Java按公用值合并流

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<Person> persons = Arrays.asList(
                new Person("John", "A"),
                new Person("Jane", "A"),
                new Person("Tom", "B"),
                new Person("Alice", "B")
        );

        Map<String, String> mergedMap = persons.stream()
                .collect(Collectors.groupingBy(Person::getGroup, Collectors.mapping(Person::getName, Collectors.reducing("", (s1, s2) -> s1 + ", " + s2))))
                .entrySet()
                .stream()
                .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().orElse("")));

        System.out.println(mergedMap);
    }

    static class Person {
        private String name;
        private String group;

        public Person(String name, String group) {
            this.name = name;
            this.group = group;
        }

        public String getName() {
            return name;
        }

        public String getGroup() {
            return group;
        }
    }
}