代码太乱了，当自己写时，懵逼了

// 创建学生对象并选课

public void createStudentAndSelectCours() {

//创建一个学生对象

student = new Student("1", "小明");

System.out.println("欢迎学生：" + student.name + "选课！");

//创建一个Scanner对象，用来接收从键盘输入的课程ID

Scanner console = new Scanner(System.in);

for (int i = 0; i < 3; i++) {

System.out.println("请输入课程ID");

String courseId = console.next();

for (Course cr : coursesToSelect) {

if(cr.id.equals(courseId)) {

student.courses.add(cr);        //这里报错了，

}

}

}

}

报错原因：

The method add(Collection_ArrayList.Course) in the type Set<Course> is not applicable for the arguments (CollectionMethod.Course)