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| title = Object-Oriented Simulation of systems with sophisticated control
| publisher = International Journal of General Systems
| year = 2011 | pages = 313–343}}</ref><ref>{{Cite book|last1=Lewis|first1=John|last2=Loftus|first2= William|title=Java Software Solutions Foundations of Programming Design 6th ed|publisher=Pearson Education Inc.|year=2008|isbn=978-0-321-53205-3}}, section 1.6 "Object-Oriented Programming"</ref>
Many of the most widely used programming languages (such as [[C++]], [[Java (programming language)|Java]],{{sfn|Bloch|2018|loc=Foreword|pp=xi-xii}} [[Python (programming language)|Python]], etc.) are [[multi-paradigm programming language|multi-paradigm]] and they support object-oriented programming to a greater or lesser degree, typically in combination with [[imperative programming]], [[procedural programming]] and [[functional programming]].
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[[Modular programming]] support provides the ability to group procedures into files and modules for organizational purposes. Modules are [[namespace]]d so identifiers in one module will not conflict with a procedure or variable sharing the same name in another file or module.
===Objects
Objects sometimes correspond to things found in the real world. For example, a graphics program may have objects such as "circle", "square", and "menu". An online shopping system might have objects such as "shopping cart", "customer", and "product".<ref>{{cite book|last=Booch|first=Grady|title=Software Engineering with Ada|year=1986|publisher=Addison Wesley|isbn=978-0-8053-0608-8|page=220|url=https://en.wikiquote.org/wiki/Grady_Booch|quote=Perhaps the greatest strength of an object-oriented approach to development is that it offers a mechanism that captures a model of the real world.}}</ref> Sometimes objects represent more abstract entities, like an object that represents an open file, or an object that provides the service of translating measurements from U.S. customary to metric. Objects are accessed somewhat like variables with complex internal structures, and in many languages are effectively [[Pointer (computer programming)|pointers]], serving as actual references to a single instance of said object in memory within a heap or stack. Procedures are known as [[Method (computer science)|methods]]; variables are also known as [[Field (computer science)|fields]], members, attributes, or properties.▼
Languages that support object-oriented programming (OOP) typically use [[Inheritance (object-oriented programming)|inheritance]] for code reuse and extensibility in the form of either [[Class-based programming|classes]] or [[Prototype-based programming|prototypes]]. Those that use classes support two main concepts:▼
Objects can contain other objects in their instance variables; this is known as [[object composition]]. For example, an object in the Employee class might contain (either directly or through a pointer) an object in the Address class, in addition to its own instance variables like "first_name" and "position". Object composition is used to represent "has-a" relationships: every employee has an address, so every Employee object has access to a place to store an Address object (either directly embedded within itself or at a separate ___location addressed via a pointer).▼
▲Objects sometimes correspond to things found in the real world. For example, a graphics program may have objects such as "circle", "square", and "menu". An online shopping system might have objects such as "shopping cart", "customer", and "product".<ref>{{cite book|last=Booch|first=Grady|title=Software Engineering with Ada|year=1986|publisher=Addison Wesley|isbn=978-0-8053-0608-8|page=220|url=https://en.wikiquote.org/wiki/Grady_Booch|quote=Perhaps the greatest strength of an object-oriented approach to development is that it offers a mechanism that captures a model of the real world.}}</ref> Sometimes objects represent more abstract entities, like an object that represents an open file, or an object that provides the service of translating measurements from U.S. customary to metric.
===Inheritance===
▲
In [[class-based programming]], the most popular style, each object is required to be an [[instance (computer science)|instance]] of a particular ''class''. The class defines the data format or [[data type|type]] (including member variables and their types) and available procedures (class methods or member functions) for a given type or class of object. Objects are created by calling a special type of method in the class known as a [[Constructor (object-oriented programming)|constructor]]. Classes may inherit from other classes, so they are arranged in a hierarchy that represents "is-a-type-of" relationships. For example, class Employee might inherit from class Person. All the data and methods available to the parent class also appear in the child class with the same names. For example, class Person might define variables "first_name" and "last_name" with method "make_full_name()". These will also be available in class Employee, which might add the variables "position" and "salary". It is guaranteed that all instances of class Employee will have the same attributes, such as the name, position, and salary. Procedures and variables can be specific to either the class or the instance; this leads to the following terms:
* [[Class variable]]s – belong to the ''class as a whole''; there is only one copy of each variable, shared across all instances of the class
* [[Instance variable]]s or attributes – data that belongs to individual ''objects''; every object has its own copy of each one
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* Instance methods – belong to ''individual objects'', and have access to instance variables for the specific object they are called on, inputs, and class variables
[[Abstract class]]es cannot be instantiated into objects; they exist only for inheritance into other "concrete" classes that can be instantiated. In Java, the <code>[[final (Java)|final]]</code> keyword can be used to prevent a class from being subclassed.{{sfn|Bloch|2018|loc=Chapter §2 Item 4 Enforce noninstantiability with a private constructor|p=19}}▼
In contrast, in [[
The doctrine of [[composition over inheritance]] advocates implementing has-a relationships using composition instead of inheritance. For example, instead of inheriting from class Person, class Employee could give each Employee object an internal Person object, which it then has the opportunity to hide from external code even if class Person has many public attributes or methods. Some languages like [[Go (programming language)|Go]] do not support inheritance at all. Go states that it is object-oriented,<ref>{{Cite web |url=https://golang.org/doc/faq#Is_Go_an_object-oriented_language |title=Is Go an object-oriented language? |access-date=April 13, 2019 |quote=Although Go has types and methods and allows an object-oriented style of programming, there is no type hierarchy.}}</ref> and Bjarne Stroustrup, author of C++, has stated that it is possible to do OOP without inheritance.<ref>{{cite conference |last1=Stroustrup |first1=Bjarne |title=Object-Oriented Programming without Inheritance (Invited Talk) |date=2015 |doi=10.4230/LIPIcs.ECOOP.2015.1|url=https://www.youtube.com/watch?v=xcpSLRpOMJM|conference=29th European Conference on Object-Oriented Programming (ECOOP 2015)|at=1:34}}</ref> [[Delegation (object-oriented programming)|Delegation]] is another language feature that can be used as an alternative to inheritance.
▲In [[Prototype-based programming|prototype-based languages]] the ''objects'' are the primary entities. No ''classes'' even exist. The ''prototype'' of an object is just another object to which the object is linked. Every object has one ''prototype'' link (and only one). New objects can be created based on already existing objects chosen as their prototype. You may call two different objects ''apple'' and ''orange'' a fruit if the object ''fruit'' exists, and both ''apple'' and ''orange'' have ''fruit'' as their prototype. The idea of the ''fruit'' class does not exist explicitly but as the [[equivalence class]] of the objects sharing the same prototype. The attributes and methods of the ''prototype'' are [[Delegation (object-oriented programming)|delegated]] to all the objects of the equivalence class defined by this prototype. The attributes and methods ''owned'' individually by the object may not be shared by other objects of the same equivalence class; e.g. the attribute ''sugar_content'' may be unexpectedly not present in ''apple''. Only [[single inheritance]] can be implemented through the prototype.
===Dynamic dispatch/message passing===
It is the responsibility of the object, not any external code, to select the procedural code to execute in response to a method call, typically by looking up the method at run time in a table associated with the object. This feature is known as [[dynamic dispatch]]. If the call variability relies on more than the single type of the object on which it is called (i.e. at least one other parameter object is involved in the method choice), one speaks of [[multiple dispatch]]. A method call is also known as ''[[message passing]]''. It is conceptualized as a message (the name of the method and its input parameters) being passed to the object for dispatch.
Dispatch interacts with inheritance; if a method is not present in a given object or class, the dispatch is [[Delegation (object-oriented programming)|delegated]] to its parent object or class, and so on, going up the chain of inheritance.
===Data abstraction and encapsulation===
Data [[Abstraction (computer science)|abstraction]] is a design pattern in which data are visible only to semantically related functions, to prevent misuse. The success of data abstraction leads to frequent incorporation of [[Information hiding|data hiding]] as a design principle in object-oriented and pure functional programming. Similarly, [[Encapsulation (computer programming)|encapsulation]] prevents external code from being concerned with the internal workings of an object. This facilitates [[code refactoring]], for example allowing the author of the class to change how objects of that class represent their data internally without changing any external code (as long as "public" method calls work the same way). It also encourages programmers to put all the code that is concerned with a certain set of data in the same class, which organizes it for easy comprehension by other programmers. Encapsulation is a technique that encourages [[Coupling (computer programming)|decoupling]].
If a class does not allow calling code to access internal object data and permits access through methods only, this is a form of information hiding known as [[Abstraction (computer science)|abstraction]]. Some languages (Java, for example) let classes enforce access restrictions explicitly, for example, denoting internal data with the <code>private</code> keyword and designating methods intended for use by code outside the class with the <code>public</code> keyword.{{sfn|Bloch|2018|loc=Chapter §4 Item15 Minimize the accessibility of classes and members|pp=73-77}} Methods may also be designed public, private, or intermediate levels such as <code>protected</code> (which allows access from the same class and its subclasses, but not objects of a different class).{{sfn|Bloch|2018|loc=Chapter §4 Item15 Minimize the accessibility of classes and members|pp=73-77}} In other languages (like Python) this is enforced only by convention (for example, <code>private</code> methods may have names that start with an [[underscore]]). In C#, Swift & Kotlin languages, <code>internal</code> keyword permits access only to files present in the same assembly, package, or module as that of the class.<ref>{{Cite web |date=2023-01-05 |title=What is Object Oriented Programming (OOP) In Simple Words? – Software Geek Bytes |url=https://softwaregeekbytes.com/object-oriented-programming-simple-words/ |access-date=2023-01-17 |language=en-US}}</ref>▼
▲If a class does not allow calling code to access internal object data and permits access through methods only, this is also a form of information hiding
In programming languages, particularly object-oriented ones, the emphasis on abstraction is vital. Object-oriented languages extend the notion of type to incorporate data abstraction, highlighting the significance of restricting access to internal data through methods.<ref>{{Cite journal |last1=Cardelli |first1=Luca |last2=Wegner |first2=Peter |date=1985-12-10 |title=On understanding types, data abstraction, and polymorphism |journal=ACM Computing Surveys |language=en |volume=17 |issue=4 |pages=471–523 |doi=10.1145/6041.6042 |issn=0360-0300|doi-access=free }}</ref>
▲Objects can contain other objects in their instance variables; this is known as [[object composition]]. For example, an object in the Employee class might contain (either directly or through a pointer) an object in the Address class, in addition to its own instance variables like "first_name" and "position". Object composition is used to represent "has-a" relationships: every employee has an address, so every Employee object has access to a place to store an Address object (either directly embedded within itself or at a separate ___location addressed via a pointer).
▲Languages that support classes almost always support [[inheritance (object-oriented programming)|inheritance]]. This allows classes to be arranged in a hierarchy that represents "is-a-type-of" relationships. For example, class Employee might inherit from class Person. All the data and methods available to the parent class also appear in the child class with the same names. For example, class Person might define variables "first_name" and "last_name" with method "make_full_name()". These will also be available in class Employee, which might add the variables "position" and "salary". This technique allows easy re-use of the same procedures and data definitions, in addition to potentially mirroring real-world relationships intuitively. Rather than utilizing database tables and programming subroutines, the developer utilizes objects the user may be more familiar with: objects from their application ___domain.<ref>{{cite book|last=Jacobsen|first=Ivar|title=Object Oriented Software Engineering|year=1992|publisher=Addison-Wesley ACM Press|isbn=978-0-201-54435-0|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>
▲Subclasses can override the methods defined by superclasses. [[Multiple inheritance]] is allowed in some languages, though this can make resolving overrides complicated. Some languages have special support for [[mixin]]s, 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, class UnicodeConversionMixin might provide a method unicode_to_ascii() when included in class FileReader and class WebPageScraper, which do not share a common parent.
▲[[Abstract class]]es cannot be instantiated into objects; they exist only for inheritance into other "concrete" classes that can be instantiated. In Java, the <code>[[final (Java)|final]]</code> keyword can be used to prevent a class from being subclassed.{{sfn|Bloch|2018|loc=Chapter §2 Item 4 Enforce noninstantiability with a private constructor|p=19}}
The "[[open/closed principle]]" advocates that classes and functions "should be open for extension, but closed for modification".
===Polymorphism===
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