流 - 嵌套集合 - 转换为地图

Question

假设我有2节课.

课程班

public class Course {
    private int id;
    private String name;
}

学生班

public class Student {
    private int id;
    private String name;
    private List<Course> courses;
}

我有List<Student>,每个人Student都注册了多门课程.

我需要使用Java 8流API过滤掉结果,如下所示.

Map<courseId, List<Student>> 

我在下面试过,但没有成功:

第一种方法

Map<Integer, List<Student>> courseStudentMap = studentList.stream()
    .collect(Collectors.groupingBy(
        student -> student.getCourses().stream()
            .collect(Collectors.groupingBy(Course::getId))));

第二种方法

Map<Integer, List<Student>> courseStudentMap = studentList.stream()
    .filter(student -> student.getCourses().stream()
        .collect(Collectors.groupingBy(
            Course::getId, Collectors.mapping(Student::student, Collectors.toList()))));

Answer 1

 Map<Integer, List<Student>> result = studentsList
            .stream()
            .flatMap(x -> x.getCourses().stream().map(y -> new SimpleEntry<>(x, y.getId())))
            .collect(Collectors.groupingBy(
                    Entry::getValue,
                    Collectors.mapping(Entry::getKey, Collectors.toList())));