学生管理系统学号重复问题

public static void addStudent(ArrayList<Student> array) {
    Scanner sc = new Scanner(System.in);

    String sid;
    while (true) {
        System.out.println("请输入学生学号：");
        sid = sc.nextLine();

        boolean flag = isUsed(array, sid);
        if (flag) {
            System.out.println("您输入的学号已被使用，请重新输入");
        } else {
            break;
        }
    }

    System.out.println("请输入学生姓名：");
    String name = sc.nextLine();
    System.out.println("请输入学生年龄：");
    String age = sc.nextLine();
    System.out.println("请输入学生居住地：");
    String address = sc.nextLine();

    Student s = new Student();
    s.setSid(sid);
    s.setName(name);
    s.setAge(age);
    s.setAddress(address);

    array.add(s);

    System.out.println("添加学生成功");
}

public static boolean isUsed(ArrayList<Student> array, String sid) {
    boolean flag = false;
    for (int i = 0; i < array.size(); i++) {
        Student s = array.get(i);
        if (s.getSid().equals(sid)) {
            flag = true;
            break;
        }
    }

    return flag;
}