如何使用流外的值创建带有 Java 流 API 的地图? [英] How to create a map with Java stream API using a value outside the stream?

问题描述

我想初始化一个 Map<String, BigDecimal> 并希望始终从流外部放置相同的 BigDecimal 值.

I want to init a Map<String, BigDecimal> and want to always put the same BigDecimal value from outside of the stream.

BigDecimal samePrice;
Set<String> set;

set.stream().collect(Collectors.toMap(Function.identity(), samePrice));

然而 Java 抱怨如下:

However Java complains as follows:

类型 Collectors 中的方法 toMap(Function, Function) 不适用于参数(函数，BigDecimal)

The method toMap(Function, Function) in the type Collectors is not applicable for the arguments (Function, BigDecimal)

为什么我不能从外部使用 BigDecimal?如果我写:

Why can't I use the BigDecimal from outside? If I write:

set.stream().collect(Collectors.toMap(Function.identity(), new BigDecimal()));

它会起作用，但这当然不是我想要的.

it would work, but that's of course not what I want.

推荐答案

toMap(keyMapper, valueMapper) 是一个函数，它接受流元素并返回映射的值.

The second argument (like the first one) of toMap(keyMapper, valueMapper) is a function that takes the stream element and returns the value of the map.

在这种情况下，你想忽略它，这样你就可以拥有:

In this case, you want to ignore it so you can have:

set.stream().collect(Collectors.toMap(Function.identity(), e -> samePrice));

请注意，出于同样的原因，您的第二次尝试不会奏效.

Note that your second attempt wouldn't work for the same reason.

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