Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Collection Accessor MethodYou are using Indirect Variable Access.How do you provide access to an instance variable that holds a collection?The simplest solution is just to publish a Getting Method for the variable. That way, any client thatwants can add to, delete from, iterate over, or otherwise use the collection.The problem with this approach is that it opens up too much of the implementation of an object tothe outside world. If the object decides to change the implementation, say by using a different kindof collection, the client code may well break.The other problem with just offering up your private collections for public viewing is that it is hardto keep related state current when someone else is changing the collection without notifying you.Here’s a department that use a Caching Instance Variable to speed access to its total salary:Departmentsuperclass: Objectinstance variables: employees totalSalarytotalSalarytotalSalary isNil ifTrue: [totalSalary := self computeTotalSalary].^totalSalarycomputeTotalSalaryêmployeesinject: 0into: [:sum :each | sum + each salary]clearTotalSalarytotalSalary := nilWhat happens if client code deletes an employee without notifying the department?...aDepartment employees remove: anEmployee...The totalSalary cache never gets cleared. It now contains a number inconsistent with the valuereturned by computeTotalSalary.The solution to this is not to let other objects have your collections. If you use Indirect VariableAccess, make sure Getting Methods for collections are private. Instead, give clients restrictedaccess to operations on the collection through messages that you implement. This gives you achance to do whatever other processing you need to.The downside of this approach is that you have to actually implement all of these methods. Givingaccess to a collection takes one method. You might need four or five methods to provide all thenecessary protected access to the collection. In the long run, it’s worth it though, because your codewill read better and be easier to change.Implement methods that are implement with Delegation to the collection. To name themethods, add the name of the collection (in singular form) to the collection messages.If Department wanted to let others add and delete employees, it would implement CollectionAccessor Methods:Coding Patterns page 72 of 147 9/30/2006
addEmployee: anEmployeeself clearTotalSalary.employees add: anEmployeeremoveEmployee: anEmployeeself clearTotalSalary.employees remove: anEmployeeDon’t just blindly name a Collection Accessor Method after the collection message it delegates.See if you can find a word from the domain that makes more sense. For example, I prefer:employs: anEmployeeêmployees includes: anEmployeeto:includesEmployee: anEmployeeêmployees includes: anEmployeeImplement an Enumeration Method for safe and efficient general collection access.Coding Patterns page 73 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
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 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 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
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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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