Compareble自然排序器

让元素所属的类实现Comparable接口，重写compareTo(Object o)方法
Collections调用sort方法 即可按照规则排序

  public int compareTo(Object o) {
        Student current=(Student)o;
        //按照成绩升序score-current.score，降序current.score-score
        return (int)(score-current.score);
    }