➍

    public void updateTotalPrice() {
        totalPrice = this.book.getUnitPrice() * this.quantity;
    } 

    @Override

➎

    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((book == null) ? 0 : book.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        CartItem other = (CartItem) obj;
        if (book == null) {
            if (other.book != null)
                return false;
        } else if (!book.equals(other.book))
            return false;
        return true;
    }

}