Template method pattern: Difference between revisions

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In [[object-oriented programming]], the '''template method''' is one of the [[behavioral pattern|behavioral]] [[Software design pattern|design patterns]] identified by Gamma et al.<ref name=":0">{{cite book|title=Design Patterns|last1=Gamma|first1=Erich|last2=Helm|first2=Richard|last3=Johnson|first3=Ralph|last4=Vlissides|first4=John|publisher=Addison-Wesley|year=1994|isbn=0-201-63361-2|pages=[https://archive.org/details/designpatternsel00gamm/page/325 325–330]|chapter=Template Method|author-link1=Erich Gamma|author-link2=Richard Helm|author-link3=Ralph Johnson (computer scientist)|author-link4=John Vlissides|title-link=Design Patterns}}</ref> in the book ''[[Design Patterns]]''. The template method is a method in a superclass, usually an abstract superclass, and defines the skeleton of an operation in terms of a number of high-level steps. These steps are themselves implemented by additional ''helper methods'' in the same class as the template method.

The ''helper methods'' may be either ''[[Abstract method|abstract methods]]'', in which case subclasses are required to provide concrete implementations, or ''[[hook methods]],'' which have empty bodies in the superclass. [[Subclass (computer science)|Subclass]]es can (but are not required to) customize the operation by [[Method overriding|overriding]] the hook methods. The intent of the template method is to define the overall structure of the operation, while allowing subclasses to refine, or redefine, certain steps.<ref name=":2">{{cite book|url=http://shop.oreilly.com/product/9780596007126.do|title=Head First Design Patterns|last2=Freeman|first2=Elisabeth|last3=Sierra|first3=Kathy|last4=Bates|first4=Bert|publisher=O'REILLY|year=2004|isbn=978-0-596-00712-6|editor-last=Hendrickson|editor-first=Mike|volume=1|pages=289, 311|format=paperback|editor-last2=Loukides|editor-first2=Mike|last1=Freeman|first1=Eric|accessdateaccess-date=2012-09-12}}</ref>

==Overview==

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== Structure ==

=== UML class diagram ===

[[File:w3sDesign Template Method Design Pattern UML.jpg|frame|none|A sample UML class diagram for the Template Method design pattern.<ref>{{cite web|title=The Template Method design pattern - Structure |url=http://w3sdesign.com/?gr=b10&ugr=struct|website=w3sDesign.com|accessdateaccess-date=2017-08-12}}</ref>]]

In the above [[Unified Modeling Language|UML]] [[class diagram]], the <code>AbstractClass</code> defines a <code>templateMethod()</code> operation that defines the skeleton (template) of a behavior by

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==Usage==

The template method is used in frameworks, where each implements the invariant parts of a ___domain's architecture, while providing hook methods for customization. This is an example of [[inversion of control]]. The template method is used for the following reasons.<ref name=":1">{{cite web | url = http://sourcemaking.com/design_patterns/template_method | title = Template Method Design Pattern | publisher = Source Making - teaching IT professional | quote = Template Method is used prominently in frameworks. | accessdateaccess-date = 2012-09-12}}</ref>

* It lets subclasses implement varying behavior (through [[Method overriding (programming)|overriding]] of the hook methods).<ref name=":3">{{Cite book|title=Pro Objective-C Design Patterns for iOS|last=Chung|first=Carlo|publisher=Apress|year=2011|isbn=978-1-4302-3331-2|___location=Berkeley, CA|pages=266}}</ref>

* It avoids duplication in the code: the general workflow of the algorithm is implemented once in the abstract class's template method, and necessary variations are implemented in the subclasses.<ref name=":3" />