createStudentAndSelectCourse里为什么不能用Student student？而要先在外面添加属性Student再在这里面用？
public void createStudentAndSelectCourse(){
student = new Student("1","小明");
System.out.println("欢迎学生" + student.name + "选课！");
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.course.add(cr);
}
}
}
for(Course cr:student.course){
System.out.println("已选课程："+ cr.id + ":" + cr.name);
}
}
