() -> networkForTest.edgesConnecting(N1, NODE_NOT_IN_GRAPH));
    assertNodeNotInGraphErrorMessage(e);
    e =
        assertThrows(
            IllegalArgumentException.class,
            () -> networkForTest.edgesConnecting(NODE_NOT_IN_GRAPH, N2));
    assertNodeNotInGraphErrorMessage(e);
    e =
        assertThrows(
            IllegalArgumentException.class,

  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);
  }