程序描述：本章实现了一个网上书店系统，采用面向对象的思想，系统的业务逻辑层包括图书类Book、购物车类Cart、会员类User、订单类Order以及订单详细信息类OrderDetail。-Descr iption: This chapter online bookstores realize a system that uses object-oriented thinking, the Business Logic Tier System, including category Book, c