Java:如何按属性对列表元素进行分组

import java.util.*;
import java.util.stream.Collectors;

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 "Person{name='" + name + "', age=" + age + "}";
    }
}

public class GroupingExample {
    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
            new Person("Alice", 20),
            new Person("Bob", 20),
            new Person("Charlie", 30),
            new Person("David", 30)
        );

        // 按年龄分组
        Map<Integer, List<Person>> peopleByAge = people.stream()
            .collect(Collectors.groupingBy(Person::getAge));

        System.out.println(peopleByAge);
    }
}

Map<Integer, List<Person>> peopleByAge = people.stream()
    .filter(Objects::nonNull) // 排除null元素
    .collect(Collectors.groupingBy(Person::getAge));