Editing Class (computer programming) (section)

=== 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.