Java wildcard generic not allowed?

Question

So I want to make a util method, that returns a List with errors from ConstraintValidator s. I have this method:

public static List<String> getViolationsToList(Set<ConstraintViolation<?>> violations) {
    List<String> errors = new ArrayList<>();
    for(ConstraintViolation<?> violation: violations) {
        errors.add(violation.getMessage());
    }

    return errors;
}

However when I call it,

RestUtil.getViolationsToList(violations);

It does not compile. Violations is of type:

Set<ConstraintViolation<UserDto>> violations;

any ideas? Eclipse suggest to remove the wildcard with UserDto or create new method, but I want to use generics.

Answer 1

Use

Set<<ConstraintViolation<T>>

Then:

public static <T> List<String> getViolationsToList(Set<ConstraintViolation<T>> violations) {

Answer 2

Generics are invariant. Set<ConstraintViolation<UserDto>> is not a subtype of Set<ConstraintViolation<?>> , even though ConstraintViolation<UserDto> is a subtype of ConstraintViolation<?> . This is just like how List<String> is not a subtype of List<Object> , even though String is a subtype of Object .

You need a wildcard at the top level in order to get polymorphism of type arguments:

List<String> getViolationsToList(Set<? extends ConstraintViolation<?>> violations) {

Answer 3

To dig a bit further,

OP does not mention it, but note that the code is working in Java 8 due to better type inference.

In java 8 you could do the following :

return violations.stream().map(Violation::getMessage).collect(Collectors.toList());

which is a bit more elegant

Answer 4

May be you are trying to do something like this :

public static <T> List<String> getViolationsToList(Set<ConstraintViolation<T>>    violations) {
        List<String> errors = new ArrayList<>();
        for(ConstraintViolation<T> violation: violations) {
            errors.add(violation.getMessage());
        }

        return errors;
    }