public void addEdge(Edge edge) {

                // check for already added removed
                // if (adjacentEdges.contains(edge)) {
                // throw new
                // IllegalArgumentException("Edge has already been added");
                // } else

                if (howManyEdges() >= 4) {
                        throw new IllegalArgumentException("Cannot add more than 4 edges");
                }
                else if (edge == null) {
                        throw new IllegalArgumentException("You tried to add an adjacent null edge");
                }

                adjacentEdges.add(edge);
        }