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{{Short description|Software abstraction with state, behavior, and identity}} |
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In strictly [[mathematical]] branches of [[computer science]] the term ''object'' is used in a purely mathematical sense to refer to any "thing". While this interpretation is useful in the discussion of abstract theory, it is not concrete enough to serve as a primitive in the discussion of more concrete branches (such as [[programming]]) that are closer to actual computation and [[information processing]]. There, objects are still conceptual entities, but generally correspond directly to a contiguous block of [[computer memory]] of a specific size at a specific location. This is because computation and information processing ultimately require a form of computer memory. Objects in this sense are fundamental primitives needed to accurately define concepts such as [[reference (computer science)|reference]]s, [[variable]]s, and [[name binding]]. This is why the rest of this article will focus on the concrete interpretation of ''object'' rather than the abstract one. |
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In [[software development]], an '''object''' is an [[entity]] that has [[State (computer science)|state]], [[behavior]], and [[Identity (object-oriented programming)|identity]].<ref name="ooa">{{cite book|title=Object-Oriented Analysis and Design with Applications |edition=3 |date=April 30, 2007 |author1=Grady Booch |author2=Robert Maksimchuk |author3=Michael Engle |author4=Bobbi Young |author5=Jim Conallen |author6=Kelli Houston |isbn=020189551X |publisher= Addison-Wesley Professional}}</ref>{{rp|78}} An object can [[model]] some part of [[reality]] or can be an [[invention]] of the [[design process]] whose collaborations with other such objects serve as the mechanisms that provide some higher-level behavior. Put another way, an object represents an individual, identifiable item, unit, or entity, either real or abstract, with a well-defined role in the problem domain.<ref name="ooa"></ref>{{rp|76}} |
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Note that although a block of computer memory can appear contiguous on one level of abstraction and incontiguous on another, the important thing is that it appears contiguous to the program that treats it as an object. That is, as far as the program is concerned the object must be free of internal references, because otherwise it is no longer a primitive. In other words, object's private storage details must not be exposed to clients of the object, and must be able to change without changes to client code. |
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A [[programming language]] can be classified based on its support for objects. A language that provides an encapsulation construct for state, behavior, and identity is classified as [[object-based language|object-based]]. If the language also provides [[polymorphism (computer science)|polymorphism]] and [[inheritance (object-oriented programming)|inheritance]] it is classified as [[Object-oriented programming|object-oriented]]. A language that supports creating an object from a [[class (computer science)|class]] is classified as [[class-based programming|class-based]]. A language that supports object creation via a template object is classified as [[prototype-based programming|prototype-based]]. |
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Objects exist only within contexts that are aware of them; a piece of computer memory only holds an object if a program treats it as such (for example by reserving it for exclusive use by specific procedures and/or associating a [[data type]] with it). Thus, the lifetime of an object is the time during which it is treated as an object. This is why they are still conceptual entities, despite their physical presence in computer memory. |
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The concept of object is used in many different software contexts, including: |
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In other words, abstract concepts that do not occupy memory space at runtime are, according to the definition, not objects; e.g., design patterns exhibited by a set of classes, [[data type]]s in statically typed programs. |
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* Possibly the most common use is [[Computer memory|in-memory]] objects in a [[computer program]] written in an object-based language. |
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To emphasize that an object actually contains meaningful data, a term ''data object'' is sometimes used to refer to such an object. |
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* [[Information systems]] can be [[object-oriented analysis and design|modeled]] with objects representing their components and interfaces.<ref name="ooa"/>{{rp|39}} |
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== Objects in Object-Oriented Programming == |
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* In the [[relational model]] of [[database]] management, aspects such as [[Table (database)|table]] and [[Column (database)|column]] may act as objects.<ref name=Oppel>{{cite book |first=Andy |last=Oppel |title=SQL Demystified |publisher=McGraw Hill |year=2005| page=7 |isbn=0-07-226224-9}}</ref> |
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* [[Distributed object|Objects]] of a [[distributed computing]] system tend to be larger grained, longer lasting, and more service-oriented than programming objects. |
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In [[Object-Oriented Programming]] (OOP), an instance of a program (i.e. a program running in a computer) is treated as a dynamic set of interacting objects. Objects in OOP extend the more general notion of objects described above to include a very specific kind of [[datatype|typing]], which among other things allows for: |
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# data members that represent the data associated with the object. |
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# [[method (computer science)|method]]s that access the data members in predefined ways. |
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In the case of most objects, one can access the data members only through the method members, making it easy to guarantee that the data will always remain in a well-defined state ([[class invariant]]s will be enforced). Some languages do not make distinctions between data members and methods. |
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In a language where each object is created from a class, an object is called an '''instance''' of that class. If each object has a type, two objects with the same class would have the same [[datatype]]. Creating an instance of a class is sometimes referred to as '''[[instantiation|instantiating]]''' the class. |
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*{{annotated link|Actor model}} |
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*{{annotated link|Business object}} |
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*{{annotated link|Object copying}} |
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*{{annotated link|Semantic Web}} |
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==References== |
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A real-world example of an object would be "my dog", which is an [[Instance_%28programming%29|instance]] of a [[Datatype|type]] (a [[Class_%28computer_science%29|class]]) called "dog", which is a [[subclass]] of a class "animal". In the case of a [[Polymorphism_%28computer_science%29|polymorphic]] object, some details of its type can be selectively ignored, for example a "dog" object could be used by a function looking for an "animal". So could a "cat", because it too belongs to the class of "animal". While being accessed as an "animal", some member attributes of a "dog" or "cat" would remain unavailable, such as the "tail" attribute, because not all animals have tails. |
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{{Reflist}} |
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==External links== |
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Three properties characterize objects: |
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*[http://java.sun.com/docs/books/tutorial/java/concepts/object.html ''What Is an Object?''] from ''The Java Tutorials'' |
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# [[identity (object-oriented programming)|identity]] - the property of an object that distinguishes it from other objects |
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# [[state]] - describes the data stored in the object |
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# [[behaviour]] - describes the methods in the object's [[Interface_%28computer_science%29|interface]] by which the object can be used |
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{{Software engineering}} |
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Some terms for specialized kinds of objects include: |
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{{Data types}} |
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*[[Singleton pattern|Singleton]] object - An object that is the only instance of its class during the lifetime of the program. |
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{{Authority control}} |
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*Functor ([[function object]]) - an object with a single method (in C++, this method would be the function operator, "operator()") that acts much like a function (like a C/C++ pointer to a function). |
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*[[Immutable object]] - an object set up with a fixed state at creation time and which does not vary afterward. |
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*[[First-class object]] - an object that can be used without restriction. |
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*[[Container object]] - an object that can contain other objects. |
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*[[Factory object]] - a metaobject that can create other objects. |
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*[[Metaobject]] - an object from which other objects can be created (Compare with [[class (computer science)|class]], which is not necessarily an object) |
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*[[Prototype (computer science)|Prototype]] - a specialized metaobject from which other objects can be created by copying |
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*[[Object copy]] |
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*[[Design_pattern_%28computer_science%29|Design patterns]] |
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*[[Actor model]] |
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{{DEFAULTSORT:Object (computer science)}} |
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[[Category:Object-oriented programming]] |
[[Category:Object-oriented programming]] |
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[[Category:Data types]] |
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[[Category:Composite data types]] |
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[[de:Objekt_(objektorientierte_Programmierung)]] |
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[[lt:Objektas (programavimas)]] |
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[[nl:Object (informatica)]] |
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[[ja:インスタンス]] |
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[[pl:Obiekt (programowanie obiektowe)]] |
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[[zh:对象]] |
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Latest revision as of 13:40, 31 October 2024
In software development, an object is an entity that has state, behavior, and identity.[1]: 78 An object can model some part of reality or can be an invention of the design process whose collaborations with other such objects serve as the mechanisms that provide some higher-level behavior. Put another way, an object represents an individual, identifiable item, unit, or entity, either real or abstract, with a well-defined role in the problem domain.[1]: 76
A programming language can be classified based on its support for objects. A language that provides an encapsulation construct for state, behavior, and identity is classified as object-based. If the language also provides polymorphism and inheritance it is classified as object-oriented. A language that supports creating an object from a class is classified as class-based. A language that supports object creation via a template object is classified as prototype-based.
The concept of object is used in many different software contexts, including:
- Possibly the most common use is in-memory objects in a computer program written in an object-based language.
- Information systems can be modeled with objects representing their components and interfaces.[1]: 39
- In the relational model of database management, aspects such as table and column may act as objects.[2]
- Objects of a distributed computing system tend to be larger grained, longer lasting, and more service-oriented than programming objects.
See also
[edit]- Actor model – Model of concurrent computation
- Business object – Entity within a multi-tiered software application
- Object lifetime – Time period between the creation and destruction of an object-oriented programming instance
- Object copying – Technique in object-oriented programming
- Semantic Web – Extension of the Web to facilitate data exchange
References
[edit]- ^ a b c Grady Booch; Robert Maksimchuk; Michael Engle; Bobbi Young; Jim Conallen; Kelli Houston (April 30, 2007). Object-Oriented Analysis and Design with Applications (3 ed.). Addison-Wesley Professional. ISBN 020189551X.
- ^ Oppel, Andy (2005). SQL Demystified. McGraw Hill. p. 7. ISBN 0-07-226224-9.
External links
[edit]- What Is an Object? from The Java Tutorials
