如何使用Java流将一组对象转换为该对象的字段映射到一组字段？

Question

If I have some class A

public class A {
   String name;

   String value;
}

I have a Set which I want to convert into a Map<String, Set<String>> , which maps A.name to a set of A.value, using the stream class, since many instances of A will share the same name String, but none will share the same value String. Is there a way to do this?

Answer 1

To create a Map<String, Set<String>> , which maps A.name to a set of A.value , you need to use the following methods:

• stream()

• collect(Collector<? super T,A,R> collector)

• groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)

• mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)

• toSet()

Map<String, Set<String>> map = set.stream()
        .collect(Collectors.groupingBy(A::getName,
                                       Collectors.mapping(A::getValue,
                                                          Collectors.toSet())));

The code uses method references to getter methods on your A class, though lambda expressions can be used instead, eg A::getName could be replaced with o -> o.name .

Answer 2

您可以在转换之前实例化一个空的Map，然后将使用匿名函数收集转换，该函数将检查该Map以查看它是否应该放置一个新集合或更新一个现有集合。