Object-oriented programming (OOP) is a programming paradigm that uses "objects" to design applications and computer programs. It is based on several techniques, including inheritance, modularity, polymorphism, and encapsulation. It was not commonly used in mainstream software application development until the early 1990s. Many modern programming languages now support OOP.
Object-oriented programming roots reach all the way back to the 1960s, when the nascent field of software engineering had begun to discuss the idea of a software crisis. As hardware and software became increasingly complex, researchers studied how software quality could be maintained. Object-oriented programming was deployed to address this problem by strongly emphasizing modularity (discrete units of programming logic) in software.
The Simula programming language was the first to introduce the concepts underlying object-oriented programming (objects, classes, subclasses, virtual methods, coroutines, garbage collection, and discrete event simulation) as a superset of Algol. Smalltalk was the first programming language to be called "object-oriented".
Object-oriented programming may be seen as a collection of cooperating objects, as opposed to a traditional view in which a program may be seen as a list of instructions to the computer. In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects. Each object can be viewed as an independent little machine with a distinct role or responsibility.
By way of "objectifying" software modules, object-oriented programming is intended to promote greater flexibility and maintainability in programming, and is widely popular in large-scale software engineering.  By virtue of its strong emphasis on modularity, object oriented code is intended to be simpler to develop and easier to understand later on, lending itself to more direct analysis, coding, and understanding of complex situations and procedures than less modular programming methods. Core Java Examples
The concept of objects and instances in computing had its first major breakthrough with the PDP-1 system at MIT which was probably the earliest example of capability based architecture. Another early example was Sketchpad made by Ivan Sutherland in 1963; however, this was an application and not a programming paradigm. Objects as programming entities were introduced in the 1960s in Simula 67, a programming language designed for making simulations, created by Ole-Johan Dahl and Kristen Nygaard of the Norwegian Computing Center in Oslo. (Reportedly, the story is that they were working on ship simulations, and were confounded by the combinatorial explosion of how the different attributes from different ships could affect one another. The idea occurred to group the different types of ships into different classes of objects, each class of objects being responsible for defining its own data and behavior.) Such an approach was a simple extrapolation of concepts earlier used in analog programming. On analog computers, such direct mapping from real-world phenomena/objects to analog phenomena/objects (and conversely), was (and is) called 'simulation'. Simula not only introduced the notion of classes, but also of instances of classes, which is probably the first explicit use of those notions.
The Smalltalk language, which was developed at Xerox PARC in the 1970s, introduced the term Object-oriented programming to represent the pervasive use of objects and messages as the basis for computation. Smalltalk creators were influenced by the ideas introduced in Simula 67, but Smalltalk was designed to be a fully dynamic system in which classes could be created and modified dynamically rather than simply using static ones as in Simula 67. The ideas in Simula 67 were also used in many other languages, from derivatives of Lisp to Pascal.
Object-oriented programming developed as the dominant programming methodology during the mid-1990s, largely due to the influence of C++. Its dominance was further cemented by the rising popularity of graphical user interfaces, for which object-oriented programming is well-suited. An example of a closely related dynamic GUI library and OOP language can be found in the Cocoa frameworks on Mac OS X, written in Objective C, an object-oriented, dynamic messaging extension to C based on Smalltalk. OOP toolkits also enhanced the popularity of event-driven programming (although this concept is not limited to OOP). Some feel that association with GUIs (real or perceived) was what propelled OOP into the programming mainstream.
OOP also became increasingly popular for developing computer games during the 1990s. As the complexity of games grew, as faster hardware became more widely available and compilers (especially C++) matured, more and more games and their engines were written in OOP languages. Prominent C++ examples include Starcraft, Diablo, and Warcraft III. Since almost all video games feature virtual environments which contain many, often thousands of objects that interact with each other in complex ways, OOP languages are particularly suited for game development.
At ETH Zürich, Niklaus Wirth and his colleagues had also been investigating such topics as data abstraction and modular programming. Modula-2 included both, and their succeeding design, Oberon, included a distinctive approach to object orientation, classes, and such. The approach is unlike Smalltalk, and very unlike C++.
Object-oriented features have been added to many existing languages during that time, including Ada, BASIC, Lisp, Fortran, Pascal, and others. Adding these features to languages that were not initially designed for them often led to problems with compatibility and maintainability of code.
In the past decade Java has emerged in wide use partially because of its similarity to C and to C++, but perhaps more importantly because of its implementation using a virtual machine that is intended to run code unchanged on many different platforms. This last feature has made it very attractive to larger development shops with heterogeneous environments. Microsoft's .NET initiative has a similar objective and includes/supports several new languages, or variants of older ones.
More recently, a number of languages have emerged that are primarily object-oriented yet compatible with procedural methodology, such as Python and Ruby. Besides Java, probably the most commercially important recent object-oriented languages are Visual Basic .NET and C# designed for Microsoft's .NET platform.
Just as procedural programming led to refinements of techniques such as structured programming, modern object-oriented software design methods include refinements such as the use of design patterns, design by contract, and modeling languages (such as UML).
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