Object-oriented programming (OOP) is a programming paradigm that uses "objects" to design applications and computer programs. It utilizes several techniques from previously established paradigms, including inheritance, modularity, polymorphism, and encapsulation. Today, many popular programming languages (such as Ada, C++, Delphi, Java, Lisp, SmallTalk, Perl, PHP, Python, Ruby, VB.Net, Visual FoxPro, and Visual Prolog) support OOP.
Object-oriented programming's 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, how could software quality be maintained? Object-oriented programming addresses this problem by strongly emphasizing modularity 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. Object-oriented programming came into existence because human consciousness, understanding and logic are highly object-oriented. By way of "objectifying" software modules, it 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.
A survey of computing literature, identified a number of "quarks," or fundamental concepts, identified in the strong majority of definitions of OOP. They are:
Class
A class defines the abstract characteristics of a thing (object), including the thing's characteristics (its attributes or properties) and the things it can do (its behaviors or methods or features). For example, the class Dog would consist of traits shared by all dogs, for example breed, fur color, and the ability to bark. 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. Collectively, the properties and methods defined by a class are called members.
Object
A particular instance of a class. The class of Dog defines all possible dogs by listing the characteristics that they can have; the object Lassie is one particular dog, with particular versions of the characteristics. A Dog has fur; Lassie has brown-and-white fur. In programmer jargon, the object Lassie is an instance of the Dog class. The set of values of the attributes of a particular object is called its state.
Method
An object's abilities. 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 sit() or eat(). Within the program, using a method should only affect one particular object; all Dogs can bark, but you need one particular dog to do the barking.
Message passing
"The process by which an object sends data to another object or asks the other object to invoke a method."
Encapsulation
Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior ; encapsulation serves to separate the contractual interface of an abstraction and its implementation.
In short, Encapsulation :
- Hides the implementation details of a class.
- Forces the user to use an interface to access data.
- Makes the code more maintainable.
Polymorphism is the existence of the classes or methods in different forms or single name denoting different implementations.
Abstraction
Abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.
OOP in scripting
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 4. The Document Object Model of HTML, XHTML, and XML documents on the Internet have bindings to the popular JavaScript/ECMAScript language. JavaScript is perhaps the best known prototype-based programming language.
Problems and patterns
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 more general 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.
Gang of Four design patterns
Design Patterns: Elements 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.
Object-orientation and databases
Both object-oriented programming and relational database management systems (RDBMSs) are extremely common in software today. 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.
Matching real world
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 modelling 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 a model of some part of the world; "Reality is a cousin twice removed".
Formal definition
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: coalgebraic datatypes existential quantification and modules recursion records and record extensions F-bounded polymorphism Attempts to find a consensus definition or theory behind objects have not proven very successful, 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").
