}
    return previousValue;
  }

  @Override
  @CanIgnoreReturnValue
  @CheckForNull
  public V putEdgeValue(EndpointPair<N> endpoints, V value) {
    validateEndpoints(endpoints);
    return putEdgeValue(endpoints.nodeU(), endpoints.nodeV(), value);
  }

  @Override
  @CanIgnoreReturnValue
  public boolean removeNode(N node) {
    checkNotNull(node, "node");

    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
    e =
        assertThrows(
            IllegalArgumentException.class, () -> graph.edgeValue(EndpointPair.unordered(2, 1)));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void edgeValue_undirected_correct() {

  /**
   * Removes the edge connecting {@code endpoints}, if it is present.
   *
   * <p>If this graph is directed, {@code endpoints} must be ordered.
   *
   * @return the value previously associated with the edge connecting {@code endpoints}, or null if
   *     there was no such edge.
   * @since 27.1
   */
  @CanIgnoreReturnValue
  @CheckForNull
  V removeEdge(EndpointPair<N> endpoints);

  @Override
  public Set<E> edgesConnecting(EndpointPair<N> endpoints) {
    return delegate().edgesConnecting(endpoints);
  }

  @Override
  @CheckForNull
  public E edgeConnectingOrNull(N nodeU, N nodeV) {
    return delegate().edgeConnectingOrNull(nodeU, nodeV);
  }

  @Override
  @CheckForNull
  public E edgeConnectingOrNull(EndpointPair<N> endpoints) {
    return delegate().edgeConnectingOrNull(endpoints);

/**
 * An immutable pair representing the two endpoints of an edge in a graph. The {@link EndpointPair}
 * of a directed edge is an ordered pair of nodes ({@link #source()} and {@link #target()}). The
 * {@link EndpointPair} of an undirected edge is an unordered pair of nodes ({@link #nodeU()} and
 * {@link #nodeV()}).
 *
 * <p>The edge is a self-loop if, and only if, the two endpoints are equal.
 *
 * @author James Sexton
 * @since 20.0

  /**
   * Adds an edge connecting {@code endpoints} (in the order, if any, specified by {@code
   * endpoints}) if one is not already present.
   *
   * <p>If this graph is directed, {@code endpoints} must be ordered and the added edge will be
   * directed; if it is undirected, the added edge will be undirected.
   *
   * <p>If this graph is directed, {@code endpoints} must be ordered.
   *

    /**
     * Adds an edge connecting {@code endpoints} (in the order, if any, specified by {@code
     * endpoints}) if one is not already present.
     *
     * <p>If this graph is directed, {@code endpoints} must be ordered and the added edge will be
     * directed; if it is undirected, the added edge will be undirected.
     *
     * <p>If this graph is directed, {@code endpoints} must be ordered.
     *

     * must also be non-null.
     *
     * <p>If either or both endpoints are not already present in this network, this method will
     * silently {@link #addNode(Object) add} each missing endpoint to the network.
     *
     * <p>If {@code edge} already connects an endpoint pair equal to {@code endpoints}, then this
     * method will have no effect.
     *
     * @return this {@code Builder} object

    assume().that(graphIsMutable()).isTrue();

    EndpointPair<Integer> endpoints = EndpointPair.unordered(N1, N2);
    IllegalArgumentException e =
        assertThrows(IllegalArgumentException.class, () -> graphAsMutableGraph.putEdge(endpoints));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  /**

    checkNotNull(endpoints);
    checkArgument(isOrderingCompatible(endpoints), ENDPOINTS_MISMATCH);
  }

  /**
   * Returns {@code true} iff {@code endpoints}' ordering is compatible with the directionality of
   * this graph.
   */
  protected final boolean isOrderingCompatible(EndpointPair<?> endpoints) {
    return endpoints.isOrdered() == this.isDirected();
  }