docs: public <K> Graph<K, NullValue, NullValue> keyType(Class<K> vertexKey) {
  
  		if (edgeReader == null) {
  			throw new RuntimeException("The edge input file cannot be null!");
  		}
  
  		DataSet<Edge<K, NullValue>> edges = edgeReader
  			.types(vertexKey, vertexKey)
  				.name(GraphCsvReader.class.getName())
  			.map(new Tuple2ToEdgeMap<>())
  				.name("Type conversion");
  
  		return Graph.fromDataSet(edges, executionContext);
  	}