Design Patterns Explained: A New Perspective on Object-Oriented Design

Handling New Requirements

After writing this application, suppose I receive a new requirement to change the way I have to handle taxes. Now, I have to be able to handle taxes on orders from customers outside the United States. At a minimum, I will need to add new rules for computing these taxes.

How can I handle these new rules? I could attempt to reuse the existing SalesOrder object, processing this new situation like a new kind of sales order, only with a different set of taxation rules. For example, for Canadian sales, I could derive a new class called CanadianSalesOrder from SalesOrder that would override the tax rules. I show this solution in Figure 14-2.

Figure 14-2. Sales order architecture for an e-commerce system.

Now, design patterns ...