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principles:encapsulate_the_concept_that_varies [2021-09-02 12:26]
65.21.179.175 old revision restored (2021-05-19 11:22) 		principles:encapsulate_the_concept_that_varies [2021-10-18 21:31]
christian old revision restored (2019-10-07 08:29)
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 ===== Context ===== ===== Context =====
 /* fill in contexts here: */ /* fill in contexts here: */
-  * [[contexts:Object-Oriented Design]]  +  * [[contexts:Object-Oriented Design]] 
 +  * [[contexts:API Design]] 
 +  * [[contexts:Architecture]]
  
 ===== Principle Statement ===== ===== Principle Statement =====
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 ===== Description ===== ===== Description =====
 +
 +This principle has two aspects that roughly correspond to the two sub-principles [[Single Responsibility Principle|SRP]] and [[Open-Closed Principle|OCP]]. The first one is about making changes local. Everything which is supposed to change in the future should be encapsulated in a single module. This means [[glossary:cross-cutting concerns]] are avoided as much as possible. This is not completely possible but in many cases it is.
 +
 +The second aspect is about introducing abstractions. Sometime the varying concept is one which varies at runtime rather than by maintenance. So at runtime it is decided upon a certain variation or there can be even several variations at the same time. In this case there has to be an abstract base class or an interface which encapsulates the varying concept. Several concrete descendant classes then specify the concrete variation.
 +
 +The difference between the two aspects is whether the varying concept is one that changes over time during maintenance or one that may change at runtime. Nevertheless the advice is the same: encapsulate the concept that varies.
  
  
 ===== Rationale ===== ===== Rationale =====
 +
 +There are two reasons for this principle. The first reason is locality. When a varying concept is properly encapsulated in a single module, only this module is affected in case of a change. This reduces maintenance effort and [[glossary:ripple effects]].
 +
 +The second reason comes to play when the varying concept is implemented as an abstract class or interface. In this case a variation can be introduced without changing existing and tested code. This reduces testing effort (as already tested code does not need to be retested as it is not changed) as well as ripple effects (as the enhancement is done simply by adding a class. Note that for this rationale to work, the [[Liskov Substitution Principle]] also has to be adhered to.
  
  
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     * Visitor: New operations have to be added to a given more or less static inheritance structure of classes.     * Visitor: New operations have to be added to a given more or less static inheritance structure of classes.
     * ...     * ...
 +
 +===== Caveats =====
 +
 +See section [[#contrary principles]].
 +
  
 ===== Origin ===== ===== Origin =====
 +
 +The principle is stated, explained and used in the GoF book:
  
 Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: //[[wp>Design Patterns|Design Patterns: Elements of Reusable Object-Oriented Software]]// ("GoF book"), p. 29 Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: //[[wp>Design Patterns|Design Patterns: Elements of Reusable Object-Oriented Software]]// ("GoF book"), p. 29
 +
 +But the idea if ECV is actually much older. It was first presented in
 +
 +David Parnas: //[[http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.132.7232|On the Criteria To Be Used in Decomposing Systems into Modules]]//
 +
  
 ===== Evidence ===== ===== Evidence =====
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 ==== Generalizations ==== ==== Generalizations ====
- 
-  * [[Generalization Principle]] (GP): Encapsulating a varying concept typically results in a more generally applicable solution. This is especially true when an abstract concept is encapsulated by introducing an interface or an abstract class. 
  
 ==== Specializations ==== ==== Specializations ====
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 ==== Contrary Principles ==== ==== Contrary Principles ====
  
-  * [[More Is More Complex]] (MIMC): ECV demands adding a new class for a new varying concept. +  * **[[More Is More Complex]] (MIMC)**: ECV demands adding a new class for a new varying concept. 
-  * [[Keep It Simple Stupid]] (KISS): ECV demands adding a new class for a new varying concept. This adds complexity. +  * [[Model Principle]] (MP): ECV sometimes results in classes which do not correspond to a real-world concept in the sense of MP. A "concept that varies" can also be a technical concept.
-  * [[Model Principle]] (MP): ECV sometimes results in classes which do not correspond top a real-world concept in the sense of MP. A "concept that varies" can also be a technical concept.+
  
 ==== Complementary Principles ==== ==== Complementary Principles ====
  
-  * [[Dependency Inversion Principle]] (DIP): ECV may result in the creation of abstract classes (i.e. the concepts) and descendant concrete classes (i.e. the variations). DIP now tells that other classes should only depend on the abstractions. +  * [[Low Coupling]] (LC): ECV results in the creation of a new module. When introducing such a new module, LC has to be adhered to. 
- +  * [[Liskov Substitution Principle]] (LSP): ECV may result in the introduction of an abstract base class. Here it is important to get the abstraction right. Otherwise LSP may be violated. 
 +  * [[Generalization Principle]] (GP): Encapsulating a varying concept typically results in a more generally applicable solution. This is especially true when an abstract concept is encapsulated by introducing an interface or an abstract class. 
 +  * [[Dependency Inversion Principle]] (DIP): ECV may result in the introduction of an abstract base class. Here DIP demands that other classes should only depend on this new abstract base class and not on the concrete subclasses. 
 +  * [[Information Hiding/Encapsulation]] (IH/E): ECV tells that varying concepts should be encapsulated. IH/E then tells how encapsulation is done.
 ==== Principle Collections ==== ==== Principle Collections ====
  
 {{page>collections:OOD Principle Language#Box}} {{page>collections:OOD Principle Language#Box}}
-{{page>collections:SOLID#Box}} 
-{{page>collections:Principles in "Object-Oriented Software Construction"#Box}} 
  
-===== Example =====+ 
 +===== Examples =====
  
  
 ===== Description Status ===== ===== Description Status =====
 /* Choose one of the following and comment out the rest: */ /* Choose one of the following and comment out the rest: */
-[[wiki:Stub]] +/*[[wiki:Stub]]*/ 
-/*[[wiki:Incomplete]]*/+[[wiki:Incomplete]]
 /*[[wiki:Complete]]*/ /*[[wiki:Complete]]*/
  
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   * Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides: //[[wp>Design Patterns|Design Patterns:   * Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides: //[[wp>Design Patterns|Design Patterns:
  Elements of Reusable Object-Oriented Software]]//  Elements of Reusable Object-Oriented Software]]//
-  * Bertrand Meyer: //[[wp>Object-Oriented Software Construction]]//, p. 57pp. +  * [[Single Responsibility Principle]] 
-  * Robert C. Martin: //Agile Software Development, Principles, Patterns, and Practices//, p. 99pp. +  * [[Open-Closed Principle]] 
-  * [[http://www.butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod|ButUncleBob: Principles of OOD]]+ 
 +===== Discussion ===== 
 + 
 +Discuss this wiki article and the principle on the corresponding [[talk:principles:Encapsulate the Concept that Varies|talk page]].
principles/encapsulate_the_concept_that_varies.txt · Last modified: 2021-10-18 21:31 by christian

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