public CreateTableBuilder addColumn(ColumnDef columnDef) {
    columnDefs.add(requireNonNull(columnDef, "column def can't be null"));
    return this;
  }
@Test
  public void addColumn_throws_NPE_if_ColumnDef_is_null() {
    assertThatThrownBy(() -> underTest.addColumn(null))
      .isInstanceOf(NullPointerException.class)
      .hasMessageContaining("column def can't be null");
  }