子类型和泛型函数<? super T, ? extends T>在 Java 中

Question

I have an interface A and class B implementing A.

I have a method in A, called doX(Function<? super A, ? extends A> f) . When I tried to implement it in B, by writing <T extends A> doX(Function<? super B, T> f) , it doesn't compile. I tried a variant of other possible generics also doesn't work. Basically, my function wants to take in itself, and spits out some other item implementing A. By using PECS, it becomes <? super B> <? super B> , however it is wrong. Now I am totally clueless about what I should write.

Answer 1

...my function wants to take in itself, and spits out some other item implementing A

If this is your only requirement, we can do away with super clause.

1. You said the function should take its current instance as input. We can acheive it by introducing a type parameter at interface A level.
2. For returning another implementation of interface A , we can introduce a type parameter at the method level.

Below is the complete code

public class Subtyping {
    private interface A<T extends A<T>> {
        <S extends A<S>> S doX(Function<T, S> f);
    }
    private static class B implements A<B> {
        @Override
        public <S extends A<S>> S doX(Function<B, S> f) {
                return f.apply(this);
        }
    }
    private static class C implements A<C> {
        @Override
        public <S extends A<S>> S doX(Function<C, S> f) {
            return null;
        }
    }
    public static void main(String[] args) {
        B b = new B();
        C result = b.doX(v -> new C());
        System.out.println(result);
    }
}