assertThat(graph.incidentEdges(N4)).isEmpty();

    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.adjacentNodes(N1)));
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.predecessors(N1)));
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.successors(N1)));

    addNode(N1);
    addNode(N2);
    assertNodeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.edgesConnecting(N1, NODE_NOT_IN_GRAPH)));
    assertNodeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.edgesConnecting(NODE_NOT_IN_GRAPH, N2)));
    assertNodeNotInGraphErrorMessage(
        assertThrows(

  private static final String EDGE_STRING = "Edge";

  enum EdgeType {
    UNDIRECTED,
    DIRECTED;
  }

  private TestUtil() {}

  static void assertNodeNotInGraphErrorMessage(Throwable throwable) {
    assertThat(throwable).hasMessageThat().startsWith(NODE_STRING);
    assertThat(throwable).hasMessageThat().contains(ERROR_ELEMENT_NOT_IN_GRAPH);
  }