CODEWITHSUNDEEP
javagenericsinheritanceinterfacemultiple-inheritance
Ganz
Ganz

Reputation: 33

Java generics: Bound mismatch.Looking for the valid substitute

I have a couple of generic classes:

public interface Data<E> {}
public interface Clonable<E extends Clonable<E>> {}
public interface NaturalNumberInterface extends Data<NaturalNumberInterface> {}
public class NaturalNumber implements NaturalNumberInterface {}
public interface SetInterface<E extends Data<E>> extends Clonable<SetInterface<E>> {}
public class Set<E extends Data<E>> implements SetInterface<E> {}

When I'm trying to create the new instance of Set Set<NaturalNumber> s=new Set<NaturalNumber>(); compiler says:

NaturalNumber is not valid substitute for the type parameter <E extends Data<E>> of the type Set<E>

Maybe you can help me to find the mistake, cause I spent a long time and didn't find the solution.

Upvotes: 2

Views: 69

Answers (1)

Tagir Valeev
Tagir Valeev

Reputation: 100209

I assume that your SetInterface is defined in the same way as ListInterface and Data is just interface Data<T>.

The generic argument of SetInterface is F-bounded: E extends Data<E>. In your current code NaturalNumber type extends Data<NaturalNumberInterface>. So if E is NaturalNumber, then condition is violated as it should extend more specific type Data<NaturalNumber>.

You should use F-bounds for NaturalNumberInterface as well:

public interface NaturalNumberInterface<T extends NaturalNumberInterface<T>> extends Data<T>
public class NaturalNumber implements NaturalNumberInterface<NaturalNumber>

This way it will work.

Upvotes: 2

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