Classes should only exhibit nil or export coupling with other classes, that is, a class should only use operations in the public interface of another class or have nothing to do with that class.

• 类之间应该零耦合，或者只有导出耦合关系。也即，一个类要么同另一个类毫无关系，要么只使用另一个类的公有接口中的操作。