Smalltalk Best Practice Patterns Volume 1: Coding - Free

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You may need Double Dispatch for complex multi-class equality tests. Testing is often done interms of Simple Delegation with one or more components of an objectCoding Patterns page 92 of 147 9/30/2006
Hashing MethodYou have written an Equality Method. You may be putting new objects into a Set. You may beusing a new object as a key in a Dictionary.How do you ensure that new objects work correctly with hashed Collections?Once you understand the idea that you can override “=“, the world becomes your oyster. Newobjects that have their own definition of equality automatically begin to play nicely with the rest ofSmalltalk. Almost.Collection>>includes: uses equality:Collection>>includes: anObjectself do: [:each | each = anObject ifTrue: [^true]].^falseSo you start out with an OrderedCollection and your newly equal objects and everything worksfine. Then you decide you need a Set. All of a sudden, you start getting bugs. You look at a Set andit has two objects in it that you know are equal. Weren’t Sets supposed to avoid that problem?The problem is that Set>>includes: doesn’t just send “=“, it also sends “hash”. If two objects areequal but they don’t return the same hash value, the Set is likely to miss the fact that they are equal.If you override “=“ and use the object with a hashed Collection, override “hash” so that twoobject that are equal return the same hash value.As it stands, this rule is kind of abstract. In practice, though, it is simple to satisfy this constraint.The Equality Method typically relies on some set of messages returning results that are equal:Book>>= aBook^self title = aBook title & (self author = aBook author)My first cut implementation of “hash” is to get the hash values of all the messages that the EqualityMethod relies on and put them together with bit-wise exclusive-or.Book>>hash^self title hash bitXor: self author hashThis works whether the messages return Strings, Numbers, Arrays, or whatnot, because all the subcomponentsobey the Hashing Method pattern.Hashing is often implemented in terms of Simple Delegation.Coding Patterns page 93 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
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 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 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
Page 136 and 137: self listPane parentcolor: Color bl
Page 138 and 139: Point>>printOn: aStreamaStreamprint
Page 140 and 141: 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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