final DropDownChoice ddc = new DropDownChoice("authors",
new Model(CollectionUtils.isEmpty(authors) ? null : authors.iterator().next()), persons, renderer);
ddc.add(new IValidator<Person>() {
public void validate(final IValidatable<Person> validatable) {
final Person author = validatable.getValue();
if (author == null) {
final ValidationError error = new ValidationError();
error.setMessage("Author must be filled in");
validatable.error(error);
}