if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException("Node " + node + " is not an element of this graph.");
    }
    return connections;
  }

  final boolean containsNode(@Nullable N node) {
    return nodeConnections.containsKey(node);
  }

  private final boolean hasEdgeConnectingInternal(N nodeU, N nodeV) {
    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);