Vertex c = graph.getVertex("c");
        Vertex d = graph.getVertex("d");
        assertEquals("a", a.getLabel());
        assertEquals("b", b.getLabel());
        assertEquals("c", c.getLabel());
        assertEquals("d", d.getLabel());
        assertFalse(hasEdge(graph, "b", "a"));
        assertFalse(hasEdge(graph, "a", "c"));
        assertFalse(hasEdge(graph, "a", "d"));

        final List<Vertex> children = new ArrayList<>();
        final List<Vertex> parents = new ArrayList<>();

        Vertex(String label) {
            this.label = label;
        }

        String getLabel() {
            return label;
        }

        List<Vertex> getChildren() {
            return children;
        }

        List<Vertex> getParents() {
            return parents;
        }

        final List<Vertex> children = new ArrayList<>();
        final List<Vertex> parents = new ArrayList<>();

        Vertex(String label) {
            this.label = label;
        }

        String getLabel() {
            return label;
        }

        List<Vertex> getChildren() {
            return children;
        }

        List<Vertex> getParents() {
            return parents;
        }