Smalltalk Best Practice Patterns Volume 1: Coding - Free

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CollectionHow do you represent a one-to-many relationship?Every programming language provides facilities to represent one-to-many relationships. The firstdata structure in FORTRAN was the array.Computer science has made a franchise of representing one-to-many relationships. How manythousands of varieties of trees, lists, and tables are there hidden away in the Journals of the ACM?Such an important topic has picked up a vast quantity of idiom in every programming language.Even though I hardly use C any more, I can instantly recognize:for (i = 0; i < n; i++) ...as iterating over an array.One of the most brilliant early developments in Smalltalk was presenting a unified protocol to allthe varieties of ways of representing one-to-many relationships. No longer does all client codedirectly encode how to iterate over elements. Iteration is the responsibility of the objectrepresenting the relationship itself. This results in tremendous power throughout development. Youcan change from a linear list to a hash table by changing “OrderedCollection” to “Set”, with theconfidence that no other code will be affected.The down side of the Collection classes is their very power. Because you can write code involvingCollections in a very few words, you have to be initiated in the special meaning of those wordsbefore you have a chance of understanding such code. Once you understand the vocabulary,though, you will never be able to write code in a language without support for Collections withouta wistful sigh about “how easy this would be in Smalltalk”.There is an alternative to using the Collection classes. Any class can be made indexable, that ishave both named variables and variables you access through at: and at:put:. Using this feature, anyclass can be made to act like both a regular object and a collection at the same time. This would beuseful if you were extremely tight on space, but copying the Collection code just isn’t worth it.Create a Collection to represent the one-to-many relationship.Use an OrderedCollection for Collections that change size dynamically. Use an Array for fixedsizedCollections. Use a Set to ensure element uniqueness. Use a Dictionary to map from one kindof object to another. Use a Temporarily Sorted Collection for ordering elements. Use aSortedCollection to keep elements in a computed order.Coding Patterns page 86 of 147 9/30/2006
OrderedCollectionYou need a dynamically sized Collection.How do you represent Collections whose size can’t be determined when they are created?Many of the hassles programs give users come from the need for flexibility. Primitive memorymanagement has lead to a lack of flexibility in the sizes of data. Every time I read “you can have upto 99 files” or “each function can have no more than 256 lines” I imagine a software engineerpreallocating memory to handle just that many items. While arbitrary limits like “99” or “256” aretypically chosen because the engineers can’t imagine the need for any more, just as typically theybecome a hindrance to a user somewhere down the road (sometimes years down the road).There is no excuse for arbitrary data size limitations in Smalltalk. Two factors work in your favoras you try to eliminate such limits. First, the Collection classes give you tremendous leverage tochange your mind about representing one-to-many relationships. Second, the garbage collectorfrees you from the drudgery of maintaining references as data structures change size.This flexibility comes at a cost. The implementation of OrderedCollection, the most commondynamically sized Collection, allocates more memory than it strictly needs at first, so that somegrowth can be accommodated at little cost. The implementation also uses indirection to accesselements. In some Collection-intensive code I have found this to be a bottleneck. While you’re justtrying to get the program working, though, you shouldn’t worry about such issues. Time enough toaddress them later.Use an OrderedCollection as your default dynamically sized Collection.Change to a Set if you need to ensure that every element appears only once. Change to a RunArrayif you want to compactly represent a collection with long runs of the same element. Change to anArray if you don’t need dynamic sizing.Coding Patterns page 87 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
Page 36 and 37: MessagesMessages are the heartbeat
Page 38 and 39: Choosing MessageYou are using a Mes
Page 40 and 41: Decomposing MessageYou are using a
Page 42 and 43: Intention Revealing SelectorYou may
Page 44 and 45: Integer>>addFloat: aFloat^self asFl
Page 46 and 47: Class: SuperFindingClientsuperclass
Page 48 and 49: Extending SuperYou are using Super.
Page 50 and 51: DelegationA Composed Method needs w
Page 52 and 53: Self DelegationYou are using Delega
Page 54 and 55: Pluggable BehaviorHow can you param
Page 56 and 57: Pluggable BlockYou need complex Plu
Page 58 and 59: marriedMenAndUnmarriedWomen^self ma
Page 60 and 61: Instance VariablesI wrote the secti
Page 62 and 63: Variable StateHow do you represent
Page 64 and 65: Timer>>defaultMillisecondPeriod^100
Page 66 and 67: Default Value MethodYou have a comp
Page 68 and 69: Direct Variable AccessYou need to a
Page 70 and 71: Getting MethodYou are using Lazy In
Page 72 and 73: 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 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 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
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self listPane parentcolor: Color bl
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Point>>printOn: aStreamaStreamprint
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Development ExampleIn this chapter,
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Money>>setAmount: aNumber currency:
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MoneySum>>printOn: aStreammonies do
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| m1 m2 |m1 := Moneyamount: 5curren
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Smalltalk Best Practice Patterns Volume 1: Coding - Free

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