选择费用代码，如飞机票、租车费、差旅费或停车费。
Any code you write is likely to spend most of its lifetime in maintenance; application maintenance is a large expense.编写出来的代码可能需要大量的维护；应用程序的维护是一笔很大的开支。
At the expense of defining an additional interface and making the client code a little more verbose, this approach certainly has the edge for me when it comes to code clarity.虽然代价是定义一个额外接口，还会使客户代码更长一些，但是这项技术对于代码清晰性来说具有优势。
