Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Qualified Subclass NameYou have created a Simple Superclass Name.What should you name a new subclass?One way to name classes is to give each a unique name, indicative of its purpose. Unique namesgive you the opportunity to convey a lot of information without creating names that are long andunwieldy.This is exactly the right thing to do if the names are in common use. Array is a subclass ofCollection, because lots of software engineers guess correctly about the nature of the class byreading “Array”. Number is a subclass of Magnitude, because Number-ness is far more importantto communicate than Magnitude-ness. String is a subclass of Collection, because everyone knowswhat a String is.In general, if I am using inheritance strictly for code sharing, but the role of the subclass is differentthan the role of the superclass, I go back to Simple Superclass Name.More often, the inheritance structure of your code is important for a reader to understand,particularly where a subclass is both conceptually a variation on the superclass and they shareimplementation. The two pieces of information you need to communicate are:How the new class is the sameHow the new class is differentYou can communicate how the new class is the same by naming some superclass. It need not be theimmediate superclass, if some distant ancestor communicates more clearly.You can communicate how the new class is different by finding a word that accurately highlightsthe reason the new class isn’t just the superclass.Name subclasses in your hierarchies by prepending an adjective to the superclass name.For example:OrderedCollection is a Collection in which elements are OrderedSortedCollection is a Collection in which elements are Sorted. Even though SortedCollectionsubclasses OrderedCollection you wouldn’t call it SortedOrderedCollection. The choice tosubclass OrderedCollection is strictly for implementation reasons, not because the twocollections play similar roles.LargeInteger is an Integer that takes many bits to represent.Coding Patterns page 124 of 147 9/30/2006
Factored SuperclassHow do you design inheritance hierarchies?Of the three pillars of objects- encapsulation, polymorphism, and inheritance, inheritance provokesthe most controversy. Opinions of what it is and how it should be used are legion. For some reason,inheritance debates seem to generate intense emotions. This makes any concrete statement abouthow to use inheritance a dangerous proposition. I’m bound to offend someone, or at least challengetheir current thinking.The purpose of patterns is to demystify, to reduce to repeatable practice. Ignoring or waffling oninheritance would be a breach of my responsibility as an author. Buckle your seat belts, here comesheresy!The inheritance debate is clouded by the many legitimate uses to which it is put in differentcontexts. As originally conceived in artificial intelligence, subclasses are similar to superclasses.Thus, mammal is subclass of animal and employee is a subclass of person. This relationship oftendescribed as “is-a”. A mammal “is-an” animal and an employee “is-a” person. Knowledgerepresentation languages use inheritance to represent conceptual classification.As implemented in object programming languages, from Simula on down, inheritance doessomething quite different. Inheritance shares implementation. An instance of a subclass with nomethods responds to all messages exactly as an instance of the superclass would. As the subclassoverrides or extends superclass methods, its implementation begins to diverge. This use ofinheritance can be summarized as “is-implemented-like”. Thus, a code editor “is-implemented-like”a text editor and a socket stream “is-implemented-like” a serial stream.C++ and Eiffel further cloud the picture by using inheritance to specify types. Sets of messages arerepresented as classes, often with no implementation. If an object is to be used in a context wherethose messages will be sent, it must inherit from one of these “mix-in” classes. In these languages,inheritance means both “is-implemented-like” and “responds-to-the-same-messages-as”.Just to be sure the picture isn’t at all clear, communicating protocols (sets of messages that oneobject expects of another) is very important. Often, protocols are really the heart of how a design isto be reused. If you can create an Impostor for an object in a framework, you can suddenly unlock awhole world of functionality.Well, then, how should Smalltalk inheritance be used?Make all of your classes subclasses of Object at first. Create a superclass to hold commoncode of two or more existing classes.Coding Patterns page 125 of 147 9/30/2006
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Smalltalk Best Practice PatternsVol
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STATE..............................
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PrefaceThis preface will explain wh
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AcknowledgmentsI would like to than
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Many of the patterns tell you what
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day to day in development and docum
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What’s Missing?There are a few co
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AdoptionHow can you learn and use t
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Why Patterns WorkHere is the Big As
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Role of PatternsReadingThese patter
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BehaviorObjects model the world thr
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Composed MethodYou are implementing
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Complete Creation MethodA Composed
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ConversionHow do you convert inform
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Converter Creation MethodYou need t
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Query MethodA Composed Method has h
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Execute Around MethodHow do you rep
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MessagesMessages are the heartbeat
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Choosing MessageYou are using a Mes
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Decomposing MessageYou are using a
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Intention Revealing SelectorYou may
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Integer>>addFloat: aFloat^self asFl
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Class: SuperFindingClientsuperclass
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Extending SuperYou are using Super.
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DelegationA Composed Method needs w
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Self DelegationYou are using Delega
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Pluggable BehaviorHow can you param
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Pluggable BlockYou need complex Plu
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marriedMenAndUnmarriedWomen^self ma
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Instance VariablesI wrote the secti
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Variable StateHow do you represent
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Timer>>defaultMillisecondPeriod^100
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Default Value MethodYou have a comp
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Direct Variable AccessYou need to a
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Getting MethodYou are using Lazy In
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Collection Accessor MethodYou are u
Page 74 and 75: Enumeration MethodYou are using Ind
Page 76 and 77: Role Suggesting Instance Variable N
Page 78 and 79: Temporary VariableA Composed Method
Page 80 and 81: Caching Temporary VariableA Perform
Page 82 and 83: Reusing Temporary VariableTemporary
Page 84 and 85: CollectionsThe collection hierarchy
Page 86 and 87: CollectionHow do you represent a on
Page 88 and 89: RunArrayYou are using an OrderedCol
Page 90 and 91: | s |s := Set new.Like an OrderedCo
Page 92 and 93: You may need Double Dispatch for co
Page 94 and 95: DictionaryYou need a Collection tha
Page 96 and 97: SortedCollectionYou have a Collecti
Page 98 and 99: ByteArrayYou need to represent an A
Page 100 and 101: (1 to: anArray size) collect:[:each
Page 102 and 103: IsEmptyHow do you test if a collect
Page 104 and 105: ConcatentationHow do you put two co
Page 106 and 107: DoHow do you execute code for each
Page 108 and 109: Select/RejectHow do you execute cod
Page 110 and 111: Inject:into:You need an Enumeration
Page 112 and 113: Duplicate Removing SetYou have a Co
Page 114 and 115: StackHow do you implement a stack?O
Page 116 and 117: implementation of queues in Pascal
Page 118 and 119: Lookup CacheHow do you optimize com
Page 120 and 121: Parsing StreamHow do you write a si
Page 122 and 123: ClassesThere is probably no coding
Page 126 and 127: FormattingNo other topic generates
Page 128 and 129: Type Suggesting Parameter NameYou a
Page 130 and 131: Rectangular BlockYou are writing an
Page 132 and 133: Conditional ExpressionHow do you fo
Page 134 and 135: Simple Enumeration ParameterWhat sh
Page 136 and 137: self listPane parentcolor: Color bl
Page 138 and 139: Point>>printOn: aStreamaStreamprint
Page 140 and 141: Development ExampleIn this chapter,
Page 142 and 143: Money>>setAmount: aNumber currency:
Page 144 and 145: MoneySum>>printOn: aStreammonies do
Page 146: | m1 m2 |m1 := Moneyamount: 5curren
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Smalltalk Best Practice Patterns Volume 1: Coding - Free

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