要求用面向对象的思想实现。该系统包括以下基本功能：

(1)创建多种商品，每种都有一定数量以供交易；

(2)商品之间有共性，但代码和单价不同，也许还有其他不同；

(3)一个交易过程包括：用户选商品及数量，系统给出价格，用户付款，系统判断是否货源充足，或提示付款金额不足，或进行回找零钱等，货源足且金额够则自然出货；

(4)可随时查询交易明细，可以是某个时间段内的，也可以是最近20笔，或其他范围的；

(5)可随时查询各商品的出货情况；

(6)程序退出后数据应存在文件中，下次运行程序后可以从文件中提取各数据；

-Asked to use object-oriented ideology. The system includes the following basic functions: (1) create a variety of products, each with a certain amount for the transaction　(2) there is commonality between the products, but the code and price are different, maybe there are other different　(3) a transaction process comprising: a user selection of goods and quantity, the system gives the price, user payments, the system determines whether the supply of adequate, inadequate or prompt payments, or back to looking for change, etc., enough to supply enough of the nature and amount of the ship　(4) can always check the transaction details, can be a period of time, it can last 20 pens, or other ranges　(5) can always check the product shipments　(6) after exiting the program data files should exist , the next time you run the program after each data extracted from the file