How does one define a generic wrapper interface type and use it as method return type in java?

Question

I'm trying to do the following: defining a wrapper interface type and have a calculate method that returns that wrapper, but it doesn't work:

public interface Wrapper<T> {
}

public interface Calculator<WrapperType extends Wrapper<?> {
   <T> WrapperType<T> calculate(Supplier<T> calculator);
}

The error is something like "Type Wrapper does not have type parameter". Is this something possible in Java?

Answer 1

This won't compile like this. You can't have this many levels of genericity in java, you're going to have to decide between

public interface Calculator<T> {
   Wrapper<T> calculate(Supplier<T> calculator);
}

and

public interface Calculator<T, WrapperType extends Wrapper<T>> {
   WrapperType calculate(Supplier<T> calculator);
}

The latter gives more flexibility at the cost of an additional complexity.