eventA, halfA)); // 9722 ????

        // Short Period
        LocalDate birthB = LocalDate.of(2012, Month.SEPTEMBER, 10);
        LocalDate eventB = LocalDate.of(2012, Month.SEPTEMBER, 12);
        LocalDate halfB = eventB.plus(Period.between(birthB, eventB));
        System.out.println(halfB); // 2018-09-14
        System.out.println(ChronoUnit.DAYS.between(birthB, eventB)); // 2
        System.out.println(ChronoUnit.DAYS.between(eventB, halfB)); // 2
    }
}

Answer 1

A Period由数年，数月和数天组成。您的情况Period.between(birthA, eventA)是26年7个月12天。

如果将其添加到中birthA，则会得到：

• 1965年+ 26年-> 1991年
• 1991年9月+ 7个月-> 1991年4月
• 1991年4月27日+ 12天-> 1992年5月9日

哪个按预期工作。

If you apply the same calculation, starting from May 9, 1992, you get December 21, 2018.

If you want to add a certain number of days instead, you can't simply add the period (as years and months don't always have the same length). One option is to use ChonoUnit.DAYS.between instead:

LocalDate halfA = eventA.plusDays(ChronoUnit.DAYS.between(birthA, eventA));

That returns 2018-12-20 which I think is what you expected.

Answer 2

To complement assylias' answer, here is a simplified example to show why this happens:

    public static void main(String[] args)
    {
        LocalDate a = LocalDate.of(1992, Month.APRIL, 1);
        LocalDate b = LocalDate.of(1992, Month.MAY, 1);
        // Calculate the period. It will return "One month"
        Period period = Period.between(a, b);
        // Add one month to b. It will return June 1, 1992
        LocalDate c = b.plus(period);
        System.out.println(ChronoUnit.DAYS.between(a, b)); // 30 days as April has 30 days
        System.out.println(ChronoUnit.DAYS.between(b, c)); // 31 days as May has 31 days
    }