Java Stream Collectors.toMap值是一个Set [英] Java Stream Collectors.toMap value is a Set

问题描述

我想使用Java Stream来运行POJO列表，例如下面的列表 List< A> ，并将其转换为Map Map< String，Set< String>> 。

I want to use a Java Stream to run over a List of POJOs, such as the list List<A> below, and transform it into a Map Map<String, Set<String>>.

例如，A类是：

class A {
    public String name;
    public String property;
}

我写了下面的代码，将值收集到地图中 Map< String，String> ：

I wrote the code below that collects the values into a map Map<String, String>:

final List<A> as = new ArrayList<>();
// the list as is populated ...

// works if there are no duplicates for name
final Map<String, String> m = as.stream().collect(Collectors.toMap(x -> x.name, x -> x.property));

但是，因为可能有多个POJO具有相同的名称，我希望地图的值为 Set 。所有属性相同密钥名称的字符串应该进入相同的集合。

However, because there might be multiple POJOs with the same name, I want the value of the map be a Set. All property Strings for the same key name should go into the same set.

如何做到这一点？

// how do i create a stream such that all properties of the same name get into a set under the key name
final Map<String, Set<String>> m = ???

推荐答案

groupingBy 完全符合您的要求：

groupingBy does exactly what you want:

import static java.util.stream.Collectors.*;
...
as.stream().collect(groupingBy((x) -> x.name, mapping((x) -> x.property, toSet())));

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