A very common format of graphs is adjacency lists with no values associated to edges or vertices. For instance a line in the input can be of the type:
1 2 3
which represents a vertex with id 1 that has edges to vertices 2 and 3 with no values associated.
I've created a vertex reader named AdjacencyListVertexReader which is essentially a copy of the AdjacencyListVertexReader modified to handle this format. It's an abstract class and subclasses can override the defaultVertexValue() and defaultEdgeValue() methods to provide default values for vertices and edges correspondingly (otherwise values are initialized to null).
I've also created an example subclass.