@Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    Vertex other = (Vertex) obj;
    if (id != other.id) {
      return false;
    }
    return true;
  }