Nested wildcard generics variable affectation

Question

Given the following Java code:

public class Test {
    public static class A<T> {
        private T t;

        public A(T t) {
            this.t = t;
        }

        public T getT() {
            return t;
        }

        public void setT(T t) {
            this.t = t;
        }
    }

    public static class B<T> {
        private T t;

        public B(T t) {
            this.t = t;
        }

        public T getT() {
            return t;
        }

        public void setT(T t) {
            this.t = t;
        }
    }

    public static class F<T> {
        private T t;

        public F(T t) {
            this.t = t;
        }

        public A<B<T>> construct() {
            return new A<>(new B<>(t));
        }

        public T getT() {
            return t;
        }

        public void setT(T t) {
            this.t = t;
        }
    }

    public static void main(String[] args) {
        F<?> f = new F<>(0);
        // 1: KO
        // A<B<?>> a = f.construct();
        // 2: KO
        // A<B<Object>> a = f.construct();
        // 3: OK
        // A<?> a = f.construct();
    }
}

In the main method of the Test class, what is the correct type of a variable that will receive the result of f.construct() ? This type should be something like A<B<...>> where ... is what I'm looking for.

There are 3 commented lines of code above which represent my attempts to solve this problem. The first and second lines aren't valid. The third is but I loose the B type information and I have to cast a.getT().

Why are you using `F> f = new F<>(0);` instead of, for example, `F f = new F<>(0);` — Tunaki, Mar 30 '16 at 12:17
Because `f` will not always be affected with instances of `F`. — Romain Moreau, Mar 30 '16 at 12:20
I can't check right now but I think `A extends B>> a = f.construct();` is what you want. — Paul Boddington, Mar 30 '16 at 12:23
Just remember that when you nest generics you usuallt need a ? at every level. — Paul Boddington, Mar 30 '16 at 12:30
I can't really answer properly as I'm on my mobile. Don't worry about the points. I'm glad I could help — Paul Boddington, Mar 30 '16 at 12:32

score 1 · Accepted Answer · answered Mar 30 '16 at 12:36

1

A<? extends B<?>> a = f.construct(); is the right syntax, as stated by Paul Boddington.

answered Mar 30 '16 at 12:36

Romain Moreau

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Nested wildcard generics variable affectation

1 Answers1