}

      @Override
      public boolean isDirected() {
        return AbstractBaseGraph.this.isDirected();
      }

      @Override
      public boolean allowsParallelEdges() {
        return false; // Graph doesn't allow parallel edges
      }

      @Override
      public boolean allowsSelfLoops() {
        return AbstractBaseGraph.this.allowsSelfLoops();
      }

      GraphBuilder.directed().<String>immutable().addNode("B").build();

  private static final NetworkBuilder<?, ?> networkBuilderA =
      NetworkBuilder.directed().allowsParallelEdges(true).expectedNodeCount(10);
  private static final NetworkBuilder<?, ?> networkBuilderB =
      NetworkBuilder.directed().allowsSelfLoops(true).expectedNodeCount(16);

   * an edge connecting node A to node B if this {@link Network} has an edge connecting A to B.
   *
   * <p>If this network {@link #allowsParallelEdges() allows parallel edges}, parallel edges will be
   * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node

        EndpointPair.ordered(N4, N4));
  }

  @Test
  public void endpointPair_undirectedNetwork() {
    MutableNetwork<Integer, String> undirectedNetwork =
        NetworkBuilder.undirected().allowsParallelEdges(true).allowsSelfLoops(true).build();
    undirectedNetwork.addNode(N0);
    undirectedNetwork.addEdge(N1, N2, E12);
    undirectedNetwork.addEdge(N2, N1, E12_A); // adds parallel edge, won't be in Graph edges

        EndpointPair.ordered(N4, N4));
  }

  @Test
  public void endpointPair_undirectedNetwork() {
    MutableNetwork<Integer, String> undirectedNetwork =
        NetworkBuilder.undirected().allowsParallelEdges(true).allowsSelfLoops(true).build();
    undirectedNetwork.addNode(N0);
    undirectedNetwork.addEdge(N1, N2, E12);
    undirectedNetwork.addEdge(N2, N1, E12_A); // adds parallel edge, won't be in Graph edges

    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,

    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,

    Random gen = new Random(42); // Fixed seed so test results are deterministic.

    for (int trial = 0; trial < NUM_TRIALS; ++trial) {
      MutableNetwork<Integer, Object> network =
          networkBuilder.allowsParallelEdges(true).allowsSelfLoops(true).build();

      assertThat(network.nodes()).isEmpty();
      assertThat(network.edges()).isEmpty();
      AbstractNetworkTest.validateNetwork(network);

    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,

    Random gen = new Random(42); // Fixed seed so test results are deterministic.

    for (int trial = 0; trial < NUM_TRIALS; ++trial) {
      MutableNetwork<Integer, Object> network =
          networkBuilder.allowsParallelEdges(true).allowsSelfLoops(true).build();

      assertThat(network.nodes()).isEmpty();
      assertThat(network.edges()).isEmpty();
      AbstractNetworkTest.validateNetwork(network);