principles:tell_don_t_ask_information_expert
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| principles:tell_don_t_ask_information_expert [2013-02-11 18:59] – +IH/E christian | principles:tell_don_t_ask_information_expert [2021-09-02 20:51] – old revision restored (2021-03-28 06:12) 65.21.179.175 | ||
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| ===== Variants and Alternative Names ===== | ===== Variants and Alternative Names ===== | ||
| - | * Expert | + | * Information |
| - | * Do It Myself | + | * Do It Myself |
| + | * Tell, don't Ask in ((Andrew Hunt and David Thomas: // | ||
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| ===== Principle Statement ===== | ===== Principle Statement ===== | ||
| - | Assign a responsibility to this [[glossary: | + | * Assign a responsibility to that [[glossary: |
| + | * Don't ask an object for information, | ||
| ===== Description ===== | ===== Description ===== | ||
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| ===== Rationale ===== | ===== 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: | + | 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: |
| ===== 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 bas 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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| ==== 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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| - | ===== Example | + | ===== Examples |
principles/tell_don_t_ask_information_expert.txt · Last modified: 2021-10-18 21:42 by christian