Comment by yarg

Valhalla is what you want, at least for some of it.

https://openjdk.org/jeps/402

    interface Box<T> {
        T get();
        void set(T val);
    }

    interface IntBox extends Box<int> {
        int get();
        void set(int val);
    }

https://openjdk.org/projects/valhalla/