使用Java Streams按Collection属性进行分组

Question

我有一个包含字符串集合的对象,比方说一个人说的语言.

public class Person {
   private String name;
   private int age;
   private List<String> languagesSpoken;

   // ...
}

现在,创建像这样的一些实例......

Person p1 = new Person("Bob", 21, Arrays.asList("English", "French", "German"));
Person p2 = new Person("Alice", 33, Arrays.asList("English", "Chinese", "Spanish"));
Person p3 = new Person("Joe", 43, Arrays.asList("English", "Dutch", "Spanish", "German"));

//put them in list
List<Person> people = Arrays.asList(p1,p2,p3);

...我想要的是Map<String, List<Person>>,对于每种语言列出说这种语言的人:

["English" -> [p1, p2, p3],
 "German"  -> [p1, p3],
 etc. ]

当然,这可以通过命令式方式轻松编程,但如何使用Java Streams实现功能？我尝试了类似的东西people.stream.collect(groupingBy(Person::getLanguagesSpoken))但当然给了我一个Map<List<String>, List<Person>>.我能找到的所有例子都groupingBy在Primitives或Strings上使用.

Answer 1

您可以将Person实例分解为成对的语言和Person(使用flatMap),然后根据需要对它们进行分组:

Map<String, List<Person>> langPersons =
    people.stream()
          .flatMap(p -> p.getLanguagesSpoken()
                         .stream()
                         .map(l -> new SimpleEntry<>(l,p)))
          .collect(Collectors.groupingBy(Map.Entry::getKey,
                                         Collectors.mapping(Map.Entry::getValue,
                                                            Collectors.toList())));