code: public Set<T> getEdges(int vertex1, int vertex2) {
          EdgeSet<T> e = getEdgeSet(vertex1);
          if (e == null)
              return Collections.<T>emptySet();
          Set<T> edges = e.getEdges(vertex2);
          return edges.isEmpty() ? Collections.<T>emptySet() : edges;
      }
docs: {@inheritDoc}