Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Development ExampleIn this chapter, I will pretend to develop a piece of software guided by the Best Practice Patterns. Itwill likely seem incredibly slow to you, since I won’t write more than a couple of words of codewithout pointing out another pattern. Not to worry, once you get used to the patterns you don’treally think of them individually. You build higher levels of structure in your mind that let you justwrite clean code quickly and without thinking consciously of the smaller steps, much as you canspeak without consulting a grammar book.Coding Patterns page 140 of 147 9/30/2006
ProblemThe problem we will solve is representing multi-currency monetary values. It’s not as simple as itmight seem. There are lots of unit-value computations in the world. Any time you use a numberyou generally have at least an implied set of units behind it, whether inches, pixels, or kilograms.Some units can be freely converted into others like pounds to kilograms, others just don’t makeany sense. If you add four meters to two grams you should get an error. One simplifyingassumption that makes unit-values easy to program is that for conversions that make sense, there isa single, immutable conversion ratio. You don’t have to go look up the New York spot quote onpound to kilograms every morning.Money is different. There is no single conversion rate. There isn’t really a single sequence ofconversion rates. Sometimes you want to answer questions like, “How does the value of myportfolio differ if I liquidate it in Hong Kong or Zurich?” This need for flexibility is the primarydriving force in designing objects to represent currency.StartThe first object we’ll give the Simple Superclass Name of Money. It represents a value in a singlecurrency.Class: Moneysuperclass: Objectinstance variables: amount currencyThe variable with the Role Suggesting Instance Variable Name “amount” will hold any oldNumber. This lets us defers issues of numerical accuracy and stability to the Number classes. If youwant a super accurate but slow Money, you can make the Amount a FixedPoint. If you want lessaccuracy but more speed, you can use a Double. As long as the amount responds to number-likemessages, Money won’t care.The variable “currency” will hold Symbol for now, the name of a currency. Currency traders youstandard three letter abbreviations for the currencies of the world, USD for United States Dollars,for example. A complete currency system needs a real Currency object, because differentcurrencies act different computationally, but for now all we care about is whether two currenciesare equal or not.How do we create a Money? We need a Complete Creation Method.Money class>>amount: aNumber currency: aSymbol^self newsetAmount: aNumbercurrency: aSymbolThe method communicates the types of its parameters with Type Suggesting Parameter Names. Thebody of the method uses Indented Control Flow to make it clear to the reader that the CreationParameter Method takes two arguments. Complete Creation Methods also always have anInteresting Return Value.Now we need to set the instance variables to the objects provided to the Complete CreationMethod. We write a Creation Parameter Method:Coding Patterns page 141 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
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Enumeration MethodYou are using Ind
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Role Suggesting Instance Variable N
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Temporary VariableA Composed Method
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Caching Temporary VariableA Perform
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Reusing Temporary VariableTemporary
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CollectionsThe collection hierarchy
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CollectionHow do you represent a on
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RunArrayYou are using an OrderedCol
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Page 94 and 95: DictionaryYou need a Collection tha
Page 96 and 97: SortedCollectionYou have a Collecti
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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
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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 124 and 125: Qualified Subclass NameYou have cre
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 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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