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 ''[[Abstractabstract method|abstract methods]]s'', 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|access-date=2012-09-12}}</ref>

==Overview==

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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

* implementing the invariant parts of the behavior and

* sending to '''self''' the messages <code>primitive1()</code> and <code>primitive2()</code> , which, because they are implemented in <code>SubClass1</code> , allow that subclass to provide a variant implementation of those parts of the algorithm.

[[Image:Template Method pattern in LePUS3.gif|thumb|none|300px|Template Method in [[Lepus3|LePUS3]].<ref>LePUS3 legend. Retrieved from http://lepus.org.uk/ref/legend/legend.xml.</ref>]]

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=== Use with code generators ===

The template pattern is useful when working with auto-generated code. The challenge of working with generated code is that changes to the source code will lead to changes in the generated code; if hand-written modifications have been made to the generated code, these will be lost. How, then, should the generated code be customized?

The Template pattern provides a solution. If the generated code follows the template method pattern, the generated code will all be an abstract superclass. Provided that hand-written customizations are confined to a subclass, the code generator can be run again without risk of over-writing these modifications. When used with code generation, this pattern is sometimes referred to as the [[Generation gap (pattern)|generation gap pattern]].<ref>{{cite book|last1=Vlissides|first1=John|title=Pattern Hatching: Design Patterns Applied|date=1998-06-22|publisher=Addison-Wesley Professional|isbn=978-0201432930|pages=85–101|url=http://www.informit.com/store/pattern-hatching-design-patterns-applied-9780201432930}}</ref>