Java语言。向类抛出groupingBy

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("Alice", 20),
                new Person("Bob", 25),
                new Person("Charlie", 20),
                new Person("David", 25)
        );

        Map<Integer, List<Person>> groupedByAge = persons.stream()
                .collect(Collectors.groupingBy(Person::getAge));

        System.out.println(groupedByAge);
    }
}

class Person {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return name;
    }
}

{20=[Alice, Charlie], 25=[Bob, David]}