将Long列表转换为Map，其中long项为键，String列表为值[duplicate]

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• how to use java 8 stream and lambda to flatMap a groupingBy result 2 answers

如何使用java 8 stream和lambda来flatMap一个groupingBy结果2个答案

I have a person

我有一个人

class Person{
  private Long id;
  private String firstName;
  private String lastName;
  //with getters and setters
}

I have a following method that receives a person's id and returns a list of tags that are associated with the person

我有一个以下方法接收一个人的id并返回与该人相关联的标签列表

public List<String> getTags(Long personId){
 /*ExampleImplementation*/
   someCollection.stream()
     .filter(Objects::nonNull)
     .map(SomeType::someMethod)
     .filter(Objects:nonNull)
     .collect(Collectors.toList());
}

I am building a method in order to get a map of all the personIds and the list of tags associated with the personId.

我正在构建一个方法,以获取所有personIds的映射以及与personId关联的标记列表。

public Map<Long, List<String>>getPersonTags(List<Long> personIds){
  return personIds.stream()
            .collect(Collectors.groupingBy(
            (personId) -> personId,
            Collectors.mapping(this::getTags, Collectors.toList())));
}

But getPersonTags() is returning a Map<Long, List<List<String>>> but I expect a Map<Long, List<String>> What should I change to get my expected return type.

但getPersonTags()返回一个Map >>但我希望Map >我应该更改什么来获得我期望的返回类型。 ,list> ,list>

1 个解决方案

#1

If personIds contains distinct values, you don't need to use groupingBy(). Use toMap() instead:

如果personIds包含不同的值,则不需要使用groupingBy()。使用toMap()代替:

return personIds.stream()
        .collect(Collectors.toMap(Function.identity(), this::getTags));