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The delegation pattern is a technique where an object outwardly expresses certain behaviour but in reality delegates responsibility for implementing that behavior to an associated object in an Inversion of Responsibility. This simulates mixins, delegation, and some kinds of aspects in traditional object-oriented languages like C++ and Java. Aggregation must be used with it if inheritance is not applicable.

1 Simple Java Example

In this example, the class C has method stubs that forward the methods f() and g() to class A. Class C pretends that it has attributes of class A.

class A { void f() { System.out.println("A: doing f()"); } void g() { System.out.println("A: doing g()"); } } class C { // delegation A a = new A(); void f() { a.f(); } void g() { a.g(); } // normal attributes X x = new X(); void y() { /* do stuff */ } } void main() { C c = new C(); c.f(); c.g(); }

2 Complex Java Example

By using interfaces, delegation can be made more flexible and typesafe. In this example, class C can delegate to either class A or class B. Class C has methods to switch between classes A and B. Including the implements clauses improves type safety, because each class must implement the methods in the interface. The main tradeoff is more code.

interface I { void f(); void g(); } class A implements I { void f() { System.out.println("A: doing f()"); } void g() { System.out.println("A: doing g()"); } } class B implements I { void f() { System.out.println("B: doing f()"); } void g() { System.out.println("B: doing g()"); } }

class C implements I { // delegation I i = new A(); void f() { i.f(); } void g() { i.g(); } // normal attributes void toA() { i = new A(); } void toB() { i = new B(); } } void main() { C c = new C(); c.f(); c.g(); }

3 Criticisms

Because this is a pattern, developers can make many kinds of mistakes. The developer could forget to delegate a method in the simple version. The developer could mistype the name of one attributeAn attribute is the following: Generally, an attribute is an abstraction characteristic of an entity In database management, an attribute is a property inherent in an entity or associated with that entity for database purposes. In network management, an a.

This pattern typically sacrifices speed optimization in favor of enhanced clarity of abstractionThis article is about the concept of abstraction in general. For other uses, please see abstract (disambiguation). Abstraction is the thought process wherein ideas are distanced from objects. Abstraction uses a strategy of simplification of detail, wherei.

4 References

See also Design pattern and Post-object programmingPost-object programming is a new generation of techniques that evolved out of the object-oriented programming generation. Post-object programming emphasizes mixins, delegation, and aspects; as well as heterogeneous groups and multimethods. Post-object lan.

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Topics: Delegation Pattern, Simple Java Example, Complex Java Example...

Delegation patternThe delegation pattern is a technique where an object outwardly expresses certain behaviour but in reality delegates responsibility for implementing that behavior to an associated object in an Inversion of Responsibility''. This simulates mixins, delegati	DelegationFor the programming technique, see Delegation (programming). Delegation is handing a task over to a subordinate. It is the assignment of authority and responsibility to another person to carry out specific activities. It allows a subordinate to make decis	Delegation (programming)In object-oriented programming there are two notions of delegation . In older times delegation was refering to passing on the execution to some other object. In recent times however, it means the method lookup rules when dispatching so-called self-calls a
Delegation for the Adoption of an International Auxiliary LanguageThe Delegation for the Adoption of an International Auxiliary Language was a group of linguists who presented proposals for a revision of the Esperanto language in 1907. The resulting language became known as Ido.

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