CODEWITHSUNDEEP
javagenericsinheritance
Pau
Pau

Reputation: 16116

Inheritance in bounded type parameters to limite types

I have an abstract class with a generic using a bounded type parameter to limit this type:

public abstract class AbstractCreator<T extends Request> {
     ....
}

Then I have multiple children from AbstractCreator and one uses another bounded type parameter, like next:

public abstract class AbstractBigCreator<U extends BigRequest> extends AbstractCreator<U> {
    U request;
    ...
}

The interfaces bounded types are like next:

public interface Request<T extends Some> {
     T getRequest();
}
public interface BigRequest<T extends SomeOther> extends Request<T> {

}

My problem is this child with another bounded type parameter is limiting to its the father class bounded type parameter in the class itself instead of this type parameter. So if I call the method getRequest()from U request I get that it is <T extends Some> instead of <T extends SomeOther>.

I expect:

U request;
request.getRequest(); //Class <T extends SomeOther>

But instead I get:

U request;
request.getRequest(); //Class <T extends Some>

Anyone knows what's happening?

Upvotes: 1

Views: 254

Answers (1)

fps
fps

Reputation: 34470

The problem is not that the request attribute is of type Request instead of BigRequest in your AbstractBigCreator class.

The actual problem is that your BigRequest interface is extending the Request interface, so you are inheriting the definition of the T getRequest() method. And in Request, the T type is bounded to Some, and so is in BigRequest.

The easiest solution is to redefine the T getRequest() method in the BigRequest interface, so that covariance is applied to the return type of the getRequest method. This means that the getRequest method of the BigRequest interface would return SomeOther instead of Some.

Just do this:

public interface BigRequest<T extends SomeOther> extends Request<T> {

    @Override
    T getRequest();
}

And you'll be able to access SomeOther after calling request.getRequest() in AbstractBigCreator.

Upvotes: 1

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