@Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (!(obj instanceof PostID))
      return false;
    PostID other = (PostID) obj;
    if (author == null) {
      if (other.author != null)
        return false;
    } else if (!author.equals(other.author))
      return false;
    if (timestamp != other.timestamp)
      return false;
    return true;
  }