return edgeValueOrDefaultInternal(checkNotNull(nodeU), checkNotNull(nodeV), defaultValue);
  }

  @Override
  public @Nullable V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue) {
    validateEndpoints(endpoints);
    return edgeValueOrDefaultInternal(endpoints.nodeU(), endpoints.nodeV(), defaultValue);
  }

  @Override
  protected long edgeCount() {
    return edgeCount;
  }

    edgeToReferenceNode.put(edge, nodeU);
    return true;
  }

  @Override
  @CanIgnoreReturnValue
  public boolean addEdge(EndpointPair<N> endpoints, E edge) {
    validateEndpoints(endpoints);
    return addEdge(endpoints.nodeU(), endpoints.nodeV(), edge);
  }

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