Design Patterns Explained

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242 Part V • Handling Variations with Design Patterns New requirement: add a header One approach: use switches in SalesTicket Figure 15-1 SalesOrder using SalesTicket. In the process of writing the application, suppose I get a new requirement to add header information to the SalesTicket. How can I handle this new requirement? If I am writing the system to be used by just one company, it may be easiest simply to add the control of headers and footers in the SalesTicket class. This is shown in Figure 15-2. Figure 15-2 SalesOrder using SalesTicket with different options.
Chapter 15 • The Decorator Pattern 243 In this solution, I have put the control in SalesTicket, with flags saying whether it is to print the headers or the footers. This works quite well if I do not have to deal with a lot of options or if the sales orders using these headers do not change. If I have to deal with many different types of headers and footers, printing only one each time, then I might consider using one Strategy pattern for the header and another Strategy pattern for the footer. What happens if I have to print more than one header and/or footer at a time? Or what if the order of the headers and/or footers needs to change? The number of combinations can quickly overwhelm. In situations like this, the Decorator pattern can be very useful. Instead of controlling added functionality by having a control method, the Decorator pattern says to control it by chaining together the functions desired in the correct order needed. The Decorator pattern separates the dynamic building of this chain of functionality from the client that uses it, in this case, the SalesOrder. The Decorator Pattern According to Gang of Four, the Decorator pattern's intent is to Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. 1 The Decorator pattern works by allowing me to create a chain of objects that starts with the decorator objects— the objects responsible for the new function— and ends with the original object. Figure 15-3 illustrates this. 1. Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of Reusable Object-Oriented Software, Reading, Mass.: Addison-Wesley, 1995, p. 315. The approach is not flexible The Decorator pattern helps The intent, according to the Gang of Four How it works
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xvi Preface From Object Orientation
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xviii Preface Patterns are supposed
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xx Preface A slight digression. The
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xxii Preface abstractly and because
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xxiv Preface Approach Rationale A N
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xxvi Preface Acknowledgments Almost
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Overview CHAPTER 1 The Object-Orien
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Chapter 1 The Object-Oriented Parad
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Table 1-2 Objects and Their Respons
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The main program would now look lik
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Table 1-4 Object-Oriented Terminolo
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3O Part I • An Introduction to Ob
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32 Part I • An Introduction to Ob
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4O Part I • An Introduction to Ob
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PART II The Limitations of Traditio
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CHAPTER 3 A Problem That Cries Out
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Chapter 3 • A Problem That Cries
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Figure 3-6 Class diagram of my solu
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Summary Chapter 3 • A Problem Tha
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58 Part II • The Limitations of T
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6O Part II • The Limitations of T
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Chapter 4 • A Standard Object-Ori
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Chapter 4 • A Standard Object-Ori
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Example 4-6 C++ Code Fragments: Imp
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Part Overview PART III Design Patte
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Overview CHAPTER 5 An Introduction
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Chapter 5 An Introduction to Design
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Table 5-2 Key Features of Patterns
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Figure 5-2 A miter joint Chapter 5
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Overview CHAPTER 6 The Facade Patte
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This person would determine how I a
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Field Notes: The Facade Pattern Fac
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Summary Chapter 6 • The Facade Pa
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96 Part III • Design Patterns Int
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98 Part III • Design Patterns How
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1OO Part III • Design Patterns /
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1O2 Part III • Design Patterns Th
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1O4 Part III • Design Patterns Co
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1O6 Part III • Design Patterns Ad
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CHAPTER 8 Expanding Our Horizons Ov
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1. Make a preliminary design withou
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Chapter 8 • Expanding Our Horizon
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124 Part III • Design Patterns It
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126 Part III Design Patterns Proper
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128 Part III Design Patterns But, t
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130 Part III Design Patterns Unders
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134 Part III Design Patterns An alt
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136 Part III Design Patterns It sti
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138 Part III Design Patterns Learni
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140 Part III Design Patterns Strate
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142 Part III Design Patterns Tying
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144 Part III Design Patterns One ru
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146 Part III Design Patterns From a
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148 Part III Design Patterns Exampl
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150 Part III Design Patterns The es
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152 Part III Design Patterns Compou
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154 Part III Design Patterns A usef
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156 Part III Design Patterns Concep
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CHAPTER 10 The Abstract Factory Pat
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Chapter 1O • The Abstract Factory
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Figure 1O-5 The ResFactory encapsul
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Figure 1O-6 Intermediate solution u
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In this pattern, Chapter 1O • The
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CHAPTER 11 How Do Experts Design? O
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Design is often thought of as a pro
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Design is often thought of as a pro
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294 Part V • Handling Variations
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Figure 2O-1 Using Strategy patterns
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3O2 Part V • Handling Variations
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CHAPTER 21 Design Patterns Reviewed
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Chapter 21 • Design Patterns Revi
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Chapter 21 • Design Patterns Revi
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Figure 21-1 The relationships betwe
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CHAPTER 22 Bibliography This book h
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• Meyer, B., Object-Oriented Soft
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• See http://java.sun.com/j2se/L3
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Personal Favorites It is my belief
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Spradely, J. P., The Ethnographic I
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Section Overview Section 3: Design
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How do we get good quality repeated
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Or again, people are creatures of h
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this book is not a book about writi
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…The details can confuse the solu
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Patterns help us see the forest for
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Design Pattern Matrix Design Patter
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Design Pattern Matrix How it is imp
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Design Pattern Matrix Is there vari
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: (colon), connecting object and cl
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usiness rules defining, 238 encapsu
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decompression object, 249 decorator
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FilterlnputStream class, 250 flexib
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notifyObservers method, 272 NULL te
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sales-order architecture, e-tail sy
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