Design Patterns Explained

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150 Part III Design Patterns The essence of the pattern The Bridge pattern often incorporates the Adapter pattern The Bridge Pattern in Retrospect Now that you’ve seen how the Bridge pattern works, it is worth looking at it from a more conceptual point of view. As shown in Figure 9-13, the pattern has an abstraction part (with its derivations) and an implementation part. When designing with the Bridge pattern, it is useful to keep these two parts in mind. The implementation’s interface should be designed considering the different derivations of the abstract class that it will have to support. Note that a designer shouldn’t necessarily put in an interface that will implement all possible derivations of the abstract class (yet another possible route to paralysis by analysis). Only those derivations that actually are being built need be supported. Time and time again, the authors have seen that the mere consideration of flexibility at this point often greatly improves a design. Note: In C++, the Bridge pattern’s implementation must be implemented with an abstract class defining the public interface. In Java, either an abstract class or an interface can be used. The choice depends upon whether implementations share common traits that abstract classes can take advantage of. See Peter Coad’s Java Design, discussed on page 316 of the Bibliography, for more on this. Field Notes: Using the Bridge Pattern Note that the solution presented in Figures 9-12 and 9-13 integrates the Adapter pattern with the Bridge pattern. I do this because I was given the drawing programs that I must use. These drawing programs have preexisting interfaces with which I must work. I must use the Adapter to adapt them so that they can be handled in the same way. While it is very common to see the Adapter pattern incorporated into the Bridge pattern, the Adapter pattern is not part of the Bridge pattern.
The Bridge Pattern: Key Features Chapter 9 The Bridge Pattern 151 Intent Decouple a set of implementations from the set of objects using them. Problem The derivations of an abstract class must use multiple implementations without causing an explosion in the number of classes. Solution Define an interface for all implementations to use and have the derivations of the abstract class use that. Participants and Collaborators The Abstraction defines the interface for the objects being implemented. The Implementor defines the interface for the specific implementation classes. Classes derived from the Abstraction use classes derived from the Implementor without knowing which particular ConcreteImplementor is in use. Consequences The decoupling of the implementations from the objects that use them increases extensibility. Client objects are not aware of implementation issues. Implementation • Encapsulate the implementations in an abstract class. Contain a handle to it in the base class of the abstraction being implemented. Note: In Java, you can use interfaces instead of an abstract class for the implementation. GoF Reference Pages 151–162. Figure 9-15 Standard, simplified view of the Bridge pattern.
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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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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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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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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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2O4 Part IV • Putting It AM Toget
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Chapter 12 • Solving the CAD/CAM
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CHAPTER 13 The Principles and Strat
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Chapter 13 • The Principles and S
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228 Part V • Handling Variations
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23O Part V • Handling Variations
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Overview CHAPTER 18 The Template Me
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Chapter 18 • The Template Method
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Intent Problem Solution Participant
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CHAPTER 19 The Factory Method Patte
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The Factory Method Pattern Chapter
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Summary Chapter 19 • The Factory
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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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