principles:encapsulate_the_concept_that_varies
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principles:encapsulate_the_concept_that_varies [2021-05-19 11:23] – old revision restored (2021-05-11 21:51) 2a04:ac00:4:d29:5054:ff:fe00:d387 | principles:encapsulate_the_concept_that_varies [2021-09-02 19:36] – old revision restored (2021-05-11 21:52) 65.21.179.175 | ||
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===== Context ===== | ===== Context ===== | ||
/* fill in contexts here: */ | /* fill in contexts here: */ | ||
- | * [[contexts: | + | * [[contexts: |
- | * [[contexts: | + | |
- | * [[contexts: | + | |
===== Principle Statement ===== | ===== Principle Statement ===== | ||
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===== Description ===== | ===== Description ===== | ||
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- | 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: | ||
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- | 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. | ||
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- | 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 ===== | ||
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- | 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: | ||
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- | 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. | ||
===== Strategies ===== | ===== Strategies ===== | ||
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- | * Introduce a separate module for the concept that may change in the future. In that way the future change will only affect that particular module. If the varying concept is properly encapsulated, | ||
- | * Introduce an interface encapsulating the varying concept. The interface may be implemented differently by several classes and code that only relies on the interface can handle any class implementing the interface. In case of another variation, just another class has to be introduced and this class has to implement the interface. If the abstraction is done properly, no module has to change. | ||
- | * Introduce an abstract base class encapsulating the varying concept. This is basically the same as introducing an interface. But here, implementation can also be inherited. So common parts can remain in the abstract base class whereas only the actual variations are defined in the subclasses. By means of method overriding, the implementation of the base class methods can be changed without touching the base class directly. | ||
- | * Use design patterns. Several design patterns use the above techniques to encapsulate varying concepts. For example: | ||
- | * Abstract Factory: A family of objects changes. | ||
- | * Factory Method: The exact type of an object to create changes. | ||
- | * Adapter: The interface of a module changes. | ||
- | * Bridge: A concept varies in more than one aspect. | ||
- | * Decorator: The behavior of certain methods may need to be enhanced. | ||
- | * Iterator: The traversal algorithm of a structure changes. Or the structure itself changes resulting in the need for a different traversal algorithm. | ||
- | * Observer: The objects interested in a certain event may change. | ||
- | * State: The behavior in a certain state or the state machine (states and transitions) of a certain module changes. | ||
- | * Strategy: An algorithm changes. | ||
- | * Template Method: The concrete steps in an algorithms change. | ||
- | * Visitor: New operations have to be added to a given more or less static inheritance structure of classes. | ||
- | * ... | ||
- | |||
- | ===== Caveats ===== | ||
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- | See section [[#contrary principles]]. | ||
===== Origin ===== | ===== Origin ===== | ||
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- | The principle is stated, explained and used in the GoF book: | ||
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- | Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: // | ||
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- | But the idea if ECV is actually much older. It was first presented in | ||
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- | David Parnas: // | ||
===== Evidence ===== | ===== Evidence ===== | ||
/* Comment out what is not applicable and explain the rest: */ | /* Comment out what is not applicable and explain the rest: */ | ||
- | /* * [[wiki: | + | /* |
- | + | * [[wiki: | |
- | * [[wiki:Accepted]]: This principle was popularly described in the GoF book and can thus be regarded accepted. | + | * [[wiki:Examined]] |
- | * [[wiki:Examined]]: Many of the patterns in the GoF book are precisely about encapsulating varying concepts. See strategies section. | + | * [[wiki:Accepted]] |
- | + | * [[wiki: | |
- | /* * [[wiki: | + | */ |
===== Relations to Other Principles ===== | ===== Relations to Other Principles ===== | ||
==== Generalizations ==== | ==== Generalizations ==== | ||
+ | |||
+ | * [[Generalization Principle]] | ||
+ | * [[Open-Closed Principle]] | ||
+ | |||
==== Specializations ==== | ==== Specializations ==== | ||
- | * [[Single Responsibility Principle]] (SRP): | + | * [[Single Responsibility Principle]] (SRP): |
- | * [[Open-Closed Principle]] (OCP): The OCP demands encapsulating abstract concepts in base classes (or interfaces) in order to be able to enhance the module by subclassing which is possible without changing the previously written code. In this case several variations of a concept may exist in the code at the same time. There is always the abstract base class plus one or usually more concrete subclasses. So the OCP is about encapsulating abstract concepts that vary "in space" | + | |
==== Contrary Principles ==== | ==== Contrary Principles ==== | ||
- | | + | * [[More Is More Complex]] (MIMC): ECV demands adding a new class for a new varying concept. |
* [[Model Principle]] (MP): ECV sometimes results in classes which do not correspond top a real-world concept in the sense of MP. A " | * [[Model Principle]] (MP): ECV sometimes results in classes which do not correspond top a real-world concept in the sense of MP. A " | ||
==== Complementary Principles ==== | ==== Complementary Principles ==== | ||
- | * [[Low Coupling]] (LC): ECV results in the creation of a new module. When introducing such a new module, LC has to be adhered to. | + | * [[Dependency Inversion |
- | * [[Liskov Substitution | + | |
- | * [[Generalization Principle]] | + | |
- | * [[Dependency Inversion Principle]] (DIP): ECV may result in the introduction of an abstract base class. Here DIP demands | + | |
- | * [[Information Hiding/ | + | |
==== Principle Collections ==== | ==== Principle Collections ==== | ||
{{page> | {{page> | ||
+ | {{page> | ||
+ | {{page> | ||
- | + | ===== 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: | + | [[wiki: |
- | [[wiki: | + | /*[[wiki: |
/ | / | ||
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* Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides: // | * Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides: // | ||
- | | + | |
- | * [[Single Responsibility Principle]] | + | * Bertrand Meyer: //[[wp> |
- | * [[Open-Closed Principle]] | + | * Robert C. Martin: //Agile Software Development, |
- | + | * [[http:// | |
- | ===== Discussion ===== | + | |
- | + | ||
- | Discuss this wiki article | + |
principles/encapsulate_the_concept_that_varies.txt · Last modified: 2021-10-18 21:31 by christian