====== Extract Method ======

===== Alternative Names =====

===== Context =====
  * [[contexts:Implementation]]

===== Purpose =====

Extract a portion of code from a (probably long) method into a new one in order to improve cohesion and readability of the source.

===== Description =====

===== Mechanics =====

TODO: explain mechanics

This refactoring is often automated by modern [[glossary:IDEs]].

===== Origin =====

{{page>resources:Refactoring#reference}}

===== Reasons For Applying This Refactoring =====

  * Readability and Reduced Complexity
    * [[principles:More Is More Complex|MIMC]]: Applying this refactoring reduces the code in the original method.
    * [[principles:Keep It Simple Stupid|KISS]]: The resulting methods are typically shorter and simpler than the original one.
    * [[principles:High Cohesion|HC]], [[principles:Single Responsibility Principle|SRP]]: The resulting methods are typically more cohesive.
  * Flexibilty, Reuse, and Robustness
    * [[principles:Don't Repeat Yourself|DRY]]: The extracted method can be reused in other places eliminating code duplication.
    * [[principles:Generalization Principle|GP]], [[principles:Encapsulate the Concept that Varies|ECV]]: The extracted method typically has parameters so it can be invoked with different parameter values, too. This makes the method more generally applicable. Furthermore the extracted method may change separately.
  * Other Reasons
    * [[principles:Linguistic Modular Units|LMU]]: The extracted method carries an identifier.

===== Reasons Against Applying This Refactoring =====

  * [[principles:More Is More Complex|MIMC]]: Applying this refactoring adds another (private) method and another method call.
  * [[principles:More Is More Complex|MIMC]], [[principles:Low Coupling|LC]]: If the extracted method has very many parameters (see [[anti-patterns:long parameter list]]), it has a high coupling to the calling method. In such a case it is either the wrong piece of code which is extracted or the parameter list should be shortened by applying further refactorings.
  * [[principles:Model Principle|MP]]: If you cannot find a suitable name for the new method, it it probably the wrong piece of code to extract.
  * [[principles:Principle of Separate Understandability|PSU]]: Technically it is possible to extract almost arbitrary portions of code into a new method. But extracting the wrong piece of code results in a PSU violation. Either the extracted method is not understandable on its own, i.e. you have to read the calling method in order to understand the extracted one. Or the calling method is not understandable anymore without reading the extracted method. A typical problem here is that the extracted method takes parameters where it's not clear why they are needed without reading the code of the extracted method. In either case the refactoring didn't improve the code. Rather you still have to read both methods so there are no readability and simplicity benefits. Moreover the code which has to be read together is now separated so reading both methods may involve jumping back and forth in different parts of the source. In such a case extracting the method is a bad thing.

===== Patterns the Refactoring May Lead to =====

===== Examples =====

===== Description Status =====
/* Choose one of the following and comment out the rest: */
[[wiki:Stub]]

/*[[wiki:Incomplete]]*/
/*[[wiki:Complete]]*/

===== Further Reading =====

  * {{page>resources:Refactoring#reference}}
  * Reafctoring.com: [[http://www.refactoring.com/catalog/extractMethod.html|Extract Method]]
===== Discussion =====

Discuss this wiki article and the refactoring on the corresponding [[talk:refactorings:Extract Method|talk page]].