I have an interface ProductService with method findByCriteria. This method had a long list of nullable parameters, like productName, maxCost, minCost, producer and so on.

I refactored this method by introducing Parameter Object. I created class SearchCriteria and now method signature looks like this:

findByCriteria (SearchCriteria criteria)

I thought that instances of SearchCriteria are only created by method callers and are only used inside findByCriteria method, i.e.:

void processRequest() {
   SearchCriteria criteria = new SearchCriteria ()
                                  .withMaxCost (maxCost)
                                  .......
                                  .withProducer (producer);

   List<Product> products = productService.findByCriteria (criteria);
   ....
}

and

List<Product> findByCriteria(SearchCriteria criteria) {
    return doSmthAndReturnResult(criteria.getMaxCost(), criteria.getProducer());    
}

So I did not want to create separate public class for SearchCriteria and put it inside ProductServiceInterface:

public interface ProductService {
    List<Product> findByCriteria (SearchCriteria criteria);

    static class SearchCriteria {
       ...
    }
}

Is there anything bad in this interface? Where whould you place SearchCriteria class?