Editing Class (computer programming) (section)

====Inheritance====

{{Main|Inheritance (object-oriented programming)|Superclass (computer science)|Subclass (computer science)}}

Conceptually, a superclass is a [[superset]] of its subclasses. For example, {{Mono|GraphicObject}} could be a superclass of {{Mono|Rectangle}} and {{Mono|Ellipse}}, while {{Mono|Square}} would be a subclass of {{Mono|Rectangle}}. These are all [[Subset|subset relations]] in set theory as well, i.e., all squares are rectangles but not all rectangles are squares.

A common conceptual error is to mistake a ''part of'' relation with a subclass. For example, a car and truck are both kinds of vehicles and it would be appropriate to model them as subclasses of a vehicle class. However, it would be an error to model the parts of the car as subclass relations. For example, a car is composed of an engine and body, but it would not be appropriate to model an engine or body as a subclass of a car.

In [[object-oriented modeling]] these kinds of relations are typically modeled as object properties. In this example, the {{Mono|Car}} class would have a property called {{Mono|parts}}. {{Mono|parts}} would be typed to hold a collection of objects, such as instances of {{Mono|Body}}, {{Mono|Engine}}, {{Mono|Tires}}, etc.
Object modeling languages such as [[Unified Modeling Language|UML]] include capabilities to model various aspects of "part of" and other kinds of relations – data such as the cardinality of the objects, constraints on input and output values, etc. This information can be utilized by developer tools to generate additional code besides the basic data definitions for the objects, such as error checking on [[Mutator method|get and set methods]].<ref>{{cite web|title=UML-to-Java transformation in IBM Rational Software Architect editions and related software|url=http://www.ibm.com/developerworks/rational/library/08/1202_berfeld/|publisher=[[IBM]]|date=2 December 2008|first=Marya|last=Berfeld|access-date=20 December 2013}}</ref>

One important question when modeling and implementing a system of object classes is whether a class can have one or more superclasses. In the real world with actual sets, it would be rare to find sets that did not intersect with more than one other set. However, while some systems such as Flavors and CLOS provide a capability for more than one parent to do so at run time introduces complexity that many in the object-oriented community consider antithetical to the goals of using object classes in the first place. Understanding which class will be responsible for handling a message can get complex when dealing with more than one superclass. If used carelessly this feature can introduce some of the same system complexity and ambiguity classes were designed to avoid.<ref>{{cite book|last=Jacobsen|first=Ivar|title=Object Oriented Software Engineering|year=1992|publisher=Addison-Wesley ACM Press|isbn=0-201-54435-0|pages=[https://archive.org/details/objectorientedso00jaco/page/43 43–69]|author2=Magnus Christerson|author3=Patrik Jonsson|author4=Gunnar Overgaard|url=https://archive.org/details/objectorientedso00jaco/page/43}}</ref>

Most modern object-oriented languages such as Smalltalk and Java require single inheritance at run time. For these languages, multiple inheritance may be useful for modeling but not for an implementation.

However, [[semantic web]] application objects do have multiple superclasses. The volatility of the Internet requires this level of flexibility and the technology standards such as the [[Web Ontology Language|Web Ontology Language (OWL)]] are designed to support it.

A similar issue is whether or not the class hierarchy can be modified at run time. Languages such as Flavors, CLOS, and Smalltalk all support this feature as part of their [[meta-object protocol]]s. Since classes are themselves first-class objects, it is possible to have them dynamically alter their structure by sending them the appropriate messages. Other languages that focus more on strong typing such as Java and C++ do not allow the class hierarchy to be modified at run time. Semantic web objects have the capability for run time changes to classes. The rationale is similar to the justification for allowing multiple superclasses, that the Internet is so dynamic and flexible that dynamic changes to the hierarchy are required to manage this volatility.<ref>{{cite web|url=http://www.w3.org/2001/sw/BestPractices/SE/ODSD/|title=A Semantic Web Primer for Object-Oriented Software Developers|last1=Knublauch|first1=Holger|last2=Oberle|first2=Daniel|last3=Tetlow|first3=Phil|last4=Wallace|first4=Evan|publisher=[[W3C]]|date=2006-03-09|access-date=2008-07-30}}</ref>

Although many class-based languages support inheritance, inheritance is not an intrinsic aspect of classes. An [[object-based language]] (i.e. [[Classic Visual Basic]]) supports classes yet does not support inheritance.<!-- do not provide the structural benefits of statically type-checked interfaces for objects. This is because, in object-based languages, it is possible to use and extend data structures and attach methods to them at runtime. This precludes the compiler or interpreter from being able to check the type information specified in the source code as the type is built dynamically and not defined statically. Most of these languages allow for ''instance behavior'' and complex ''operational polymorphism'' (see [[dynamic dispatch]] and [[Polymorphism (computer science)|polymorphism]]). -->