Object-oriented programming (OOP) is a programming paradigm that uses "objects" and their interactions to design applications and computer programs. Programming techniques may include features such as information hiding, data abstraction, encapsulation, modularity, polymorphism, and inheritance. It was not commonly used in mainstream software application development until the early 1990s. Many modern programming languages now support OOP.
Object-oriented programming can trace its roots to the 1960s. As hardware and software became increasingly complex, quality was often compromised . Researchers studied ways to maintain software quality and developed object-oriented programming in part to address common problems by strongly emphasizing discrete, reusable units of programming logic. The methodology focuses on data rather than processes, with programs composed of self-sufficient modules (objects) each containing all the information needed to manipulate its own data structure. This is in contrast to the existing modular programming which had been dominant for many years that focused on the function of a module ,rather than specifically the data, but equally provided for code reuse, and self-sufficient reusable units of programming logic, enabling collaboration through the use of linked modules (/subroutines). This more conventional approach, which still persists, tends to consider data and behavior separately.
An object-oriented program may thus be viewed as a collection of cooperating objects, as opposed to the conventional model, in which a program is seen as a list of tasks (subroutines) to perform. In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects and can be viewed as an independent 'machine' with a distinct role or responsibility. The actions (or "operators") on these objects are closely associated with the object. For example, the data structures tend to carry their own operators around with them (or at least "inherit" them from a similar object or class).
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. Simula was used for physical modeling, such as models to study and improve the movement of ships and their content through cargo ports. Smalltalk was the first programming language to be called "object-oriented".
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. (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, 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 ideas of Simula 67 influenced many later languages, especially Smalltalk and derivatives of Lisp and Pascal.
The Smalltalk language, which was developed at Xerox PARC (by Alan Kay and others) 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 statically as in Simula 67 . Smalltalk and with it OOP were introduced to a wider audience by the August 1981 issue of Byte magazine.
In the 1970s, Kay's Smalltalk work had influenced the Lisp community to incorporate object-based techniques which were introduced to developers via the Lisp machine. In the 1980s, there were a few attempts to design processor architectures which included hardware support for objects in memory but these were not successful. Examples include the Intel iAPX 432 and the Linn Smart Rekursiv.
Object-oriented programming developed as the dominant programming methodology during the mid-1990s, largely due to the influence of C++. Its dominance was further enhanced by the rising popularity of graphical user interfaces, for which object-oriented programming seems to be 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.
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, 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, and also uses the idea of a virtual machine, which enables it to run on other platforms besides Microsoft's (one implementation for Linux and Mac OS X being the Mono Project).
More recently, a number of languages have emerged that are primarily object-oriented yet compatible with procedural methodology, such as Python and Ruby. Probably the most commercially important recent object-oriented languages are Visual Basic .NET (VB.NET) and C#, both designed for Microsoft's .NET platform, and Java, developed by Sun Microsystems. VB.NET and C# both support cross-language inheritance, allowing classes defined in one language to subclass classes defined in the other language.
Recently many universities have begun to teach Object-oriented design in introductory computer science classes.
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).
A survey by Deborah J. Armstrong  of nearly 40 years of computing literature identified a number of "quarks", or fundamental concepts, found in the strong majority of definitions of OOP. They are the following:
Dogwould consist of traits shared by all dogs, such as breed and fur color (characteristics), and the ability to bark and sit (behaviors). Classes provide modularity and structure in an object-oriented computer program. A class should typically be recognizable to a non-programmer familiar with the problem domain, meaning that the characteristics of the class should make sense in context. Also, the code for a class should be relatively self-contained (generally using encapsulation). Collectively, the properties and methods defined by a class are called members.
Dogdefines all possible dogs by listing the characteristics and behaviors they can have; the object
Lassieis one particular dog, with particular versions of the characteristics. A
Lassiehas brown-and-white fur.
Lassieobject is an instance of the
Dogclass. The set of values of the attributes of a particular object is called its state. The object consists of state and the behaviour that's defined in the object's class.
Lassie, being a
Dog, has the ability to bark. So
bark()is one of
Lassie's methods. She may have other methods as well, for example
save_timmy(). Within the program, using a method usually affects only one particular object; all
Dogs can bark, but you need only one particular dog to do the barking.
Breedermay tell the
Lassieobject to sit by passing a "sit" message which invokes Lassie's "sit" method. The syntax varies between languages, for example:
[Lassie sit]in Objective-C. In Java, code-level message passing corresponds to "method calling". Some dynamic languages use double-dispatch or multi-dispatch to find and pass messages.
Dogmight have sub-classes called
GoldenRetriever. In this case,
Lassiewould be an instance of the
Colliesubclass. Suppose the
Dogclass defines a method called
bark()and a property called
furColor. Each of its sub-classes (
GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once.
Collieclass might specify that the default
furColorfor a collie is brown-and-white. The
Chihuahuasubclass might specify that the
bark()method produces a high pitch by default. Subclasses can also add new members. The
Chihuahuasubclass could add a method called
tremble(). So an individual chihuahua instance would use a high-pitched
Chihuahuasubclass, which in turn inherited the usual
Dog. The chihuahua object would also have the
Lassiewould not, because she is a
Collie, not a
Chihuahua. In fact, inheritance is an "a... is a" relationship between classes, while instantiation is an "is a" relationship between an object and a class: a
Dog("a... is a"), but
Collie("is a"). Thus, the object named
Lassiehas the methods from both classes
Cats, and a
Chimeraobject could be created from these two which inherits all the (multiple) behavior of cats and dogs. This is not always supported, as it can be hard both to implement and to use well.
Dogmay be treated as a
Dogmuch of the time, a
Colliewhen necessary to access
Collie-specific attributes or behaviors, and as an
Animal(perhaps the parent class of
Dog) when counting Timmy's pets.
Carwould be made up of an Engine, Gearbox, Steering objects, and many more components. To build the
Carclass, one does not need to know how the different components work internally, but only how to interface with them, i.e., send messages to them, receive messages from them, and perhaps make the different objects composing the class interact with each other.
Dogclass has a
bark()method. The code for the
bark()method defines exactly how a bark happens (e.g., by
exhale(), at a particular pitch and volume). Timmy,
Lassie's friend, however, does not need to know exactly how she barks. Encapsulation is achieved by specifying which classes may use the members of an object. The result is that each object exposes to any class a certain interface — those members accessible to that class. The reason for encapsulation is to prevent clients of an interface from depending on those parts of the implementation that are likely to change in future, thereby allowing those changes to be made more easily, that is, without changes to clients. For example, an interface can ensure that puppies can only be added to an object of the class
Dogby code in that class. Members are often specified as public, protected or private, determining whether they are available to all classes, sub-classes or only the defining class. Some languages go further: Java uses the default access modifier to restrict access also to classes in the same package, C# and VB.NET reserve some members to classes in the same assembly using keywords internal (C#) or Friend (VB.NET), and Eiffel and C++ allow one to specify which classes may access any member.
Dogis commanded to
speak(), this may elicit a
bark(). However, if a
Pig<code> is commanded to <code>speak(), this may elicit an
oink(). They both inherit
Animal, but their derived class methods override the methods of the parent class; this is Overriding Polymorphism. Overloading Polymorphism is the use of one method signature, or one operator such as "+", to perform several different functions depending on the implementation. The "+" operator, for example, may be used to perform integer addition, float addition, list concatenation, or string concatenation. Any two subclasses of
Number, such as
Double, are expected to add together properly in an OOP language. The language must therefore overload the addition operator, "+", to work this way. This helps improve code readability. How this is implemented varies from language to language, but most OOP languages support at least some level of overloading polymorphism. Many OOP languages also support Parametric Polymorphism, where code is written without mention of any specific type and thus can be used transparently with any number of new types. Pointers are an example of a simple polymorphic routine that can be used with many different types of objects.
Not all of the above concepts are to be found in all object-oriented programming languages, and so object-oriented programming that uses classes is called sometimes class-based programming. In particular, prototype-based programming does not typically use classes. As a result, a significantly different yet analogous terminology is used to define the concepts of object and instance.
In recent years, object-oriented programming has become especially popular in scripting programming languages. Python and Ruby are scripting languages built on OOP principles, while Perl and PHP have been adding object oriented features since Perl 5 and PHP 3, and ColdFusion since version 6.
There are a number of programming challenges which a developer encounters regularly in object-oriented design. There are also widely accepted solutions to these problems. The best known are the design patterns codified by Gamma et al, but in a broader sense the term "design patterns" can be used to refer to any general, repeatable solution to a commonly occurring problem in software design. Some of these commonly occurring problems have implications and solutions particular to object-oriented development.
See main article: Design Patterns. Design Patterns: Elements of Reusable Object-Oriented Software is an influential book published in 1995 by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, sometimes casually called the "Gang of Four". Along with exploring the capabilities and pitfalls of object-oriented programming, it describes 23 common programming problems and patterns for solving them.As of April 2007, the book was in its 36th printing.Typical design patterns are as follows:
See main article: Object-Relational impedance mismatch, Object-relational mapping and Object database. Both object-oriented programming and relational database management systems (RDBMSs) are extremely common in software . Since relational databases don't store objects directly (though some RDBMSs have object-oriented features to approximate this), there is a general need to bridge the two worlds. There are a number of widely used solutions to this problem. One of the most common is object-relational mapping, as found in libraries like Java Data Objects and Ruby on Rails' ActiveRecord.
There are also object databases which can be used to replace RDBMSs, but these have not been as commercially successful as RDBMSs.
OOP can be used to translate from real-world phenomena to program elements (and vice versa). OOP was even invented for the purpose of physical modeling in the Simula-67 programming language. However, not everyone agrees that direct real-world mapping is facilitated by OOP, or is even a worthy goal; Bertrand Meyer argues in Object-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". At the same time, some principal limitations of OOP had been noted. 
However, Niklaus Wirth said of OOP in his paper "Good Ideas through the Looking Glass", "This paradigm closely reflects the structure of systems 'in the real world', and it is therefore well suited to model complex systems with complex behaviours."
There have been several attempts at formalizing the concepts used in object-oriented programming. The following concepts and constructs have been used as interpretations of OOP concepts:
Attempts to find a consensus definition or theory behind objects have not proven very successful (however, see "Abadi & Cardelli: A Theory of Objects"  for formal definitions of many OOP concepts and constructs), and often diverge widely. For example, some definitions focus on mental activities, and some on mere program structuring. One of the simpler definitions is that OOP is the act of using "map" data structures or arrays that can contain functions and pointers to other maps, all with some syntactic and scoping sugar on top. Inheritance can be performed by cloning the maps (sometimes called "prototyping").
The term OOPS, which refers to an object-oriented programming system, is widely used and was common in early development of object-oriented programming.
In the academic realm of MIT there is a long tradition of criticising object-oriented programming as a false anunciation. Usually this claim is founded upon the observation that there are no formal innovations in object-oriented programming -- that the paradigm never brought out a technique or characteristic of programming languages not known to academics by another name. Classes and inheritance are syntactic sugar, whereas polymorphism has long been known to Lisp developers as "dispatching on type." Overall, object-oriented programming, generic programming, and whatever else the designers of static programming languages add to augment the expressivity of these languages have been formally known to programmers of Lisp for many years. What has been wanting in Lisp is efficiency, and the addition of these techniques to static languages brings their efficiency to the fore.
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