@Override
    public void addEdge(E e, V v1, V v2) {
        if (!vertexToNeighbours.containsKey(v1)) {
            throw new IllegalArgumentException("Graph doesn't contain vertex " + v1);
        }
        if (!vertexToNeighbours.containsKey(v2)) {
            throw new IllegalArgumentException("Graph doesn't contain vertex " + v2);
        }
        UnorderedPair<V> endpoints = new UnorderedPair<>(v1, v2);
        if (edgeToEndpoints.containsValue(endpoints)) {
            throw new IllegalArgumentException("Graph already contains directed"
                    + " edge from vertex " + v1 + " to vertex " + v2 + ".");
        } else if (edgeToEndpoints.containsKey(e)) {
            throw new IllegalArgumentException("Graph already containts edge "
                    + e + "connecting these guys: " + edgeToEndpoints.get(e));
        } else {
            edgeToEndpoints.put(e, endpoints);
            vertexToNeighbours.get(v1).addNeighbour(v2);
            vertexToNeighbours.get(v2).addNeighbour(v1);
        }
    }