@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 == null) {
	      if (other.id != null)
	        return false;
	    } else if (!id.equals(other.id))
	      return false;
	    return true;
	  }