principles:tell_don_t_ask_information_expert
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principles:tell_don_t_ask_information_expert [2020-10-12 16:38] – old revision restored (2015-06-29 12:02) 159.69.186.191 | principles:tell_don_t_ask_information_expert [2021-09-02 12:51] – old revision restored (2021-03-28 06:13) 65.21.179.175 | ||
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===== Variants and Alternative Names ===== | ===== Variants and Alternative Names ===== | ||
- | * Information | + | * Expert |
- | * Do It Myself | + | * Do It Myself |
- | * Tell, don't Ask in ((Andrew Hunt and David Thomas: // | + | |
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/* fill in contexts here: */ | /* fill in contexts here: */ | ||
* [[contexts: | * [[contexts: | ||
- | * [[contexts: | + | |
===== Principle Statement ===== | ===== Principle Statement ===== | ||
- | * Assign a responsibility to that [[glossary: | + | Assign a responsibility to this [[glossary: |
- | * Don't ask an object for information, | + | |
===== Description ===== | ===== Description ===== | ||
- | Each module has a set of responsibilities so there is a kind of mapping between | + | Each module has a set of responsibilities. Subsystems have specific tasks, packages group several related classes, classes have methods and attributes, and so on. So there is a kind of mapping between |
- | Another view on the principle is that responsibility mapping is bad when one module has to ask another module for information (getter invocation), | ||
- | The following reasoning shows that //Tell don't Ask// and // | + | ===== Rationale ===== |
- | * Suppose TdA is adhered to, but IE is neglected. | + | When this principle |
- | * When IE is neglected, then there is a module which is not the information | + | |
- | * But then the module has to ask for the information | + | |
- | * As this violates | + | |
- | * Suppose IE is adhered to but TdA is neglected | + | |
- | * When TdA is neglected, then there is a module '' | + | |
- | * But then '' | + | |
- | * As this violates the assumption that IE is adhered | + | |
- | * So TdA and IE are equivalent views on the same principle. | + | |
- | + | ||
- | Despite of its proof-like form this is not a formal proof as there is no formal definition of TdA and IE. Nevertheless TdA and IE can be seen as two views on the same principle. | + | |
- | ===== Rationale ===== | + | |
- | When this principle is not adhered to, then a module has a responsibility for which it is lacking some information. So in order to fulfill the task the module has to first acquire the needed information by invoking other modules. This increases the dependencies between the modules (which may lead to [[glossary: | ||
- | Furthermore adhering to this principle distributes responsibilities among several classes instead of having one central [[anti-patterns: | ||
===== Strategies ===== | ===== Strategies ===== | ||
- | |||
- | * Assign a responsibility to the class that has the largest subset of the needed information. | ||
- | * Mirror functionality of composed objects to the interface of the class instead of having a getter-method returning the composed object | ||
- | * Have the objects operate on their own data using appropriate methods. Avoid getters and setters. | ||
- | ===== Caveats ===== | ||
- | |||
- | Sometimes assigning responsibilities using IE results in bad solutions (high coupling, low cohesion). This is because IE just focuses on the availability of data. So for example IE would demand domain objects saving themselves to the database. This is bad since it couples the domain objects to the database interface (JDBC, SQL, etc.) and lowers cohesion by adding unrelated responsibilities to the classes. Here it is better to give the task of persisting the domain objects to a separate class.((see Craig Larman: //Applying UML and Patterns – An Introduction to Object-Oriented Analysis and Design and Iterative Development//, | ||
- | |||
- | See also section [[#contrary principles]]. | ||
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===== 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: | + | * [[wiki: |
- | + | | |
- | [[wiki: | + | * [[wiki: |
- | + | */ | |
- | /* * [[wiki: | + | |
===== Relations to Other Principles ===== | ===== Relations to Other Principles ===== | ||
==== Generalizations ==== | ==== Generalizations ==== | ||
+ | |||
+ | * [[Low Coupling]] Adhering to the information expert principle leads to low coupling as there is less need to communicate with other modules to get the necessary information. | ||
+ | * [[High Cohesion]] Adhering to information expert also means that a module only has responsibilities which belong together. So this increases cohesion. | ||
==== Specializations ==== | ==== Specializations ==== | ||
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* [[More Is More Complex]] (MIMC): Adhering to TdA/IE sometimes results in adding further methods. | * [[More Is More Complex]] (MIMC): Adhering to TdA/IE sometimes results in adding further methods. | ||
+ | * [[Principle of Separate Understandability]] (PSU): Assigning responsibilities using TdA/IE bears the danger of violating PSU as responsibilities are assigned based on data availability instead of separate understandability. | ||
==== Complementary Principles ==== | ==== Complementary Principles ==== | ||
- | * [[Low Coupling]] Adhering to IE typically leads to low coupling as there is less need to communicate with other modules to get the necessary information. But in some cases IE also increases coupling (see [[# | ||
- | * [[High Cohesion]] Adhering to IE typically leads to high cohesion as responsibilities which belong together typically operate on the same data. But in some cases IE also lowers cohesion (see [[# | ||
* [[Model Principle]] (MP): TdA/IE tells how to distribute functionality among the natural classes which are created according to the Model Principle. | * [[Model Principle]] (MP): TdA/IE tells how to distribute functionality among the natural classes which are created according to the Model Principle. | ||
- | * [[Information Hiding/ | + | * [[Information Hiding/ |
- | * [[Principle of Separate Understandability]] (PSU): TdA/IE is about responsibility assignment. Another aspect of this task is treated by PSU. | + | |
==== Principle Collections ==== | ==== Principle Collections ==== | ||
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- | ===== Examples | + | ===== Example |
===== 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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* Andrew Hunt and David Thomas: // | * Andrew Hunt and David Thomas: // | ||
- | ===== Discussion ===== | ||
- | |||
- | Discuss this wiki article and the principle on the corresponding [[talk: |
principles/tell_don_t_ask_information_expert.txt · Last modified: 2021-10-18 21:42 by christian