Design Patterns Explained

244 Part V • Handling Variations with Design Patterns
The Decorator
pattern is a chain of
objects
In this case
Figure 15-3 The Decorator chain.
The class diagram of the Decorator pattern in Figure 15-4 implies the
chain of objects shown in Figure 15-3. Each chain starts with a Component
(a ConcreteComponent or a Decorator). Each Decorator
is followed either by another Decorator or by the original ConcreteComponent.
A ConcreteComponent always ends the chain.
Figure 15-4 The Decorator pattern class diagram.
For example, in Figure 15-4, ConcreteDecoratorB performs its
Operation and then calls the Operation method in Decorator. This
calls ConcreteDecoratorB's trailing Component's Operation.
Applying the Decorator
Pattern to the Case Study
In the case study, the SalesTicket is the ConcreteComponent.
The concrete decorators are the headers and footers. Figure 15-5
shows the application of the Decorator pattern to the case study.