为什么Instant不支持使用ChronoUnit.YEARS进行操作？ [英] Why Instant does not support operations with ChronoUnit.YEARS?

问题描述

这对我来说意外：

> Clock clock = Clock.systemUTC();

> Instant.now(clock).minus(3, ChronoUnit.DAYS);
java.time.Instant res4 = 2016-10-04T00:57:20.840Z

> Instant.now(clock).minus(3, ChronoUnit.YEARS);
java.time.temporal.UnsupportedTemporalTypeException: Unsupported unit: Years

作为解决方法我有要做到这一点：

As a workaround I have to do this:

> Instant.now(clock).atOffset(ZoneOffset.UTC).minus(3, ChronoUnit.YEARS).toInstant();
java.time.Instant res11 = 2013-10-07T01:02:56.361Z

我很好奇为什么Instant不支持YEARS。开发人员是否放弃了它？

I am curios why Instant does not support YEARS. Did developers just give up on it?

（在我的实际代码中，我试图减去 Period.ofYears（3）但是引用的Instant方法最后被调用了。）

(In my actual code I tried to subtract a Period.ofYears(3) but the quoted Instant methods are the ones being called in the end).

推荐答案

我正在刺它看起来像是非常符合逻辑的东西。

I'm taking a stab at it in what looks to me like something very logical.

这是方法的代码 plus（long，TemporalUnit）（用于减去（...））：

Here is the code for the method plus(long, TemporalUnit) (which is used in minus(...)):

     @Override
     public Instant plus(long amountToAdd, TemporalUnit unit) {
         if (unit instanceof ChronoUnit) {
             switch ((ChronoUnit) unit) {
                 case NANOS: return plusNanos(amountToAdd);
                 case MICROS: return plus(amountToAdd / 1000_000, (amountToAdd % 1000_000) * 1000);
                 case MILLIS: return plusMillis(amountToAdd);
                 case SECONDS: return plusSeconds(amountToAdd);
                 case MINUTES: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_MINUTE));
                 case HOURS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_HOUR));
                 case HALF_DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY / 2));
                 case DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY));
             }
             throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
         }
         return unit.addTo(this, amountToAdd);
     }

我们可以看到结果是通过乘以秒的单位表示来计算的，a由于显而易见的原因，年份不能以逻辑和一致的方式表示。

We can see that the results are calculated by multiplying seconds representation of units, a year cannot be logically and consistently represented by seconds for obvious reasons.

加法

我可以看到另一个明显的原因：上述方法中使用的常量来自 java.time.LocalTime 。常量仅定义单位为天。没有定义上面的常数（在 LocalDate 和 LocalDateTime 中）。

I can see another obvious reason why : constants used in the method above come from java.time.LocalTime. The constants only define units up to days. No constant above days are defined (in LocalDate and LocalDateTime neither).

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