CODEWITHSUNDEEP
javagenerics
escapedcanadian
escapedcanadian

Reputation: 303

Java Generics capture error "? super capture#1 of ? super T"

This is a simplified, fabricated scenario intended to demonstrate my problem.

I want to create an interface MarketingScheme<T> that will have a collection of participants implementing the interface MarketingCustomers<? super T>. As an example, I might have a marketing scheme designed for Vehicles, of which a marketing customer for Cars might be a participant. I have another interface, Manufacturer<R> extends MarketingCustomer <R>, so a manufacturer of cars is also a marketing customer of cars.

My MarketingScheme interface defines a single method, public void addParticipant(MarketingCustomer<?super T> customer);. I would expect to be able to add a MarketingCustomer<Car> to a MarketingScheme<Vehicle>, assuming Car was a subclass of Vehicle. There may be many schemes, so I should be able to add the same manufacturer to a MarketingScheme<DurableGood>, assuming Vehicle extends DurableGood

If I create a ManufacturerImpl and pass in a MarketingScheme as follows:

    public class ManufacturerImpl<R> implements Manufacturer<R>
{
    public ManufacturerImpl(MarketingScheme<? super R> scheme) {
        super();
        scheme.addParticipant(this);
    }
}

... I get the error 

The method addParticipant(MarketingCustomer<? super capture#1-of ? super R>) in the type MarketingScheme<capture#1-of ? super R> is not applicable for the arguments (ManufacturerImpl<R>)

I have read up on Generic Methods (helperMethods) and tried to implement one, but I could not get it right. I have considered adding an additional type declarations to the interfaces, but none of them compiled or seemed appropriate in my model. I couldn't find any casts that worked inside of my addParticipant call either.

My interfaces are defines as follows ... 

    public interface MarketingScheme<T>
{
    public void addParticipant(MarketingCustomer<? super T> customer);

}


public interface MarketingCustomer<S>
{

}


public interface Manufacturer<R> extends MarketingCustomer<R>
{

}

Your answers greatly appreciated. Thank you.

Upvotes: 1

Views: 3568

Answers (2)

Sandeep Vaid
Sandeep Vaid

Reputation: 1459

change it for

public interface MarketingScheme <T> {

public void addParticipant(MarketingCustomer<<? extends T> customer);

}

Upvotes: 0

abacabadabacaba
abacabadabacaba

Reputation: 2692

You need to change the type signature of addParticipant:

public void addParticipant(MarketingCustomer<? extends T> customer);

This is because customer's type parameter should be a subclass, not a superclass, of T.

Upvotes: 1

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