public CreateTableBuilder addPkColumn(ColumnDef columnDef, ColumnFlag... flags) {
    pkColumnDefs.add(requireNonNull(columnDef, "column def can't be null"));
    addFlags(columnDef, flags);
    return this;
  }
@Test
  public void addPkColumn_throws_NPE_if_ColumnDef_is_null() {
    assertThatThrownBy(() -> underTest.addPkColumn(null))
      .isInstanceOf(NullPointerException.class)
      .hasMessageContaining("column def can't be null");
  }