Object Oriented Software Engineering   View all facts   Glossary   Help

subject > pattern > design pattern > delegation

delegation
subject			fact

delegation	has definition A pattern in which one procedure does nothing more than call another in a neighbouring class; this reduces total coupling in the system
	has antipatterns Overusing generalization and inheriting the method that is to be reused Instead of creating a single method in the «Delegator» that does nothing other than call a method in the «Delegate», you might consider having many different methods in the «Delegator» call the delegate's method? It would be better to ensure that only one method in the «Delegator» calls the method in the «Delegate». If many methods access the method in the «Delegate», then when it changes, you may have to change every method that accesses it a method must only accesses neighbouring classes
	has context You need an operation in a class and you realize that another class has an implementation of the operation. However, inheritance is not appropriate, either because the isa rule does not apply, or because you do not want to reuse all the methods of the other class
	has forces You want to minimize development cost and complexity by reusing methods You want to reduce the linkages between classes You want to ensure that work is done in the most appropriate class
	has problem How can you most effectively make use of a method that already exists in the other class?
	has related patterns Adapter and Proxy patterns
	has solution Create a method in the «Delegator» class that does only one thing: it calls a method in a neighbouring «Delegate» class, allowing reuse of the method for which the «Delegate» has responsibility By 'neighbouring', we mean the «Delegate» is connected to the «Delegator» by an association. Normally, in order to use delegation an association should already exist between the «Delegator» and the «Delegate». This association needs only to be unidirectional from «Delegator» to «Delegate». However it may sometimes be appropriate to create a new association just so you can use delegation - provided this does not increase the overall complexity of the system.
	is a subtopic of 6.7 - The Delegation Pattern
	is an instance of design pattern
design pattern	has name
	has references one or more references which indicate who developed or inspired a pattern
	should be illustrated using a simple diagram
	should be written using a narrative writing style
pattern	should be as general as possible
	should be described in an easy-to-understand form so that people can determine when and how to use it
	should contain a solution that has been proven to effectively solve the problem in the indicated context

Next design pattern: facade    Up: design pattern    Previous design pattern: composite