Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Integer>>addFloat: aFloat^self asFloat addFloat: aFloatFloat>>addInteger: anInteger^self addFloat: anInteger asFloatIn the worst case, Double Dispatch can lead to N x M methods, where N is the number of classes ofthe original receiver and M is the number of classes of the original argument. Practically speaking,the receiver classes are usually related by inheritance and the argument classes are also usuallyrelated by inheritance so many common implementations can be factored out.Give your messages Intention Revealing Selectors.Coding Patterns page 44 of 147 9/30/2006
Send BackYou are using Self Delegation.How do you represent a computation when it has variations for each of several classes?The natural procedural reaction to this situation is a case statement. Depending on which class youare dealing with you execute different code. This solution has the advantage that theimplementation is centralized. You can look in one place and see the logic. It has the enormousdisadvantage that it is difficult to extend. When you add a new class to the system, you have to adda class to every case statement that might encounter that class.The “object-oriented” solution is to put the pieces of variant logic into the various classes. Themessage that is sent to one or the other of the classes will choose the right logic to invoke. Thissolution is much more flexible than case statements. Someone can come in with a new class and, aslong as it implements the necessary messages, it will play without further change in theenvironment.Following control flows in such a system can be tortuous. A message comes to one object, isdelegated to another, which sends a new message to a third (or the original). Sometimes I build astructure based on Send Backs only to discover that I am completely lost.The temporary dislocation caused by such a control flow is worth it in the end, because of theadditional flexibility of the system. Once you have it set up and working, adding new functionalitybecomes simple. Looking back at our original criteria for patterns, I would say Send Back adds risk(because the system is more complex), but it improves communication (because it is easy to seehow functionality is intended to be added) and reduces long-term costs.Delegate to an object that sends a message back to the first object.In VisualWorks, there is an object called InstructionStream that will Send Back to any object youprovide a set of messages that are the symbolic version of the instructions in a CompiledMethod.One of those messages is #sendSuper:numArgs:. There is also an object called InstructionClientthat provides dummy implementations of all these messages.Let’s say we want to find out if a method invokes super, and we want to do it without access to thesource code. We can use InstructionClient as a superclass, so we only have to implement onemethod.Coding Patterns page 45 of 147 9/30/2006
Page 1 and 2: Smalltalk Best Practice PatternsVol
Page 3 and 4: STATE..............................
Page 5 and 6: PrefaceThis preface will explain wh
Page 7: AcknowledgmentsI would like to than
Page 10 and 11: Many of the patterns tell you what
Page 12 and 13: day to day in development and docum
Page 14 and 15: What’s Missing?There are a few co
Page 16 and 17: AdoptionHow can you learn and use t
Page 18 and 19: Why Patterns WorkHere is the Big As
Page 20 and 21: Role of PatternsReadingThese patter
Page 22 and 23: BehaviorObjects model the world thr
Page 24 and 25: Composed MethodYou are implementing
Page 26 and 27: Complete Creation MethodA Composed
Page 28 and 29: ConversionHow do you convert inform
Page 30 and 31: Converter Creation MethodYou need t
Page 32 and 33: Query MethodA Composed Method has h
Page 34 and 35: 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 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 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
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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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