映射结果到嵌套映射的Collectors.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) {
        // 创建一个包含多个Person对象的列表
        List<Person> persons = Arrays.asList(
                new Person("Alice", 25, "Developer"),
                new Person("Bob", 30, "Manager"),
                new Person("Charlie", 35, "Developer"),
                new Person("David", 40, "Manager")
        );

        // 使用Collectors.groupingBy对职位进行分组，并将分组结果映射为嵌套的映射结构
        Map<String, Map<Integer, List<Person>>> result = persons.stream()
                .collect(Collectors.groupingBy(Person::getPosition,
                        Collectors.groupingBy(Person::getAge)));

        // 打印分组结果
        result.forEach((position, nestedMap) -> {
            System.out.println(position + ":");
            nestedMap.forEach((age, personList) -> {
                System.out.println("  " + age + ": " + personList);
            });
        });
    }
}

class Person {
    private String name;
    private int age;
    private String position;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getPosition() {
        return position;
    }

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

Developer:
  25: [Alice]
  35: [Charlie]
Manager:
  30: [Bob]
  40: [David]