Editing Class (computer programming) (section)

== Inter-class relationships ==

A programming language may support various class relationship features.

=== Compositional ===

Classes can be composed of other classes, thereby establishing a compositional relationship between the enclosing class and its embedded classes. Compositional relationship between classes is also commonly known as a ''[[has-a]]'' relationship.{{sfn|Booch|1994|p=180}} For example, a class "Car" could be composed of and contain a class "Engine". Therefore, a Car ''has an'' Engine. One aspect of composition is containment, which is the enclosure of component instances by the instance that has them. If an enclosing object contains component instances by value, the components and their enclosing object have a similar [[Object lifetime|lifetime]]. If the components are contained by reference, they may not have a similar lifetime.{{sfn|Booch|1994|p=128-129}} For example, in Objective-C 2.0:

<syntaxhighlight lang="objc">
@interface Car : NSObject

@property NSString *name;
@property Engine *engine
@property NSArray *tires;

@end
</syntaxhighlight>

This {{Mono|Car}} class ''has'' an instance of {{Mono|NSString}} (a [[string (computer science)|string]] object), {{Mono|Engine}}, and {{Mono|NSArray}} (an array object).

=== Hierarchical ===

Classes can be ''derived'' from one or more existing classes, thereby establishing a hierarchical relationship between the derived-from classes (''base classes'', ''parent classes'' or ''{{vanchor|superclasses|SUPERCLASS}}'') and the derived class (''child class'' or ''subclass'') . The relationship of the derived class to the derived-from classes is commonly known as an ''[[is-a]]'' relationship.{{sfn|Booch|1994|p=112}} For example, a class 'Button' could be derived from a class 'Control'. Therefore, a Button ''is a'' Control. Structural and behavioral members of the parent classes are ''inherited'' by the child class. Derived classes can define additional structural members (data fields) and behavioral members (methods) in addition to those that they ''inherit'' and are therefore ''specializations'' of their superclasses. Also, derived classes can [[method overriding|override]] inherited methods if the language allows.

Not all languages support multiple inheritance. For example, Java allows a class to implement multiple interfaces, but only inherit from one class.<ref name="javainterface">{{cite web| url=http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html|title=Interfaces|work=The Java Tutorials|publisher=Oracle|access-date=2012-05-01}}</ref> If multiple inheritance is allowed, the hierarchy is a [[directed acyclic graph]] (or DAG for short), otherwise it is a [[tree (graph theory)|tree]]. The hierarchy has classes as nodes and inheritance relationships as links. Classes in the same level are more likely to be [[association (object-oriented programming)|associated]] than classes in different levels. The levels of this hierarchy are called [[Layer (object-oriented design)|layers]] or levels of abstraction.

Example (Simplified Objective-C 2.0 code, from iPhone SDK):
<syntaxhighlight lang="objc">
@interface UIResponder : NSObject //...
@interface UIView : UIResponder //...
@interface UIScrollView : UIView //...
@interface UITableView : UIScrollView //...
</syntaxhighlight>
In this example, a UITableView ''is a'' UIScrollView ''is a'' UIView ''is a'' UIResponder ''is an'' NSObject.

===Modeling===

In [[object-oriented analysis and design|object-oriented analysis]] and in [[Unified Modelling Language]] (UML), an [[Association (object-oriented programming)|association]] between two classes represents a collaboration between the classes or their corresponding instances. Associations have direction; for example, a bi-directional association between two classes indicates that both of the classes are aware of their relationship.<ref name="ibmuml">{{cite web| url=http://www.ibm.com/developerworks/rational/library/content/RationalEdge/sep04/bell/|title=UML Basics: The class diagram|last=Bell|first=Donald|work=developer Works|publisher=IBM|access-date=2012-05-02}}</ref> Associations may be labeled according to their name or purpose.{{sfn|Booch|1994|p=179}}

An association role is given end of an association and describes the role of the corresponding class. For example, a "subscriber" role describes the way instances of the class "Person" participate in a "subscribes-to" association with the class "Magazine". Also, a "Magazine" has the "subscribed magazine" role in the same association. Association role multiplicity describes how many instances correspond to each instance of the other class of the association. Common multiplicities are "0..1", "1..1", "1..*" and "0..*", where the "*" specifies any number of instances.<ref name=ibmuml/>