public class Test {

    interface Letter {

        public void Compose();
    }

    interface Postcard {

        public void Sign();
    }

    interface Stampable {

        public void putStamp();
    }

    interface Factory<T> {

        public T create();
    }

    static class LetterImpl implements Letter, Stampable {
    ...
    }

    static class PostcardImpl implements Postcard, Stampable {
    ...
    }

    static class PostcardFactory implements Factory<Postcard> {

        @Override
        public Postcard create() {
            return new PostcardImpl();
        }

    }

    static class LetterFactory implements Factory<Letter> {

        @Override
        public Letter create() {
            return new LetterImpl();
        }

    }

    static class PostOffice<T> {

        List<T> items;

        private Factory<T> factory;

        public T create() {
            return factory.create();
        }

        public PostOffice(Factory<T> factory) {
            this.factory = factory;
        }

        public void putStamp() {
            for (T item : items) {
                //item.putStamp(); //error 
            }
        }
    }

    static class PostOfficeFactory {

        public static PostOffice<Postcard> CreatePostcardDept() {
            return new PostOffice<Postcard>(new PostcardFactory());
        }

        public static PostOffice<Letter> CreateLetterDept() {
            return new PostOffice<Letter>(new LetterFactory());
        }

    }

    public static void main(String[] args) {
        PostOffice<Postcard> po = PostOfficeFactory.CreatePostcardDept();
        Postcard p = po.create();
    }

}