如何在多级嵌套类中使用stream API的groupingBy？

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

class School {
    private List<Class> classes;

    public School(List<Class> classes) {
        this.classes = classes;
    }

    public List<Class> getClasses() {
        return classes;
    }
}

class Class {
    private int classId;
    private List<Student> students;

    public Class(int classId, List<Student> students) {
        this.classId = classId;
        this.students = students;
    }

    public int getClassId() {
        return classId;
    }

    public List<Student> getStudents() {
        return students;
    }
}

class Student {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

public class Main {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
                new Student("Alice", 18),
                new Student("Bob", 19),
                new Student("Charlie", 18),
                new Student("David", 19),
                new Student("Eve", 18)
        );

        List<Class> classes = Arrays.asList(
                new Class(1, students.subList(0, 2)),
                new Class(2, students.subList(2, 4)),
                new Class(3, students.subList(3, 5))
        );

        School school = new School(classes);

        Map<Integer, Map<Integer, List<Student>>> groupedStudents = school.getClasses().stream()
                .collect(Collectors.groupingBy(Class::getClassId,
                        Collectors.groupingBy(Student::getAge)));

        System.out.println(groupedStudents);
    }
}