为什么ZoneOffset.UTC！= ZoneId.of（" UTC"）？

问题描述

为什么

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

打印 false ？

我希望两个 ZonedDateTime 实例相等。

推荐答案

答案来自（强调我的）......

The answer comes from the javadoc of ZoneId (emphasis mine) ...

• 固定偏移 - 与UTC / Greenwich完全解析的偏移量，对所有本地使用相同的偏移量date-times


• 地理区域 - 用于查找从UTC / Greenwich的偏移量的特定规则集的区域

大多数固定偏移量由ZoneOffset表示。 在任何ZoneId上调用normalized（）
将确保固定的偏移ID将表示为
作为ZoneOffset。

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.

...来自（强调我的）：

... and from the javadoc of ZoneId#of (emphasis mine):

参数id指定为UTC，因此它将返回 ZoneId 偏移量，也以字符串形式显示：

The argument id is specified as "UTC", therefore it will return a ZoneId with an offset, which also presented in the string form:

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]

当您使用等于方法进行比较时，检查对象是否等效。由于描述的差异，评估的结果是 false 。

As you use the equals method for comparison, you check for object equivalence. Because of the described difference, the result of the evaluation is false.

当方法按照文档中的建议使用，使用等于的比较将返回 true ，如 normalized（）将返回相应的 ZoneOffset ：

When the normalized() method is used as proposed in the documentation, the comparison using equals will return true, as normalized() will return the corresponding ZoneOffset:

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

如文档所述，如果使用Z或+ 0作为输入ID，的将返回 ZoneOffset 直接，无需调用 normalized（）：

As the documentation states, if you use "Z" or "+0" as input id, of will return the ZoneOffset directly and there is no need to call normalized():

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

---

检查是否为存储相同的日期时间，您可以使用方法改为：

---

To check if they store the same date time, you can use the isEqual method instead:

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

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