将Collection转换为Map< String，Collection< String>>使用Java 8流 [英] Convert a Collection to Map<String,Collection<String>> using java 8 streams

问题描述

我正在寻找使用java流将Collection<Placement>转换为Map<String, Collection<String>>的方法，其中Placement类如下

I am looking to convert a Collection<Placement> to Map<String, Collection<String>> using java streams, where the Placement class is as below

public class Placement {

   private String id;

   /* Links two placements, may be same for atleast 2 placements. */
   private String futureLinkId;
}

地图的键是将来的链接ID，值是展示位置ID.例如，如果有2个展示位置 p1 { id=1, futureLinkId=f1 }, p2 { id=2, futureLinkId=f1 }，输出应为Map { f1 - { id1, id2 } }

The key for the map is future link id and the values are placement ids. For example if there are 2 placements p1 { id=1, futureLinkId=f1 }, p2 { id=2, futureLinkId=f1 } , the output should be Map { f1 - { id1, id2 } }

推荐答案

尽管其他人提供了复杂的解决方案，但这实际上很简单:

This is actually quite simple, despite the convoluted solutions others have offered:

Map<String, List<String>> map = placements.stream()
        .collect(groupingBy(Placement::getFutureLinkId, mapping(Placement::getId, toList())));

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