Design Patterns Explained

CHAPTER 15
The Decorator Pattern
Overview
This chapter continues the e-tailing case study introduced in Chap- In this chapter
ter 14, "The Strategy Pattern."
In this chapter,
• I describe a new requirement for the case study: Add header
and footer information to the printed sales ticket.
• I show how the Decorator pattern handles the requirement
flexibly.
• I discuss how the Decorator pattern can be used to handle
input/output (especially Java I/O).
• I describe the key features of the Decorator pattern.
• I describe some of my experience using the Decorator pattern
in practice.
A Little More Detail
Figure 14-2 showed the basic structure of the case study. Figure Expanding the
15-1 shows this structure in more detail. Here, I show that the diagram
SalesOrder object uses a SalesTicket object to print a sales ticket.
As you saw in Chapter 14, SalesOrder uses a CalcTax object to
calculate the tax on the order. To implement the printing function,
SalesOrder calls the SalesTicket object, requesting that it print
the ticket. This is a fine, reasonably modular design.
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