ArrayList<Integer> nodeList = new ArrayList<>(network.nodes());
      for (int i = 0; i < NUM_EDGES; ++i) {
        // Parallel edges are allowed, so this should always succeed.
        assertThat(
                network.addEdge(
                    getRandomElement(nodeList, gen), getRandomElement(nodeList, gen), new Object()))
            .isTrue();
      }
      ArrayList<Object> edgeList = new ArrayList<>(network.edges());

      ArrayList<Integer> nodeList = new ArrayList<>(network.nodes());
      for (int i = 0; i < NUM_EDGES; ++i) {
        // Parallel edges are allowed, so this should always succeed.
        assertThat(
                network.addEdge(
                    getRandomElement(nodeList, gen), getRandomElement(nodeList, gen), new Object()))
            .isTrue();
      }
      ArrayList<Object> edgeList = new ArrayList<>(network.edges());

      for (E edge : network.outEdges(node)) {
        N successorNode = network.incidentNodes(edge).adjacentNode(node);
        if (subgraph.nodes().contains(successorNode)) {
          subgraph.addEdge(node, successorNode, edge);
        }
      }
    }
    return subgraph;
  }

  /** Creates a mutable copy of {@code graph} with the same nodes and edges. */