Prefactoring

9.2. Who's in Charge?

Are CDDiscs rented to customers or do customers rent CDDiscs? That might seem like a redundant question, but the decision of who is in charge underlies many design issues.

9.2.1. One Class in Charge

In the first version of the system, CDDisc had the start_rental and end_rental operations. When start_rental was invoked, it created a Rental. If we want to know what CDDiscs a particular customer is renting at the current time, we have to ask every CDDisc if it is rented to that Customer. CDDisc would need some additional methods:

    class CDDisc
        {
        start_rental(Customer a_customer);
            // Begins a rental for particular customer
        end_rental( );  // Ends the rental for that customer
        Customer retrieve_renting_customer( );
        Customer [] retrieve_all_customers_who_rented( );
        };

Should Customer keep track of what CDDiscs are rented to it? If so, at the time of the rental, we would call a method in the Customer class to record the rental and a corresponding method at the end of the rental:

    class Customer
        {
        begin_rental(CDDisc a_cd_disc);
        end_rental(CDDisc a_cd_disc);
        Rentals [] retrieve_current_rentals( );
        };

In Sam's current system, we have three classes: CDDisc, Rental, and Customer. A Rental object is currently a member of the CDDisc class. But Rental has a reference to the Customer class. So CDDisc is coupled to Rental and Rental is coupled to Customer. This organization worked OK when we had a one-to-one relationship between the classes. In essence, a CDDisc was tied ...