6.6. Policy Versus Implementation

Following along with the concept of doing one job well, you should separate methods that contain policy (what you are going to do) from methods that contain implementation (how you are going to do it). A policy method contains the logic and reasoning for doing something. An implementation method contains the details. For example, this code checks for CDDiscs that are returned late and performs the appropriate action:

 if (is_overdue( ))         process_rental_which_ended_late( ) 

The preceding code completely separates policy from implementation. Here's a version that progressively mixes implementation and policy:

 if (today > end_date)         {         process_rental_which_ended_late( );         } 

Here's another:

 if (today > end_date)         {         // Code for overdue actions         } 

The is_overdue( ) method in the first example might be reusable by other methods employed by other use cases. If it is, the definition of overdue will be consistent among the other use cases. Otherwise, some code might use today > end_date and others might use today > start_date.add(base_rental_period) . These types of inconsistencies can proliferate quickly where teams of programmers are involved.

To separate out policy from implementation while creating code, write down what you are going to do, not how you are going to do it. For example, first write:

 if (a_customer.is_good_customer( ))         a_customer.provide_discount( ); 

Then write the is_good_customer( ) and provide_discount( ) methods.

SEPARATE POLICY FROM IMPLEMENTATION

Keeping the what separated from the how makes the what more readable and maintainable .