Java:将子值作为值分组 [英] Java: groupingBy subvalue as value

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问题描述

比方说,我有一个对象Person,其字段类型为FirstName和LastName.现在,我还有一个 List< Person> ,并且我喜欢使用流.

Let's say, I have an object Person with fields of type FirstName and LastName. Now I also have a List<Person> and I like to use streams.

现在,我想生成一个 Map< FirstName,List< LastName>> 以便对具有相同名字的人进行分组.如何在不编写大量代码的情况下进行此操作?到目前为止,我的方法是

Now I want to generate a Map<FirstName, List<LastName>> in order to group people with the same first name. How do I go about this without writing much code? My approach so far is

personList
.stream()
.collect(Collectors.groupingBy(
    Person::getFirstName,
    person -> person.getLastName() // this seems to be wrong
));

,但是这似乎是分配地图值的错误方法.我应该改变什么?还是我应该将.reduce与 new HashMap< FirstName,List< LastName>>>()用作初始值,然后通过将元素放入其中来汇总到它?

but it seems this is the wrong way to assign the value of the map. What should I change? Or should I perhaps use .reduce with new HashMap<FirstName, List<LastName>>() as initial value and then aggregate to it by putting elements inside?

推荐答案

personList.stream()
          .collect(Collectors.groupingBy(
               Person::getFirstName,
               Collectors.mapping(Person::getLastName, Collectors.toList())));

您正在寻找具有 groupingBy

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