259: Mainline

As a general rule, you should place public instance variables and public instance methods in classes such that two criteria are satisfied:

• There is no needless duplication of a public instance variable or public instance method.
• Each public instance variable and public instance method is useful to instances of all the subclasses of the class in which the instance variable or instance method is defined.

For example, the minutes instance variable is in the Attraction class, because it is useful for all Attraction subclasses.

On the other hand, there are two rating instance methods, because the way that you compute the rating of a Movie instance is different from the way that you compute the rating of a Symphony instance. There is duplication, because there are two rating methods; however, there is no needless duplication.