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oth 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 car class would have a property called parts parts would be typed to hold a collection of objects such as instances of body engine tires etc object modeling languages such as 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 get and set methods 17 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 18 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 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 protocols 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 19 although many class based languages support inheritance inheritance is not an intrinsic aspect of classes dubious discuss non sequitur an object based language i e classic visual basic supports classes yet does not support inheritance inter class relationships edit a programming language may support various class relationship features compositional edit 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 20 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 lifetime if the components are contained by reference they may not have a similar lifetime 21 for example in objective c 2 0 interface car nsobject property nsstring name property engine engine property nsarray tires end this car class has an instance of nsstring a string object engine and nsarray an array object hierarchical edit 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 superclasses 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 22 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 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 23 if multiple inheritance is allowed the hierarchy is a directed acyclic graph or dag for short otherwise it is a tree the hierarchy has classes as nodes and inheritance relationships as links classes in the same level are more likely to be associated than classes in different levels the levels of this hierarchy are called layers or levels of abstraction example simplified objective c 2 0 code from iphone sdk interface uiresponder nsobject interface uiview uiresponder interface uiscrollview uiview interface uitableview uiscrollview in this example a uitableview is a uiscrollview is a uiview is a uiresponder is an nsobject modeling edit in object oriented analysis and in unified modelling language uml an 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 24 associations may be labeled according to their name or purpose 25 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 24 taxonomy edit there are many categories of classes some of which overlap abstract and concrete edit see also abstract type in a language that supports inheritance an abstract class or abstract base class abc is a class that cannot be directly instantiated by contrast a concrete class is a class that can be directly instantiated instantiation of an abstract class can occur only indirectly via a concrete sub class an abstract class is either labeled as such explicitly or it may simply specify abstract methods or virtual methods an abstract class may provide implementations of some methods and may also specify virtual methods via signatures that are to be implemented by direct or indirect descendants of the abstract class before a class derived from an abstract class can be instantiated all abstract methods of its parent classes must be implemented by some class in the derivation chain 26 most object oriented programming languages allow the programmer to specify which classes are considered abstract and will not allow these to be instantiated for example in java c and php the reserved word keyword abstract is used 27 28 in c an abstract class is a class having at least one abstract method given by the appropriate syntax in that language a pure virtual function in c parlance 26 a class consisting of only pure virtual methods is called a pure abstract base class or pure abc in c and is also known as an interface by users of the language 14 other languages notably java and c support a variant of abstract classes called an interface via a keyword in the language in these languages multiple inheritance is not allowed but a class can implement multiple interfaces such a class can only contain abstract publicly accessible methods 23 29 30 local and inner edit in some languages classes can be declared in scopes other than the global scope there are various types of such classes an inner class is a class defined within another class the relationship between an inner class and its containing class can also be treated as another type of class association an inner class is typically neither associated with instances of the enclosing class nor instantiated along with its enclosing class depending on the language it may or may not be possible to refer to the class from outside the enclosing class a related concept is inner types also known as inner data type or nested type which is a generalization of the concept of inner classes c is an example of a language that supports both inner classes and inner types via typedef declarations 31 32 a local class is a class defined within a procedure or function such structure limits references to the class name to within the scope where the class is declared depending on the semantic rules of the language there may be additional restrictions on local classes compared to non local ones one common restriction is to disallow local class methods to access local variables of the enclosing function for example in c a local class may refer to static variables declared within its enclosing function but may not access the function s automatic variables 33 metaclass edit main article metaclass a metaclass is a class where instances are classes 34 a metaclass describes a common structure of a collection of classes and can implement a design pattern or describe particular kinds of classes metaclasses are often used to describe frameworks 35 in some languages such as python ruby or smalltalk a class is also an object thus each class is an instance of a unique metaclass that is built into the language 5 36 37 the common lisp object system clos provides metaobject protocols mops to implement those classes and metaclasses 38 final edit a final class cannot be subclassed it is basically the opposite of an abstract class which must be subclassed to be used and cannot be instantiated directly a final class is implicitly a concrete class can be instantiated directly a class is declared as final via the keyword final in java c or php or sealed in c however this concept should not be confused with classes in java qualified with the keyword sealed that only allow inheritance from selected subclasses 39 40 41 42 in languages like kotlin all classes are final by default while to allow a class to be inherited from it must instead be marked open 43 for example java s string class is marked as final 44 final classes may allow a compiler to perform optimizations that are not available for classes that can be subclassed 45 sealed edit a sealed class is a class that restricts inheritance to a selected list of classes it should not be confused with the sealed keyword in c which denotes a final class the list of permitted classes the sealed class may inherit is specified using a permits clause 46 public sealed class quadrilateral extends shape implements renderable transformable comparable quadrilateral measurable permits parallelogram trapezoid kite non sealed is another keyword used to declare that a class or interface which extends a sealed class can be extended by unknown classes 47 sealed class parallelogram extends quadrilateral sealed class rectangle extends parallelogram non sealed class square extends rectangle because square is non sealed any class can inherit from it class redsquare extends square open edit an open class can be changed typically an executable program cannot be changed by customers developers can often change some classes but typically cannot change standard or built in ones in ruby all classes are open in python classes can be created at runtime and all can be modified afterward 48 objective c categories permit the programmer to add methods to an existing class without the need to recompile that class or even have access to its source code mixin edit some languages have special support for mixins though in any language with multiple inheritance a mixin is simply a class that does not represent an is a type of relationship mixins are typically used to add the same methods to multiple classes for example a class unicodeconversionmixin might provide a method called unicodetoascii when included in classes filereader and webpagescraper that do not share a common parent partial edit in languages supporting the feature a partial class is a class whose definition may be split into multiple pieces within a single source code file or across multiple files 49 the pieces are merged at compile time making compiler output the same as for a non partial class the primary motivation for the introduction of partial classes is to facilitate the implementation of code generators such as visual designers 49 it is otherwise a challenge or compromise to develop code generators that can manage the generated code when it is interleaved within developer written code using partial classes a code generator can process a separate file or coarse grained partial class within a file and is thus alleviated from intricately interjecting generated code via extensive parsing increasing compiler efficiency and eliminating the potential risk of corrupting developer code in a simple implementation of partial classes the compiler can perform a phase of precompilation where it unifies all the parts of a partial class then compilation can proceed as usual 50 other benefits and effects of the partial class feature include enables separation of a class s interface and implementation code in a unique way eases navigation through large classes within an editor enables separation of concerns in a way similar to aspect oriented programming but without using any extra tools enables multiple developers to work on a single class concurrently without the need to merge individual code into one file at a later time partial classes have existed in smalltalk under the name of class extensions for considerable time with the arrival of the net framework 2 microsoft introduced partial classes supported in both c 2 0 and visual basic 2005 winrt also supports partial classes 51 uninstantiable edit uninstantiable classes allow programmers to group together per class fields and methods that are accessible at runtime without an instance of the class indeed instantiation is prohibited for this kind of class for example in c a class marked static can not be instantiated can only have stati...