Object-oriented programming

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Object-oriented programming (POOP) is a programming paradigm that uses abstraction to create models based on the real world. It utilizes several techniques from previously established paradigms, including modularity, polymorphism, and encapsulation. Even though it originated in the 1960s, Andi Ruth's POOP was not commonly used in mainstream software application development until the 1990s. Today, many popular programming languages (such as Java, JavaScript, C#, C++, Python) support OOP.

Object-oriented programming's roots reach all the way back to the creation of the Simula programming language in 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 in part addresses this problem by strongly emphasizing modularity in software.[1]

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 collection of functions, or simply as a list of instructions to the computer. In POOP, 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]

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.

Contents

[edit] Fundamental concepts

A survey of nearly 40 years of computing literature by Deborah J. Armstrong[3] 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, 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."[3]
  • Inheritance — In some cases, a class will have "subclasses," more specialized versions of a class. 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. Subclasses inherit attributes and behaviors from their parent classes, and can introduce their own. 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. So, 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 is 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. When an object or class inherits its traits from more than one ancestor class, it's called multiple inheritance. This is not always supported, as it can be hard both to implement and to use well.
  • Encapsulation — conceals the exact details of how a particular class works from objects that use its code or send messages to it. So, 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 and private, determining whether they are available to all classes, sub-classes or only the defining class. Some languages go further: Java uses the protected keyword 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 allows one to specify which classes may access any member.
  • Abstraction — 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.
  • Polymorphism — polymorphism is behavior that varies depending on the class in which the behavior is invoked, that is, two or more classes can react differently to the same message. For example, if a Dog is commanded to speak() this may elicit a Bark; if a Pig is commanded to speak() this may elicit an Oink.

A so-called object-based language is a language that has most of the properties of an object-oriented language, but may lack some. For example Visual Basic lacks implementation inheritance, while a Prototype-based programming language relies on prototypes instead of classes to create objects.

[edit] 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 1960's 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 1970's, 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. 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-1980s, largely due to the influence of C++, an extension of the C programming language. 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 [4] include the Doom III engine, Starcraft, Diablo, Warcraft III and World of Warcraft. 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. "Pure" object-oriented languages, on the other hand, lacked features that many programmers had come to depend upon. To bridge this gap, many attempts have been made to create new languages based on object-oriented methods but allowing some procedural features in "safe" ways. Bertrand Meyer's Eiffel was an early and moderately successful language with those goals.

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).

[edit] 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.

[edit] 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.

[edit] 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 2003, the book was in its 26th printing. While it can make for dense reading, even for experienced programmers, and has been superseded in practice by a spate of more recent, accessibly-written 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.

[edit] 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, Ruby on Rails and the Django (web framework).

There are also object databases which can be used to replace RDBMSs, but these have not been as commercially successful as RDBMSs.

[edit] Common mistakes

There are several common mistakes which programmers can make in object oriented programming. For example, checking the type of an object rather than its membership is a common pitfall, or antipattern, that counteracts the benefits of inheritance and polymorphism.

  • Confusion between classes and roles [5]
=> Unnecessary complexity due to mismatching concepts
  • Confusion between data types and interfaces
=> Performance problems due to excessive communication
  • Confusion between levels of abstraction
=> performance and correctness problems
  • Confusion between application and platform
=> Duplication of work or trouble satisfying resource limits
  • Confusion between policy and mechanism
=> inability to reuse code
  • Confusion between "uses" and "is-allowed-to-use" relationships
=> performance problems
  • Confusion between run-time and compile-time
=> performance problems
  • Confusion between function application and message passing
=> excessive context-switch overhead or trouble utilizing concurrency
  • Confusion between aggregation and inheritance[6]
=> developers cannot understand the design
  • Overuse of inheritance when composition or aggregation may be more appropriate. It has been suggested that training materials over-emphasizes inheritance, perhaps because it is easier to illustrate than the alternatives.
  • Confusion between analysis concepts and design classes
=> implementation and requirements do not coincide
  • Confusion between interfaces and implementations [7]
=> Inability to adapt to change without rewrite
  • Confusion between levels of detail

[edit] 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[8] 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".

[edit] 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:

  • 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").

[edit] Evidence and criticism

Much of the criticism of OOP comes from proponents of relational databases or relational-like techniques such as set theory, functional programming[9], and logical programming. OOP has been accused of resulting in navigational structures, which are allegedly more difficult to manage and change than their alternatives because they are shaped by initial usage rather than universal facts about domain objects or entities.

Reliable research supporting or dismissing OOP has been difficult to come by. Part of the problem with testing is disagreement about whether OOP's benefits are objective or psychological in nature. If it is the latter, then methodologies for testing the psychology of paradigms is too nascent a field to be reliable yet, and is exacerbated by the fact that different developers may process thoughts differently, so that any research may not be universal to all human minds. Further, what is useful in one domain (industry) may not be in another.

Some have called for proof of concept[10] applications that are publicly available for analysis. In this view, anecdotes are not reliable evidence.

[edit] Quotations

  • What "object-oriented programming" means to Alan Kay, who coined the term in 1967:
"OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them."[11]
"I think that object orientedness is almost as much of a hoax as Artificial Intelligence."[12]
  • "A day without objects is a day without job security." (Original author unknown)

[edit] See also

[edit] Further reading

Wikibooks
Wikiversity
At Wikiversity you can learn about Object-oriented programming at:


[edit] Notes

  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. ^ C++ Applications, by Bjarne Stroustrup, the author of C++
  5. ^ On the representation of roles in Object-oriented and conceptual modeling
  6. ^ Meyers: "Effective C++", second edition, Item 40
  7. ^ Meyers: "Effective C++", second edition, Item 36
  8. ^ Meyer, Second Edition, p. 230
  9. ^ Paul GRAHAM.
  10. ^ DevX.
  11. ^ Kay.
  12. ^ Stepanov.

[edit] References

[edit] External links

[edit] Criticism

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