必须partitioningBy生成一个包含true和false条目的地图？ [英] Must partitioningBy produce a map with entries for true and false?

问题描述

partitioningBy 收集器将谓词应用于流中的每个元素，并生成从布尔到满足或不满足谓词的流中元素列表的映射。例如：

The partitioningBy collector applies a predicate to each element in a stream and produces a map from booleans to lists of elements from the stream that satisfied or didn't satisfy the predicate. For instance:

Stream.of(1,2,3,4).collect(partitioningBy(x -> x >= 3))
// {false=[1, 2], true=[3, 4]}

正如分区的目的是什么所述，观察行为是 partitioningBy 始终返回包含 true 和 false 条目的地图。例如：

As discussed in What's the purpose of partitioningBy, the observed behavior is that partitioningBy always returns a map with entries for both true and false. E.g.:

Stream.empty().collect(partitioningBy(x -> false));
// {false=[], true=[]}

Stream.of(1,2,3).collect(partitioningBy(x -> false));
// {false=[1, 2, 3], true=[]}

Stream.of(1,2,3).collect(partitioningBy(x -> true));
// {false=[], true=[1, 2, 3]}

这种行为实际上是在某处指定的吗？ Javadoc只说：

Is that behavior actually specified somewhere? The Javadoc only says:

返回一个收集器，它根据
Predicate对输入元素进行分区，并将它们组织成一个 Map< Boolean，List< T>> 。对于返回的Map的类型，可变性，可序列化或
线程安全性，有
无保证。

Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

符合实现可以返回这些：

Could an conforming implementation return these instead:

Stream.empty().collect(partitioningBy(x -> false));
// {}, or {false=[]}, or {true=[]}

Stream.of(1,2,3).collect(partitioningBy(x -> false));
// {false=[1, 2, 3]}

Stream.of(1,2,3).collect(partitioningBy(x -> true));
// {true=[1, 2, 3]}

相应的 JSR 335 似乎只包含相同的文档，但没有关于哪些条目的额外讨论地图将包含。

The corresponding JSR 335 only seems to include the same documentation, but not additional discussion about what entries the map will contain.

推荐答案

在方法的Java 9 Javadoc ，有一个澄清，使其更明确：

In Java 9 Javadoc of the method, there is a clarification which makes it more explicit:

返回的Map总是包含false和true键的映射。

The returned Map always contains mappings for both false and true keys.

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