Java generics wildcard operator

Question

I am trying to write a utility method to assist me with builing exception messages for any validation errors with respect to bean validation in my application. When I go to validate my object (in this case, BasicContactInformation) I do it as follows:

Set<ConstraintViolation<BasicContactInformation>> constraintViolations = validator.validate(basicContactInformation);
        if(!CollectionUtils.isEmpty(constraintViolations)){
            throw new CustomerAccountException(LoggingUtils.getLoggingOutput("Unable to update customer contact information", constraintViolations));
        }

I am doing the same thing for other beans. The utility method will take in a prefix for the exception message, along with the set of constraint validations, and will build a nicely formatted output message. The problem is, I can't figure out how to build the message such that it can accept a set of constraint violations of any type. I tried the following, but it doesn't seem to be valid, as it says it cant cast:

public static String getLoggingOutput(String prefix, Set<ConstraintViolation<?>> violations){
    StringBuilder outputBuilder = new StringBuilder(prefix);
    if(!CollectionUtils.isEmpty(violations)){
        for(ConstraintViolation<?> currentViolation: violations){
            outputBuilder.append("[");
            outputBuilder.append(currentViolation.getMessage());
            outputBuilder.append("]");
        }
    }
    return outputBuilder.toString();
}

This is the compiler error

The method getLoggingOutput(String, Set<ConstraintViolation<?>>) in the type LoggingUtils is not applicable for the arguments (String, Set<ConstraintViolation<BasicContactInformation>>)

Any idea as to what the method signature should be so that it would work for any set of constraint violations? I'm trying to avoid writing a method that takes in a set of constraint violations for foo, one for bar, one for baz, etc.

Answer 1

You can make the method generic:

public static <T> String getLoggingOutput(String prefix,
        Set<ConstraintViolation<T>> violations) {
    // ...
}

Under most circumstances, if not all, the compiler will infer the type parameter from the argument. This is not at all the same thing as using a wildcard.