在Java Lambda中返回二级列表

import java.util.ArrayList;
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 = new ArrayList<>();
        persons.add(new Person("Alice", 25));
        persons.add(new Person("Bob", 30));
        persons.add(new Person("Charlie", 35));
        persons.add(new Person("David", 40));

        Map<String, List<Person>> groupedPersons = persons.stream()
                .collect(Collectors.groupingBy(Person::getCategory));

        System.out.println(groupedPersons);
    }
}

class Person {
    private String name;
    private int age;

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

    public String getCategory() {
        if (age < 30) {
            return "Young";
        } else {
            return "Old";
        }
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

{Old=[Person{name='Bob', age=30}, Person{name='Charlie', age=35}, Person{name='David', age=40}], Young=[Person{name='Alice', age=25}]}