Object-oriented programming

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Object-oriented programming (OOP) is a programming paradigm that uses "objects" and their interactions 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.

Introduction

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) and reusability in software.[1]

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.[2]

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. [citation needed] 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.[citation needed]

Fundamental concepts

A survey by Deborah J. Armstrong [3] 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:

Class
A class defines the abstract characteristics of a thing (object), including the thing's characteristics (its attributes, fields or properties) and the thing's behaviors (the things it can do or methods or features). For example, the class Dog would consist of traits shared by all dogs, such as breed and fur color (characteristics), and the ability to bark (behavior). 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 and behaviors 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.The object consists of state and the behaviour that's defined in the object's class.
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."[3] Also known to some programming languages as interfacing
Inheritance
"Subclasses" are more specialized versions of a class, which inherit attributes and behaviors from their parent classes, and can introduce their own.
For example, the class Dog might have sub-classes called Collie, Chihuahua, and GoldenRetriever. In this case, Lassie would be an instance of the Collie subclass. Suppose the Dog class defines a method called bark() and a property called furColor. Each of its sub-classes (Collie, Chihuahua, and GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once.
Each subclass can alter its inherited traits. For example, the Collie class might specify that the default furColor for a collie is brown-and-white. The Chihuahua subclass might specify that the bark() method produces a high-pitched by default. Subclasses can also add new members. The Chihuahua subclass could add a method called tremble(). So an individual chihuahua instance would use a high-pitched bark() from the Chihuahua subclass, which in turn inherited the usual bark() from Dog. The chihuahua object would also have the tremble() method, but Lassie would not, because she is a Collie, not a Chihuahua. In fact, inheritance is an "is-a" relationship: Lassie is a Collie. A Collie is a Dog. Thus, Lassie inherits the members of both Collies and Dogs.
Multiple inheritance is inheritance from more than one ancestor class, neither of these ancestors being an ancestor of the other. For example, independent classes could define Dogs and Cats, and a Chimera object 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.
Encapsulation
Encapsulation conceals the functional details of a class from objects that send messages to it.
For example, the Dog class has a bark() method. The code for the bark() method defines exactly how a bark happens (e.g., by inhale() and then 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 Dog by 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++ allows one to specify which classes may access any member.
Abstraction
Abstraction is simplifying complex reality by modeling classes appropriate to the problem, and working at the most appropriate level of inheritance for a given aspect of the problem.
For example, Lassie the Dog may be treated as a Dog much of the time, a Collie when necessary to access Collie-specific attributes or behaviors, and as an Animal (perhaps the parent class of Dog) when counting Timmy's pets.
Abstraction is also achieved through Composition. For example, a class Car would be made up of an Engine, Gearbox, Steering objects, and many more components. To build the Car class, 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.
Polymorphism
Polymorphism allows you to treat derived class members just like their parent class's members. More precisely, Polymorphism in object-oriented programming is the ability of objects belonging to different data types to respond to method calls of methods of the same name, each one according to an appropriate type-specific behavior. One method, or an operator such as +, -, or *, can be abstractly applied in many different situations. If a Dog is commanded to speak(), this may elicit a Bark. However, if a Pig is commanded to speak(), this may elicit an Oink. They both inherit speak() from 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 Integer and Double, are expected to add together properly in an OOP language. The language must therefore overload the concatenation 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 [4].

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, although there are no objects in these languages.

History

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.[5] 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[citation needed] during the mid-1990s,[citation needed] largely due to the influence of C++[citation needed]. Its dominance [citation needed]was further cemented by the rising popularity of graphical user interfaces[citation needed], 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.[citation needed] 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[6] 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.[citation needed]

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).

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, and ColdFusion since version 6.

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 which employs cloning from prototypes rather than inheriting from a class.


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 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.

Gang of Four design patterns

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 2005, the book was in its 32nd printing. While it can make for dense reading, even for experienced programmers, and has been superseded in practice by a spate of more recent, more accessible books, it is regarded as an important source not only for design patterns but also for the object-oriented design guidelines in its initial chapter.

Object-orientation and databases

Main articles: 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 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 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[7] 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".

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:

Attempts to find a consensus definition or theory behind objects have not proven very successful (however, see "Abadi & Cardelli: A Theory of Objects"[8] for formal definitions of many OO 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").

Criticism

  • Richard Stallman wrote in 1995, "Adding OOP to Emacs is not clearly an improvement; I used OOP when working on the Lisp Machine window systems, and I disagree with the usual view that it is a superior way to program."[1]
  • A study by Potok et al. [2] has shown no significant difference in productivity between OOP and procedural approaches.
  • Richard Mansfield wrote a controversial, widely discussed critique of OOP, asserting that "Even after years of OOP, many—perhaps most—people still don't get it. One has to suspect that we're dealing with the emperor's new clothes when OOP apologists keep making the same excuses over and over..." [3].
  • Christopher J. Date stated that critical comparison of OOP to other technologies, relational in particular, is difficult because of lack of an agreed-upon and rigorous definition of OOP.[9]
  • Alexander Stepanov suggested that OOP provides a mathematically-limited viewpoint and called it, "almost as much of a hoax as Artificial Intelligence" (possibly referring to the Artificial Intelligence projects and marketing of the 1980s that are sometimes viewed as overzealous in retrospect) [4].
  • Edsger W. Dijkstra wrote:

... what society overwhelmingly asks for is snake oil. Of course, the snake oil has the most impressive names —otherwise you would be selling nothing— like "Structured Analysis and Design", "Software Engineering", "Maturity Models", "Management Information Systems", "Integrated Project Support Environments" "Object Orientation" and "Business Process Re-engineering" (the latter three being known as IPSE, OO and BPR, respectively)." — EWD 1175: The strengths of the academic enterprise

See also

References

  1. ^ Meyer, chapter 3
  2. ^ Booch, chapter 2
  3. ^ a b Armstrong, "The Quarks of Object-Oriented Development." In descending order of popularity, the "quarks" are: Inheritance, Object, Class, Encapsulation, Method, Message Passing, Polymorphism, Abstraction
  4. ^ B. Stroustrup, The C++ Programming Language, 3rd-ed., p. 158
  5. ^ Kay, Alan. "The Early History of Smalltalk". Retrieved 2007-09-13.
  6. ^ C++ Applications, by Bjarne Stroustrup, the author of C++
  7. ^ Meyer, Second Edition, p. 230
  8. ^ A Theory of Objects, Martin Abadi and Luca Cardelli
  9. ^ C. J. Date, Introduction to Database Systems, 6th-ed., Page 650

Further reading

The Wikibook [[wikibooks:|]] has a page on the topic of: Computer programming/Object oriented programming

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