public static Builder builder() {
    return new Builder();
  }
@Test
  public void testBuilderDoesNotCreateInvalidObjects() {
    assertThatThrownBy(() -> ListNamespacesResponse.builder().add(null).build())
        .isInstanceOf(NullPointerException.class)
        .hasMessage("Invalid namespace: null");

    assertThatThrownBy(() -> ListNamespacesResponse.builder().addAll(null).build())
        .isInstanceOf(NullPointerException.class)
        .hasMessage("Invalid namespace list: null");

    List<Namespace> listWithNullElement = Lists.newArrayList(Namespace.of("a"), null);
    assertThatThrownBy(() -> ListNamespacesResponse.builder().addAll(listWithNullElement).build())
        .isInstanceOf(IllegalArgumentException.class)
        .hasMessage("Invalid namespace: null");
  }