public void inDegree_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.inDegree(NODE_NOT_IN_GRAPH)));
  }

  @Test
  public void outDegree_isolatedNode() {
    addNode(N1);
    assertThat(graph.outDegree(N1)).isEqualTo(0);
  }

  @Test
  public void outDegree_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(