建立类Invoicek，创建一个名为发票Invoicek的类，商店可以使用它表示店中售出一款商品的一张发票。有四个信息：零件号，零件描述，售出量，单价。-Establish class Invoicek, create a class called Invoice Invoicek and shops can use it, said the store sold a commodity invoice. There are four pieces of information: part nu