code: public static final long fromVertexID(TitanVertex v) {
          Preconditions.checkArgument(v.hasId(), "Invalid vertex provided: %s", v);
          return fromVertexId(v.longId());
      }
docs: Converts a Titan vertex id of a given vertex to the user provided id as the inverse mapping of {@link #toVertexId(long)}.
  
  @param v Vertex
  @return original user provided id