Question

对于这个玩具示例，我有一个带有id和名称的教室列表。我从数据源中取出这些教室，然后，对于每个教室，我都需要学习和教师列表。然后，我想创建另一个对象，该对象包含在classroom.id，classroom.name和List<Student>, List<Teacher>中，称为Foo。问题是Foo是一个不可变对象，必须在所有信息可用时进行实例化。这是我尝试过的：

List<Classroom> classrooms = dataSource.fetchClassroomsById(classroomIds);
Map<Integer, Classroom> idToClassroom = new HashMap<>();
        classrooms.stream()
            .forEach(classroom -> {idToClassroom.put(classroom.getId(), classroom);});

//get all students and teachers that belong to set of classroomIds passed in
List<Student> students = dataSource.fetchStudentsById(classroomIds);
List<Teacher> teachers = dataSource.fetchTeachersById(classroomIds);

for (Student student : students) {
    //gets the classroom for which student belongs. This is where I get stuck.
    idsToClassroom.get(student.getClassRoomId())
}
...

基本上，我怎样才能有效而紧凑地构建我的Foo对象？

Answer 1

使用Collectors.groupingBy将学生和教师分组到相同教室ID的列表中。

List<Classroom> classrooms = dataSource.fetchClassroomsById(classroomIds);
List<Student> students = dataSource.fetchStudentsById(classroomIds);
List<Teacher> teachers = dataSource.fetchTeachersById(classroomIds);

Map<Integer, List<Student>> classroomIdToStudent = students.stream()
    .collect(Collectors.groupingBy(Student::getClassroomId));

Map<Integer, List<Teacher>> classroomIdToTeacher = teachers.stream()
    .collect(Collectors.groupingBy(Teacher::getClassroomId));

List<Foo> foos = classrooms.stream().map(c -> new Foo(c.getId(), c.getName(),
    classroomIdToStudent.get(c.getId()), classroomIdToTeacher.get(c.getId()))).collect(Collectors.toList());

有效地构建对象的相关层次结构

1 个答案: