Object-oriented programming (OOP) is a programming paradigm based on the object[1] – a software entity that encapsulates data and function(s). An OOP computer program consists of objects that interact with one another.[2][3] 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.[4] 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".[5]
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.[6] 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".[7] 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.[8] This can make an OOP solution more complex than one written via procedural programming.[9]
Notable languages with OOP support include Ada, ActionScript, C++, Common Lisp, C#, Dart, Eiffel, Fortran 2003, Haxe, Java,[10] JavaScript, Kotlin, Logo, MATLAB, Objective-C, Object Pascal, Perl, PHP, Python, R, Raku, Ruby, Scala, SIMSCRIPT, Simula, Smalltalk, Swift, Vala and Visual Basic (.NET).