Design Patterns Explained

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248 Part V • Handling Variations with <strong>Design</strong> <strong>Patterns</strong> This creates a Headerl object trailed by a Header2 object trailed by a Footerl object trailed by a SalesTicket object. Decomposing by The Decorator pattern helps to decompose the problem into two responsibilities parts: Stream I/O • How to implement the objects that give the new functionality • How to organize the objects for each special case This allows me to separate implementing the Decorators from the object that determines how they are used. This increases cohesion because each of the Decorators is only concerned with the func tion it adds— not in how it is added to the chain. Another Example: Input/Output A common use for the Decorator pattern is in stream I/O. Let's look at stream I/O a little before seeing how the pattern can be used here. I will limit the discussion to input since output works in an analogous way (if you see how it works in one direction, it should be clear how it works in the other direction). For any particular stream input, there is exactly one source, but there can be any number (including zero) of actions to perform on the input stream. For example, I can read from • A file • A socket and then decrypt the incoming stream • A file and then decompress the incoming data • A string • A file, decompress the input, and then decrypt it Depending upon how the data were sent (or stored) any combination of behaviors is possible. Think of it this way: Any source can be
Chapter 15 • The Decoi decorated with any combination of behaviors. Some of the possibilities available for stream input are shown in Table 15-1. Developers in object-oriented languages can take advantage of this by having source and behavior objects derive from a common abstract class. Each behavior object can be given its source or prior behavior in its constructor. A chain of actions is then built as the objects themselves are instantiated (each is given a reference to its trailing object). The sources derive from ConcreteComponent (see Figure 15-4), while the behaviors are decorators. Note that Con-creteComponent is now a misnomer because it is now abstract. Table 15-1 Kinds of Sources and Behaviors Sources Behaviors String File Socket (TCP/IP) Serial port Parallel port Keyboard Buffered input Run checksum Unzip Decrypt (any number of ways) Selection filters (any number of ways) For example, to get the behavior "read from a file, decompress the input, and then decrypt it," do the following: 1. Build the decorator chain by doing the following: a. Instantiate a file object. b. Pass a reference to it to the constructor of a decompression object. c. Pass a reference to that to the constructor of a decryption object. Languages reflect this
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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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132 Part III Design Patterns This s
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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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It is only possible to make a place
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that it shared its patterns with ot
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Alexander's Steps Discussion Identi
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Page 280 and 281: Overview CHAPTER 18 The Template Me
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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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