Design Patterns Explained

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Chapter 18 • The Template Method Pattern 281 Figure 18-1 Using the Template Method pattern to perform a query. I have created a method called doQuery that handles the query I need to perform. I pass in the name of the database and the query specification. The doQuery method follows the five general steps above, providing virtual methods for the steps (such as format-Connect and formatSelect) that must be implemented differently. The doQuery method is implemented as follows. As shown in Figure 18-1, it first needs to format the CONNECT command required to connect to the database. Although the abstract class (QueryTem-plate) knows this format needs to take place, it doesn't know how to do this. The exact formatting code is supplied by the derived classes. This is true for formatting the SELECT command as well. The Template Method manages to do this because the method call is made via a reference pointing to one of the derived classes. That is, although Query-Control has a reference of type Query-Template, it is actually referring to an OracleQT or an SQLSvrQT object. Thus, when the doQuery method is called on either of these objects, the methods resolved will first look for methods of the appropriate derived class. Let's say our QueryControl is referring to an OracleQT object. Since OracleQT does not override Query-Template, the QueryTemplate's doQuery method is invoked. This How this works: virtual methods for the steps that vary
282 Part V • Handling Variations with <strong>Design</strong> <strong>Patterns</strong> The Template Method is not coupled Strategies starts executing until it calls the format Connect method. Since the OracleQT object was requested to perform doQuery, the OracleOT's format Connect method is called. After this, control is returned to the Query-Template's doQuery method. The code common to all queries is now executed until the next variation is needed— the formatSelect method. Again, this method is located in the object that Query-Control is referring to (OracleQT in this example). When a new database is encountered, the Template Method provides us with a boilerplate (or template) to fill out. We create a new derived class and implement the specific steps required for the new database in it. Field Notes: Using the Template Method Pattern Sometimes a class will use several different Strategy patterns. When I first looked at the class diagram for the Template Method, I thought, "Oh, the Template Method is simply a collection of Strategies that work together." This is dangerous (and usually incorrect) thinking. While it is not uncommon for several Strategies to appear to be connected to each other, designing for this can lead to inflexibility. The Template Method is applicable when there are different, but conceptually similar processes. The variations for each process are coupled together because they are associated with a particular process. In the example I presented, when I need a format a CONNECT command for an Oracle database, if I need a format a QUERY command, it'll be for an Oracle database as well.
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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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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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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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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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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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Chapter 11 • How Do Experts Desig
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198 Part IV • Putting It All Toge
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2OO Part IV • Putting It All Toge
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2O2 Part IV • Putting It All Toge
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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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Page 280 and 281: Overview CHAPTER 18 The Template Me
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Page 312 and 313: CHAPTER 22 Bibliography This book h
Page 314 and 315: • Meyer, B., Object-Oriented Soft
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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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