使用stream()、collect()、groupingBy()和reducing()的多级映射

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

public class MultiLevelMappingExample {
    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stream()和collect()方法将列表转换为Map，按职位进行分组
        Map<String, List<Person>> groupByPosition = persons.stream()
                .collect(Collectors.groupingBy(Person::getPosition));

        // 输出按职位分组的结果
        System.out.println("按职位分组的结果：");
        groupByPosition.forEach((position, people) -> {
            System.out.println(position + ": " + people);
        });

        // 使用stream()、collect()、groupingBy()和reducing()方法进行多级映射
        Map<String, Map<Integer, Optional<Person>>> multiLevelMapping = persons.stream()
                .collect(Collectors.groupingBy(Person::getPosition,
                        Collectors.groupingBy(Person::getAge,
                                Collectors.reducing(BinaryOperator.maxBy(Person::compareByAge)))));

        // 输出多级映射的结果
        System.out.println("\n多级映射的结果：");
        multiLevelMapping.forEach((position, ageMap) -> {
            System.out.println(position + ":");
            ageMap.forEach((age, person) -> {
                System.out.println("  " + age + ": " + person.orElse(null));
            });
        });
    }

    // 定义一个Person类
    static 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;
        }

        public static int compareByAge(Person p1, Person p2) {
            return Integer.compare(p1.getAge(), p2.getAge());
        }

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