no instance(s) of type variable(s) exist so that Function conforms to Function where B is interface implemented by C

Question

I am getting the error

Required type: Function<A, B>
Provided: Function<A,C<D>>
no instance(s) of type variable(s) exist so that Function<A, C<D>> conforms to Function<A, B>

Now B is an interface defined like

public interface B <U extends X, V extends Y> {}

C implements the interface B like

public abstract class C <T extends Y> implements B <X, T>

Now if I write something as

B data = getBean(SomeFactory.class);

where getBean(SomeFactory.class) gives an instance of C<D> then this statement is valid as C implements B. But if I write

Function<A,B> supplier = getBean(SomeSupplierFactory.class);

where getBean(SomeSupplierFactory.class) gives an instance of Function<A, C<D>> it throws the above error i.e.

Required type: Function<A, B>
Provided: Function<A,C<D>>
no instance(s) of type variable(s) exist so that Function<A, C<D>> conforms to Function<A, B>

even though C is an implementation of B

Please let me know what am I missing and how to make the above work. Thanks a lot.

Why is `B` a `Function` at all? According to what you’ve posted before, it is not. Or perhaps, you should explain, how the arguments `SomeFactory.class` or `SomeSupplierFactory.class` affect what the method `getBean` returns. Further, there was no mentioning of `A` before. — Holger, Apr 12 '22 at 11:31
`B` has type parameters, so you should be writing `B data = getBean(SomeFactory.class);` instead of using `B` without type arguments. And also `Function> supplier = ...;` — Jesper, Apr 12 '22 at 11:35
@Holger B is not a function.. Later we want to change the implementation which returns a function like `aObject -> bObject` i.e. return type will be `Function`. Why I have not mentioned A is because its just a constant behavior which does not have impact on this question. — Varun Sharma, Apr 12 '22 at 11:38
B has type parameter but we cannot use it as this is a generic behavior. Having said that the issue is not there and everything works as expected even without the params `B data = objectOfC` works. Basically `getBean(SomeFactory.class)` is essentially `objectOfC` — Varun Sharma, Apr 12 '22 at 11:40
So your question is why, e.g. a `Function` is not a `Function` and all remaining stuff in your question is irrelevant. The answer is, there are no covariant “out type parameters” in Java, so the use site has to declare the variance, e.g. `Function supplier = …`. To avoid the need for such declarations, methods receiving a function should follow the [PECS rule](https://stackoverflow.com/q/2723397/2711488) and every reasonable library follows this pattern, so you can simply keep `Function >` and pass it to any method expecting `Function ` — Holger, Apr 12 '22 at 11:47

no instance(s) of type variable(s) exist so that Function conforms to Function where B is interface implemented by C

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