Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Default Value MethodYou have a complicated default value in Explicit Initialization. You need a default value for LazyInitialization.How do you represent the default value of a variable?The simplest way to provide default values is right in line with the code. It’s generally easy to readand quick and simple to write.Patterns whose purpose is creating flexibility (Lazy Initialization), or who may need additionalcommunication (Explicit Initialization), need an Intention Revealing Message. It gives you anopportunity to communicate through the selector. It gives future subclasses the option of simplyoverriding a single value.Create a method that returns the value. Prepend “default” to the name of the variable as thename of the method.If you are computing the value of a Caching Instance Variable, call the method “computeArea”rather than “defaultArea”. This will provide a clue to readers that the variable is intended as acache.Use a Constant Method for constant default values.Coding Patterns page 66 of 147 9/30/2006
Constant MethodYou are writing a Composed Method. You may need a Default Value Method.How do you represent a constant?Smalltalk provides several options for representing shared constants or variables. Pools provide aset of variables that can be used in any number of different classes. Class variables are variablesthat are shared throughout a hierarchy.I know that some folks use lots of pools in their applications. I never use them. Here’s what I don’tlike about pools- they make it too easy to stop making useful responsibilities and instead justspread data around.The IBM Smalltalk window system is a good example. There is a pool called CwConstants thatcontains hundreds of constants. Rather than being able to send a list pane a message like#singleSelect to make it select one item at a time, you send it “selectionPolicy:XmSINGLESELECT”.Of course, you could say that this is just one little thing you have to remember, and besides, Cprogrammers have to deal with stuff like this all the time. However, multiply “one little thing toremember” by several hundred and you don’t have a system that feels much like Smalltalk anymore, at least not when you’re working with the window system.I would rather limit the visibility of constants to a single class. Then that class can providemeaningful behavior to other objects. If you can refactor your code so that only a single class caresabout a constant, then you can represent that constant as a method.In the list pane example, this means leaving the original implementation of #selectionPolicy: alone,but not expecting outside objects to invoke it. Instead, for each of the possible argument values wecreate a method:ListPane>>singleSelectself selectionPolicy: 15If other methods also needed to know that 15 was the magic constant for single selection, we createa method for it, a method that just returns 15.ListPane>>singleSelectPolicy^15ListPane>>singleSelectself selectionPolicy: self singleSelectPolicyOne of the downsides of representing constants as methods is it is easier to automatically generatethe initialization for a pool than it is to automatically create a method. If you have to synchronize aset of constants with other programs, you may find it easier to use pools. However, you should stillstrive to provide rich protocol so that few classes need to know about the pool.Another question about Constant Methods is what happens when you have hundreds or thousandsof constants? A pool can easily grow to contain as many entries as necessary. A class with athousand Constant Methods would look cluttered.Create a method that returns the constant.Coding Patterns page 67 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
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 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 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
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implementation of queues in Pascal
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Lookup CacheHow do you optimize com
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Parsing StreamHow do you write a si
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ClassesThere is probably no coding
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Qualified Subclass NameYou have cre
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FormattingNo other topic generates
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Type Suggesting Parameter NameYou a
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Rectangular BlockYou are writing an
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Conditional ExpressionHow do you fo
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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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