ZoneOffset.UTC 和 ZoneId.of("UTC") 有什么区别？

Question

为什么

ZonedDateTime now = ZonedDateTime.now();
System.out.println(now.withZoneSameInstant(ZoneOffset.UTC)
        .equals(now.withZoneSameInstant(ZoneId.of("UTC"))));

打印出来false?

我希望两个 ZonedDateTime 实例相等。

Answer 1

答案来自 javadoc of ZoneId（强调我的）...

A ZoneId is used to identify the rules used to convert between an Instant and a LocalDateTime. There are two distinct types of ID:

• Fixed offsets - a fully resolved offset from UTC/Greenwich, that uses the same offset for all local date-times
• Geographical regions - an area where a specific set of rules for finding the offset from UTC/Greenwich apply

Most fixed offsets are represented by ZoneOffset. Calling normalized() on any ZoneId will ensure that a fixed offset ID will be represented as a ZoneOffset.

... 并且来自 javadoc of ZoneId#of（强调我的）：

This method parses the ID producing a ZoneId or ZoneOffset. A ZoneOffset is returned if the ID is 'Z', or starts with '+' or '-'.

参数id被指定为"UTC"，因此它会return一个带有偏移量的ZoneId，同样以字符串形式呈现：

System.out.println(now.withZoneSameInstant(ZoneOffset.UTC));
System.out.println(now.withZoneSameInstant(ZoneId.of("UTC")));

输出：

2017-03-10T08:06:28.045Z
2017-03-10T08:06:28.045Z[UTC]

当您使用 equals 方法进行比较时，您 检查对象等价性 。由于描述的差异，评估结果为false.

当按照文档中的建议使用 normalized() 方法时，使用 equals 的比较将 return true，因为 normalized() 将 return对应的是ZoneOffset:

Normalizes the time-zone ID, returning a ZoneOffset where possible.

now.withZoneSameInstant(ZoneOffset.UTC)
    .equals(now.withZoneSameInstant(ZoneId.of("UTC").normalized())); // true

如文档所述，如果您使用 "Z" 或 "+0" 作为输入 ID，of 将直接 return ZoneOffset 并且没有需要调用 normalized():

now.withZoneSameInstant(ZoneOffset.UTC).equals(now.withZoneSameInstant(ZoneId.of("Z"))); //true
now.withZoneSameInstant(ZoneOffset.UTC).equals(now.withZoneSameInstant(ZoneId.of("+0"))); //true

---

要检查它们是否存储相同的日期时间，您可以改用isEqual方法：

now.withZoneSameInstant(ZoneOffset.UTC)
    .isEqual(now.withZoneSameInstant(ZoneId.of("UTC"))); // true

---

样本

System.out.println("equals - ZoneId.of(\"UTC\"): " + nowZoneOffset
        .equals(now.withZoneSameInstant(ZoneId.of("UTC"))));
System.out.println("equals - ZoneId.of(\"UTC\").normalized(): " + nowZoneOffset
        .equals(now.withZoneSameInstant(ZoneId.of("UTC").normalized())));
System.out.println("equals - ZoneId.of(\"Z\"): " + nowZoneOffset
        .equals(now.withZoneSameInstant(ZoneId.of("Z"))));
System.out.println("equals - ZoneId.of(\"+0\"): " + nowZoneOffset
        .equals(now.withZoneSameInstant(ZoneId.of("+0"))));
System.out.println("isEqual - ZoneId.of(\"UTC\"): "+ nowZoneOffset
        .isEqual(now.withZoneSameInstant(ZoneId.of("UTC"))));

输出：

equals - ZoneId.of("UTC"): false
equals - ZoneId.of("UTC").normalized(): true
equals - ZoneId.of("Z"): true
equals - ZoneId.of("+0"): true
isEqual - ZoneId.of("UTC"): true