标识概念类和对象
原则
类，表示一组具有相同行为、属性的对象
Remember that a class represents a group (classification) of objects with the same behaviors
类，在表示对象群体的时候，一般用单数
Generally, classes that represent objects should be given names that are singular nouns
如 Coin, Student, Message
根据类，可以创建所需要数量的对象个体
We are free to instantiate as many of each object as needed
类名的选择，很重要！要能够帮助大家理解
如 Customer? Or User: 哪个更好，视情况而定