Object-oriented programming

Object-oriented programming (OOP) is a programming paradigm based on the object – a software entity that encapsulates data and function(s). An OOP computer program consists of objects that interact with one another. A programming language that provides OOP features is classified as an OOP language but as the set of features that contribute to OOP is contended, classifying a language as OOP and the degree to which it supports or is OOP, are debatable. As paradigms are not mutually exclusive, a language can be multi-paradigm; can be categorized as more than only OOP.

Sometimes, objects represent real-world things and processes in digital form. 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".

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. Bertrand Meyer argues 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". Steve Yegge noted that natural languages lack the OOP approach of naming a thing (object) before an action (method), as opposed to functional programming which does the reverse. This can make an OOP solution more complex than one written via procedural programming.

Notable languages with OOP support include Ada, ActionScript, C++, Common Lisp, C#, Dart, Eiffel, Fortran 2003, Haxe, Java, JavaScript, Kotlin, Logo, MATLAB, Objective-C, Object Pascal, Perl, PHP, Python, R, Raku, Ruby, Scala, SIMSCRIPT, Simula, Smalltalk, Swift, Vala and Visual Basic (.NET).

The idea of "objects" in programming began with the artificial intelligence group at Massachusetts Institute of Technology (MIT) in the late 1950s and early 1960s. Here, "object" referred to LISP atoms with identified properties (attributes). Another early example was Sketchpad created by Ivan 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. Later, in 1968, AED-0, MIT's version of the ALGOL programming language, connected data structures ("plexes") and procedures, prefiguring what were later termed "messages", "methods", and "member functions". Topics such as data abstraction and modular programming were common points of discussion at this time.

Meanwhile, in Norway, Simula was developed during the years 1961–1967. Simula introduced essential object-oriented ideas, such as classes, inheritance, and dynamic binding. Simula was used mainly by researchers involved with physical modelling, like the movement of ships and their content through cargo ports. Simula is generally accepted as being the first language with the primary features and framework of an object-oriented language.

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 create Smalltalk, an influential OOP language. By 1967, Kay was already using the term "object-oriented programming" in conversation. Although sometimes called the "father" of OOP, Kay has said his ideas differ from how OOP is commonly understood, and has implied that the computer science establishment did not adopt his notion. A 1976 MIT memo co-authored by Barbara Liskov lists Simula 67, CLU, and Alphard as object-oriented languages, but does not mention Smalltalk.