Validate on Generic Object, but return <?>

lukasbradley · **Posted:** Sat Aug 07, 2010 4:18 pm

Apologies, as this may be more of a generic Java question, but I've not been able to figure this out, even from Java 5.

I'm validating a standard Object, as it might be lots of different things. However, I want the return Set to be parameterized as Set<ConstraintViolation<?>>, so that I can pass it to a ConstraintViolationException.

However, the compiler is upset that Object is being passed to validate(), as it assumes the Set should be Set<ConstraintViolation<Object>>. Is there a magical way I can have the Object pretend to be a generic, non-Object Object?

Thanks for any and all help.

hardy.ferentschik · **Posted:** Tue Aug 10, 2010 12:40 pm

Hi,

Quoting from the Java Language Specification

Quote:

Note that in general, List<?> is a list of unknown type. It is not a subtype of List<T>, for any type T. Allowing such a subtype relation would be unsound

- see eg http://java.sun.com/docs/books/jls/third_edition/html/conversions.html. Or have a look at the generics primer - http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf.

I don't quite understand what you try to achieve, but could you not work with Set<ConstraintViolation<Object>>?

--Hardy

Gunnar · **Joined:** Sat Jan 24, 2009 12:46 pm **Posts:** 388

I think the problem with creating ConstraintViolationException instances you describe is addressed with

http://opensource.atlassian.com/project ... e/BVAL-198

As outlined in said issue you can help yourself by wrapping your set of constraint violations into a new HashSet:

Code:

Set<ConstraintViolation<Object>> constraintViolations = validator.validate(domainObject);

//compiler error: ("The constructor ConstraintViolationException(Set<ConstraintViolation<Object>>) is undefined")
throw new ConstraintViolationException(constraintViolations);

//this works
throw new ConstraintViolationException(new HashSet<ConstraintViolation<?>>(constraintViolations));

Regards,

Gunnar

lukasbradley · **Posted:** Wed Aug 18, 2010 5:52 pm

Thanks Gunnar. That's exactly what I ended up doing.

It's ugly, but it works.