Array List 내용 수정

public static void main(String [] as){
 Student a = new Student("a", 80);
 Student b = new Student("b", 90);
 Student c = new Student("c", 70);
 List<Student> list = new ArrayList<Student>();
 list.add(a);
 list.add(b);
 list.add(c);
 System.out.println("    :");
 for(Student s: list){
 System.out.println(s);
 }
 a.setMark(100);
 list.set(0, a);
 System.out.println("    :");
 for(Student s: list){
 System.out.println(s);
 }
}
}