abstract class Item<I extends Item<I>> {

    public abstract void add();
    public abstract void test();

    public static <T extends Item<T>> Item<T> getZero() {
        return T.ZERO();
    }

    protected static <T extends Item<T>> Item<T> ZERO() {
        class ZERO extends Item<T> {
            public void add() {}
            public void test() { System.out.println("item"); } // don't call this
        }
        return new ZERO();
    }

    public static void main(String[] args) {
        Item.<Book>getZero().test();
    }
}

class Book extends Item<Book> {

    public void add() { /* addition implementation goes here */ }
    public void test() { System.out.println("book"); } // this should be called

    protected static Item<Book> ZERO() { return new Book(); }

}