if (mark > 0)
if (mark >= 0)
  Buffer (int capacity, int limit, int position, int mark)  {    if (capacity < 0)      throw new IllegalArgumentException ();        cap = capacity;    limit (limit);    position (position);        if (mark > 0)    {      if (mark > pos)        throw new IllegalArgumentException ();            this.mark = mark;    }  }