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2014-05-09T09:06:40Z

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← Older revision		Revision as of 09:06, 9 May 2014
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	==Subclasses==		==Subclasses==

	Every object in the Smalltalk-80 system is an instance of a class. All instances of a class represent the same kind of system component. For example, each instance of Rectangle represents a rectangular area and each instance of Dictionary represents a set of associations between ~~namesand~~ values. The fact that the instances of a class all represent the same kind of component is reflected both in the way the instances respond to messages and in the form of their instance variables.		Every object in the Smalltalk-80 system is an instance of a class. All instances of a class represent the same kind of system component. For example, each instance of Rectangle represents a rectangular area and each instance of Dictionary represents a set of associations between names and values. The fact that the instances of a class all represent the same kind of component is reflected both in the way the instances respond to messages and in the form of their instance variables.

	* All instances of a class respond to the same set of messages and use the same set of methods to do so.		* All instances of a class respond to the same set of messages and use the same set of methods to do so.
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	Suppose we send an instance of DeductibleHistory a message with selector spend:for:. This method is found in the ~~supei'class~~ FinancialHistory. The method, as given in Chapter 3, is		Suppose we send an instance of DeductibleHistory a message with selector spend:for:. This method is found in the superclass FinancialHistory. The method, as given in Chapter 3, is

	<syntaxhighlight lang="smalltalk">		<syntaxhighlight lang="smalltalk">
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	===Messages to self===		===Messages to self===

	When a method contains a message whose receiver is self, the search for the method for that message begins in the instance's class, regardless of which class contains the method containing self. Thus, when the expression self totalSpentFor: reason is evaluated in the method for spend:for: found in FinancialHistory, the search for the method associated with the message selector totalSpentFor: begins in the class of self, i.e., ~~i n~~ DeductibleHistory.		When a method contains a message whose receiver is self, the search for the method for that message begins in the instance's class, regardless of which class contains the method containing self. Thus, when the expression self totalSpentFor: reason is evaluated in the method for spend:for: found in FinancialHistory, the search for the method associated with the message selector totalSpentFor: begins in the class of self, i.e., in DeductibleHistory.


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	As an example of the use of an abstract Superclass, consider two classes whose instances represent dictionaries. One class, named SmallDictionary, minimizes the space needed to store its contents; the other, named FastDictionary, stores names and values sparsely and uses a hashing technique to locate names. Both classes use two parallel lists that contain names and associated values. SmallDictionary stores the names and values contiguously and uses a simple linear search to locate a name. FastDictionary stores names and values sparsely and uses a hashing technique to locate a name. Other than the difference in how names are located, these two classes are very similar: they share identical protocol and they both use parallel ~~list s~~ to store their contents. These similarities are represented in an abstract superclass named DualListDictionary. The relationships among these three classes is shown in Figure 4.8.		As an example of the use of an abstract Superclass, consider two classes whose instances represent dictionaries. One class, named SmallDictionary, minimizes the space needed to store its contents; the other, named FastDictionary, stores names and values sparsely and uses a hashing technique to locate names. Both classes use two parallel lists that contain names and associated values. SmallDictionary stores the names and values contiguously and uses a simple linear search to locate a name. FastDictionary stores names and values sparsely and uses a hashing technique to locate a name. Other than the difference in how names are located, these two classes are very similar: they share identical protocol and they both use parallel lists to store their contents. These similarities are represented in an abstract superclass named DualListDictionary. The relationships among these three classes is shown in Figure 4.8.

	[[image:Smalltalk80LanguageImplementation_4-8.png\|그림 4-8]]		[[image:Smalltalk80LanguageImplementation_4-8.png\|그림 4-8]]
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	This description of DualListDictionary uses only messages defined in DualListDictionary itself or ones already described in this or in the previous chapters. The external protocol for a DualListDictionary consists of messages at:, at:put:, includes:, isEmpty, and initialize. A new DualListDictionary (actually ~~a n~~ instance of a subclass of DualListDictionary) is created by sending it the message new. It is then sent the message initialize so that assignments can be made to the two instance variables. The two variables are initially empty arrays (Array new: 0).		This description of DualListDictionary uses only messages defined in DualListDictionary itself or ones already described in this or in the previous chapters. The external protocol for a DualListDictionary consists of messages at:, at:put:, includes:, isEmpty, and initialize. A new DualListDictionary (actually an instance of a subclass of DualListDictionary) is created by sending it the message new. It is then sent the message initialize so that assignments can be made to the two instance variables. The two variables are initially empty arrays (Array new: 0).


	Three messages to self used in its methods are not implemented in DualListDictionary--size, indexOf:, and newIndexOf:. This is the reason that DualListDictionary is called abstract. If an instance were created, it would not be ~~a:ble~~ to respond successfully to all of the necessary messages. ~~Thetwo~~ subclasses, SmallDictionary and FastDictionary, must implement the three missing messages. The fact that the search always starts at the class of the instance referred to by self means that a ~~meth od~~ in a superclass can be specified in which messages are sent to self, but the corresponding methods are found in the subclass. In this way, a superclass can provide a framework for a method that is refined or actually implemented by the subclass.		Three messages to self used in its methods are not implemented in DualListDictionary--size, indexOf:, and newIndexOf:. This is the reason that DualListDictionary is called abstract. If an instance were created, it would not be able to respond successfully to all of the necessary messages. The two subclasses, SmallDictionary and FastDictionary, must implement the three missing messages. The fact that the search always starts at the class of the instance referred to by self means that a method in a superclass can be specified in which messages are sent to self, but the corresponding methods are found in the subclass. In this way, a superclass can provide a framework for a method that is refined or actually implemented by the subclass.


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	The implementation of newIndexOf: follows basically the same idea as ~~tha t~~ used for SmallDictionary except that when the size of an Array is changed (doubled in this case in the method for grow), each element is explicitly copied from the old Arrays into the new ones so that elements are rehashed. The size does not always have to be changed as is necessary in SmallDictionary. The size of a FastDictionary is changed only when the number of empty locations in names falls below a minimum.		The implementation of newIndexOf: follows basically the same idea as that used for SmallDictionary except that when the size of an Array is changed (doubled in this case in the method for grow), each element is explicitly copied from the old Arrays into the new ones so that elements are rehashed. The size does not always have to be changed as is necessary in SmallDictionary. The size of a FastDictionary is changed only when the number of empty locations in names falls below a minimum.

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2014-04-21T07:25:20Z

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