Programming paradigm based on the concept of objects
"Object-oriented" redirects here. For other meanings of object-oriented, seeObject-orientation.
UML notation for a class. This Button class hasvariables for data, andfunctions. Through inheritance, a subclass can be created as a subset of the Button class. Objects are instances of a class.
The idea of "objects" in programming started with theartificial intelligence group atMIT in the late 1950s and early 1960s. Here, "object" referred toLISP atoms with identified properties (attributes).[5][6]Another early example wasSketchpad created byIvan Sutherland at MIT in 1960–1961. In the glossary of his technical report, Sutherland defined terms like "object" and "instance" (with the class concept covered by "master" or "definition"), albeit specialized to graphical interaction.[7] Later, in 1968, AED-0, MIT's version of theALGOL programming language, connected data structures ("plexes") and procedures, prefiguring what were later termed "messages", "methods", and "member functions".[8][9]Topics such asdata abstraction andmodular programming were common points of discussion at this time.
Meanwhile, in Norway,Simula was developed during the years 1961–1967.[8] Simula introduced essential object-oriented ideas, such asclasses, inheritance, anddynamic binding.[10]Simula was used mainly by researchers involved withphysical modelling, like the movement of ships and their content through cargo ports.[10] Simula is generally accepted as being the first language with the primary features and framework of an object-oriented language.[11]
I thought of objects being like biological cells and/or individual computers on a network, only able to communicate with messages (so messaging came at the very beginning – it took a while to see how to do messaging in a programming language efficiently enough to be useful).
Influenced by both MIT and Simula,Alan Kay began developing his own ideas in November 1966. He would go on to createSmalltalk, an influential object-oriented programming language. By 1967, Kay was already using the term "object-oriented programming" in conversation.[1] Although sometimes called the "father" of object-oriented programming,[12] Kay has said his ideas differ from how object-oriented programming is commonly understood, and has implied that the computer science establishment did not adopt his notion.[1]A 1976 MIT memo co-authored byBarbara Liskov listsSimula 67,CLU, andAlphard as object-oriented languages, but does not mention Smalltalk.[13]
In the 1970s, the first version of theSmalltalk programming language was developed atXerox PARC byAlan Kay,Dan Ingalls andAdele Goldberg. Smalltalk-72 was notable for use of objects at the language level and its graphical development environment.[14] Smalltalk was a fully dynamic system, allowing users to create and modify classes as they worked.[15] Much of the theory of OOP was developed in the context of Smalltalk, for example multiple inheritance.[16]
In the late 1970s and 1980s, object-oriented programming rose to prominence. TheFlavors object-oriented Lisp was developed starting 1979, introducingmultiple inheritance andmixins.[17] In August 1981,Byte Magazine highlighted Smalltalk and OOP, introducing these ideas to a wide audience.[18] LOOPS, the object system forInterlisp-D, was influenced by Smalltalk and Flavors, and a paper about it was published in 1982.[19] In 1986, the firstConference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA) was attended by 1,000 people. This conference marked the beginning of efforts to consolidate Lisp object systems, eventually resulting in theCommon Lisp Object System. In the 1980s, there were a few attempts to designprocessor architectures that includedhardware support for objects inmemory, but these were not successful. Examples include theIntel iAPX 432 and theLinn SmartRekursiv.
AtETH Zürich,Niklaus Wirth and his colleagues created new approaches to OOP.Modula-2 (1978) andOberon (1987), included a distinctive approach to object orientation, classes, and type checking across module boundaries. Inheritance is not obvious in Wirth's design since his nomenclature looks in the opposite direction: It is called type extension and the viewpoint is from the parent down to the inheritor.
Many programming languages that existed before OOP have added object-oriented features, includingAda,BASIC,Fortran,Pascal, andCOBOL. This sometimes caused compatibility and maintainability issues, as these languages were not originally designed with OOP in mind.
In the new millenium, new languages likePython andRuby have emerged that combine object-oriented and procedural styles. The most commercially important "pure" object-oriented languages continue to beJava, developed bySun Microsystems, as well asC# andVisual Basic.NET (VB.NET), both designed for Microsoft's.NET platform. These languages show the benefits of OOP by creating abstractions from implementation. The .NET platform supports cross-language inheritance, allowing programs to use objects from multiple languages together.
Object-oriented programming focuses on working with objects, but not all OOP languages have every feature linked to OOP. Below are some common features of languages that are considered strong in OOP or support it along with other programming styles. Important exceptions are also noted.[24][25][26][27]Christopher J. Date pointed out that comparing OOP with other styles, likerelational programming, is difficult because there isn't a clear, agreed-upon definition of OOP.[28]
Procedures – also known as functions, methods, routines, orsubroutines – take input, generate output, and work with data. Modern languages includestructured programming constructs likeloops andconditionals.
Support formodular programming lets programmers organize related procedures into files and modules. This makes programs easier to manage. Each module has its ownnamespace, so items in one module will not conflict with items in another.
Object-oriented programming (OOP) was created to make code easier toreuse andmaintain.[29] However, it was not designed to clearly show the flow of a program's instructions—that was left to the compiler. As computers began using more parallel processing and multiplethreads, it became more important to understand and control how instructions flow. This is difficult to do with OOP.[30][31][32][33]
An object is a type ofdata structure that has two main parts:fields andmethods. Fields may also be known as members, attributes, or properties, and hold information in the form of statevariables. Methods are actions,subroutines, or procedures, defining the object's behavior in code. Objects are usually stored inmemory, and in many programming languages, they work likepointers that link directly to a contiguous block containing the object instances's data.
Objects can contain other objects. This is calledobject composition. For example, an Employee object might have an Address object inside it, along with other information like "first_name" and "position". This type of structures shows "has-a" relationships, like "an employee has an address".
Some believe that OOP places too much focus on using objects rather than onalgorithms anddata structures.[34][35] For example, programmerRob Pike pointed out that OOP can make programmers think more about type hierarchy than composition.[36] He has called object-oriented programming "theRoman numerals of computing".[37]Rich Hickey, creator ofClojure, described OOP as overly simplistic, especially when it comes to representing real-world things that change over time.[35]Alexander Stepanov said that OOP tries to fit everything into a single type, which can be limiting. He argued that sometimes we need multisorted algebras—families of interfaces that span multiple types, such as ingeneric programming. Stepanov also said that calling everything an "object" doesn't add much understanding.[34]
Sometimes, objects represent real-world things and processes in digital form.[38] 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".Niklaus Wirth said, "This paradigm [OOP] closely reflects the structure of systems in the real world and is therefore well suited to model complex systems with complex behavior".[39]
However, more often, objects represent abstract entities, like an open file or a unit converter. Not everyone agrees that OOP makes it easy to copy the real world exactly or that doing so is even necessary.Bob Martin suggests that because classes are software, their relationships don't match the real-world relationships they represent.[40]Bertrand Meyer argues inObject-Oriented Software Construction, that a program is not a model of the world but a model of some part of the world; "Reality is a cousin twice removed".[41]Steve Yegge noted that natural languages lack the OOP approach of strictly prioritizingthings (objects/nouns) beforeactions (methods/verbs), as opposed tofunctional programming which does the reverse.[42] This can sometimes make OOP solutions more complicated than those written in procedural programming.[43]
Most OOP languages allowreusing andextending code through "inheritance". This inheritance can use either "classes" or "prototypes", which have some differences but use similar terms for ideas like "object" and "instance".
Inclass-based programming, the most common type of OOP, every object is aninstance of a specificclass. The class defines the data format, like variables (e.g., name, age) and methods (actions the object can take). Every instance of the class has the same set of variables and methods. Objects are created using a special method in the class known as aconstructor.
Here are a few key terms in class-based OOP:
Class variables – belong to theclass itself, so all objects in the class share one copy.
Instance variables – belong to individualobjects; every object has its own version of these variables.
Member variables – refers to both the class and instance variables that are defined by a particular class.
Class methods – linked to theclass itself and can only use class variables.
Instance methods – belong toindividual objects, and can use both instance and class variables
Classes may inherit from other classes, creating a hierarchy of "subclasses". For example, an "Employee" class might inherit from a "Person" class. This means the Employee object will have all the variables from Person (like name variables) plus any new variables (like job position and salary). Similarly, the subclass may expand the interface with new methods. Most languages also allow the subclass tooverride the methods defined by superclasses. Some languages supportmultiple inheritance, where a class can inherit from more than one class, and other languages similarly supportmixins ortraits. For example, a mixin called UnicodeConversionMixin might add a method unicode_to_ascii() to both a FileReader and a WebPageScraper class.
Some classes areabstract, meaning they cannot be directly instantiated into objects; they're only meant to be inherited into other classes. Other classes areutility classes which contain only class variables and methods and are not meant to be instantiated or subclassed.[44]
Inprototype-based programming, there aren't any classes. Instead, each object is linked to another object, called itsprototype orparent. In Self, an object may have multiple or no parents,[45] but in the most popular prototype-based language, Javascript, every object has exactly oneprototype link, up to the base Object type whose prototype is null.
The prototype acts as a model for new objects. For example, if you have an objectfruit, you can make two objectsapple andorange, based on it. There is nofruit class, but they share traits from thefruit prototype. Prototype-based languages also allow objects to have their own unique properties, so theapple object might have an attributesugar_content, while theorange orfruit objects do not.
Some languages, likeGo, don't use inheritance at all.[46] Instead, they encourage "composition over inheritance", where objects are built using smaller parts instead of parent-child relationships. For example, instead of inheriting from class Person, the Employee class could simply contain a Person object. This lets the Employee class control how much of Person it exposes to other parts of the program.Delegation is another language feature that can be used as an alternative to inheritance.
Programmers have different opinions on inheritance. Bjarne Stroustrup, author of C++, has stated that it is possible to do OOP without inheritance.[47]Rob Pike has criticized inheritance for creating complicated hierarchies instead of simpler solutions.[48]
People often think that if one class inherits from another, it means the subclass "is a" more specific version of the original class. This presumes theprogram semantics are that objects from the subclass can always replace objects from the original class without problems. This concept is known asbehavioral subtyping, more specifically theLiskov substitution principle.
However, this is often not true, especially in programming languages that allowmutable objects, objects that change after they are created. In fact,subtype polymorphism as enforced by thetype checker in OOP languages cannot guarantee behavioral subtyping in most if not all contexts. For example, thecircle-ellipse problem is notoriously difficult to handle using OOP's concept of inheritance. Behavioral subtyping is undecidable in general, so it cannot be easily implemented by a compiler. Because of this, programmers must carefully design class hierarchies to avoid mistakes that the programming language itself cannot catch.
When a method is called on an object, the object itself—not outside code—decides which specific code to run. This process, calleddynamic dispatch, usually happens at run time by checking a table linked to the object to find the correct method. In this context, a method call is also known asmessage passing, meaning the method name and its inputs are like a message sent to the object for it to act on. If the method choice depends on more than one type of object (such as other objects passed as parameters), it's calledmultiple dispatch.
Dynamic dispatch works together with inheritance: if an object doesn't have the requested method, it looks up to its parent class (delegation), and continues up the chain until it finds the method or reaches the top.
Dataabstraction is a way of organizing code so that only certain parts of the data are visible to related functions (data hiding). This helps prevent mistakes and makes the program easier to manage. Because data abstraction works well, many programming styles, like object-oriented programming and functional programming, use it as a key principle.Encapsulation is another important idea in programming. It means keeping the internal details of an object hidden from the outside code. This makes it easier to change how an object works on the inside without affecting other parts of the program, such as incode refactoring. Encapsulation also helps keep related code together (decoupling), making it easier for programmers to understand.
In object-oriented programming, objects act as a barrier between their internal workings and external code. Outside code can only interact with an object by calling specificpublic methods or variables. If a class only allows access to its data through methods and not directly, this is calledinformation hiding. When designing a program, it's often recommended to keep data as hidden as possible. This means using local variables inside functions when possible, then private variables (which only the object can use), and finally public variables (which can be accessed by any part of the program) if necessary. Keeping data hidden helps prevent problems when changing the code later.[49] Some programming languages, like Java, control information hiding by marking variables asprivate (hidden) orpublic (accessible).[50] Other languages, like Python, rely on naming conventions, such as starting a private method's name with an underscore. Intermediate levels of access also exist, such as Java'sprotected keyword, (which allows access from the same class and its subclasses, but not objects of a different class), and theinternal keyword in C#, Swift, and Kotlin, which restricts access to files within the same module.[51]
Abstraction and information hiding are important concepts in programming, especially in object-oriented languages.[52] Programs often create many copies of objects, and each one works independently. Supporters of this approach say it makes code easier to reuse and intuitively represents real-world situations.[53] However, others argue that object-oriented programming does not enhance readability or modularity.[54][55]Eric S. Raymond has written that object-oriented programming languages tend to encourage thickly layered programs that destroy transparency.[56] Raymond compares this unfavourably to the approach taken with Unix and theC programming language.[56]
One programming principle, called the "open/closed principle", says that classes and functions should be "open for extension, but closed for modification".Luca Cardelli has stated that OOP languages have "extremely poor modularity properties with respect to class extension and modification", and tend to be extremely complex.[54] The latter point is reiterated byJoe Armstrong, the principal inventor ofErlang, who is quoted as saying:[55]
The problem with object-oriented languages is they've got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.
Leo Brodie says that information hiding can lead to copying the same code in multiple places (duplicating code),[57] which goes against thedon't repeat yourself rule of software development.[58]
Polymorphism is the use of one symbol to represent multiple different types.[59] In object-oriented programming, polymorphism more specifically refers tosubtyping or subtype polymorphism, where a function can work with a specificinterface and thus manipulate entities of different classes in a uniform manner.[60]
For example, imagine a program has two shapes: a circle and a square. Both come from a common class called "Shape." Each shape has its own way of drawing itself. With subtype polymorphism, the program doesn't need to know the type of each shape, and can simply call the "Draw" method for each shape. The programming language runtime will ensure the correct version of the "Draw" method runs for each shape. Because the details of each shape are handled inside their own classes, this makes the code simpler and more organized, enabling strongseparation of concerns.
In object-oriented programming, objects have methods that can change or use the object's data. Many programming languages use a special word, likethis orself, to refer to the current object. In languages that supportopen recursion, a method in an object can call other methods in the same object, including itself, using this special word. This allows a method in one class to call another method defined later in a subclass, a feature known aslate binding.
OOP languages can be grouped into different types based on how they support and use objects:
Pure OOP languages: In these languages, everything is treated as an object, even basic things like numbers and characters. They are designed to fully support and enforce OOP. Examples:Ruby,Scala,Smalltalk,Eiffel,Emerald,[61]JADE,Self,Raku.
Mostly OOP languages: These languages focus on OOP but also include some procedural programming features. Examples:Java,Python,C++,C#,Delphi/Object Pascal,VB.NET.
Unique OOP languages: These languages have OOP features like classes and inheritance but use them in their own way. Examples:Oberon,BETA.
Object-based languages: These support some OOP ideas but avoid traditional class-based inheritance in favor of direct manipulation of objects. Examples:JavaScript,Lua,Modula-2,CLU,Go.
Multi-paradigm languages: These support both OOP and other programming styles, but OOP is not the predominant style in the language. Examples includeTcl, where TclOO allows both prototype-based and class-based OOP, andCommon Lisp, with itsCommon Lisp Object System.
TheTIOBE programming language popularity index graph from 2002 to 2023. In the 2000s the object-orientedJava (orange) and theproceduralC (dark blue) competed for the top position.
Many popular programming languages, like C++, Java, and Python, use object-oriented programming. In the past, OOP was widely accepted,[62] but recently, some programmers have criticized it and prefer functional programming instead.[63] A study by Potok et al. found no major difference in productivity between OOP and other methods.[64]
Paul Graham, a well-known computer scientist, believes big companies like OOP because it helps manage large teams of average programmers. He argues that OOP adds structure, making it harder for one person to make serious mistakes, but at the same time restrains smart programmers.[65]Eric S. Raymond, aUnix programmer andopen-source software advocate, argues that OOP is not the best way to write programs.[56]
Richard Feldman says that, while OOP features helped some languages stay organized, their popularity comes from other reasons.[66] Lawrence Krubner argues that OOP doesn't offer special advantages compared to other styles, like functional programming, and can make coding more complicated.[67]Luca Cardelli says that OOP is slower and takes longer to compile than procedural programming.[54]
In recent years, object-oriented programming (OOP) has become very popular indynamic programming languages. Some languages, likePython,PowerShell,Ruby andGroovy, were designed with OOP in mind. Others, likePerl,PHP, andColdFusion, started as non-OOP languages but added OOP features later (starting with Perl 5, PHP 4, and ColdFusion version 6).
On the web,HTML,XHTML, andXML documents use theDocument Object Model (DOM), which works with theJavaScript language. JavaScript is a well-known example of aprototype-based language. Instead of using classes like other OOP languages, JavaScript creates new objects by copying (or "cloning") existing ones. Another language that uses this method isLua.
When computers communicate in a client-server system, they send messages to request services. For example, a simple message might include a length field (showing how big the message is), a code that identifies the type of message, and a data value. These messages can be designed as structured objects that both the client and server understand, so that each type of message corresponds to a class of objects in the client and server code. More complex messages might include structured objects as additional details. The client and server need to know how to serialize and deserialize these messages so they can be transmitted over the network, and map them to the appropriate object types. Both clients and servers can be thought of as complex object-oriented systems.
Design patterns are common solutions to problems in software design. Some design patterns are especially useful for object-oriented programming, and design patterns are typically introduced in an OOP context.
To solve this problem, developers use different methods, but none of them are perfect.[69] One of the most common solutions isobject-relational mapping (ORM), which helps connect object-oriented programs to relational databases. Examples of ORM tools includeVisual FoxPro,Java Data Objects, andRuby on Rails ActiveRecord.
Some databases, calledobject databases, are designed to work with object-oriented programming. However, they have not been as popular or successful as relational databases.
Date and Darwen have proposed a theoretical foundation that uses OOP as a kind of customizabletype system to support RDBMSs, but it forbids objects containing pointers to other objects.[70]
Inresponsibility-driven design, classes are built around what they need to do and the information they share, in the form of a contract. This is different fromdata-driven design, where classes are built based on the data they need to store. According to Wirfs-Brock and Wilkerson, the originators of responsibility-driven design, responsibility-driven design is the better approach.[71]
Open/closed principle: Software entities should be open for extension, but closed for modification.
Liskov substitution principle: Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.
GRASP (General Responsibility Assignment Software Patterns) is another set of software design rules, created byCraig Larman, that helps developers assign responsibilities to different parts of a program:[72]
Creator Principle: allows classes create objects they closely use.
Information Expert Principle: assigns tasks to classes with the needed information.
Low Coupling Principle: reduces class dependencies to improve flexibility and maintainability.
High Cohesion Principle: designing classes with a single, focused responsibility.
Controller Principle: assigns system operations to separate classes that manage flow and interactions.
Polymorphism: allows different classes to be used through a common interface, promoting flexibility and reuse.
Pure Fabrication Principle: create helper classes to improve design, boost cohesion, and reduce coupling.
In object-oriented programming, objects are things that exist while a program is running. An object can represent anything, like a person, a place, a bank account, or a table of data. Many researchers have tried to formally define how OOP works.Records are the basis for understanding objects. They can represent fields, and also methods, iffunction literals can be stored. However,inheritance presents difficulties, particularly with the interactions between open recursion and encapsulated state. Researchers have usedrecursive types andco-algebraic data types to incorporate essential features of OOP.[73] Abadi and Cardelli defined several extensions ofSystem F<: that deal with mutable objects, allowing bothsubtype polymorphism andparametric polymorphism (generics), and were able to formally model many OOP concepts and constructs.[74] Although far from trivial, static analysis of object-oriented programming languages such as Java is a mature field,[75] with several commercial tools.[76]
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