In computer science, object-oriented programming is a computer programming paradigm. Many programming languages support object-oriented programming (ref). Many programming frameworks, like the Java platform and the .NET Framework, are built on object-oriented principles. Object-oriented programming is often abbreviated as OOP.

Object-oriented programming was born at the end of 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]

The idea behind object-oriented programming is that a computer program may be seen as comprising a collection of individual units, or objects, that act on each other, 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. 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 or actor with a distinct role or responsibility.

Object-oriented programming is claimed to promote greater flexibility and maintainability in programming, and is widely popular in large-scale software engineering. Furthermore, proponents of OOP claim that OOP is easier to learn for those new to computer programming than previous approaches, and that the OOP approach is often simpler to develop and to maintain, lending itself to more direct analysis, coding, and understanding of complex situations and procedures than other programming methods. Critics dispute this, at least for some domains (industries).

Contents 1 Fundamental concepts 2 History 2.1 OOP in scripting 3 Problems and patterns 3.1 Gang of Four design patterns 3.2 Object-orientation and databases 3.3 Common mistakes 3.4 Matching real world 4 OOP as a new paradigm, point of view, and marketing term 4.1 Actor model 4.2 OOP with procedural languages 4.3 Static typing with the object-oriented paradigm 4.4 Prototype-based model 4.5 Object-based model 4.6 Multimethod model 5 Formal definition 6 Evidence and Criticism 7 See also 8 Further reading 9 Notes 10 References 11 External links 11.1 Criticism

Fundamental concepts

There is not a single, widely-agreed upon definition of object-oriented programming. However, a survey of nearly 40 years of computing literature by Deborah J. Armstrong^[2] 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 'Dogs' 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 Dogs 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."^[2]

• Inheritance — In some cases, a class will have "subclasses," more specialized versions of a class. For example, the class Dogs might have sub-classes called Collies, Chihuahuas, and Golden Retrievers. 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 Dogs class defines a method called "bark" and a property called "fur color." Each of its sub-classes (Collies, Chihuahuas, and Golden Retrievers) 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 Collies class might specify that the default furColor for a collie is brown-and-white. The Chihuahuas subclass might specify that the "bark" method is high-pitched by default. Subclasses can also add new members. The Chihuahuas subclass could add a method called "tremble." So an individual chihuahua instance would use a high-pitched "bark" from the Chihuahuas subclass, which in turn inherited the usual "bark" from Dogs. 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 Dogs class has a bark() method. The code for the bark() method defines exactly how a bark happens (e.g., by inhaling() and then exhaling(), 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. 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 Dog is commanded to speak this may elicit a Bark; if 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.

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 objects could be created, modified, and 'consumed' "on the fly" rather than having a system based on static objects. It also introduced the notion of 'inheritance.' (Thus, Smalltalk was clearly a major move beyond the analog programming models, which made no use of "instances of classes," or even Simula, which made no use of the "inheritance property.")

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 allegedly 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, 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).

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

Main article: Design Patterns

Design Patterns: Elements of Reusable Object-Oriented Software is a very influential book written by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, sometimes casually called the "Gang of Four." It presents 23 common programming problems along with templates for solving them.

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.

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

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.

1. Confusion between classes and roles ^[3]
2. Confusion between data types and interfaces
3. Confusion between levels of abstraction
4. Confusion between application and platform
5. Confusion between policy and mechanism
6. Confusion between "uses" and "is-allowed-to-use" relationships
7. Confusion between run-time and compile-time
8. Confusion between function application and message passing
9. Confusion between aggregation and inheritance^[4]
10. 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.
11. Confusion between analysis concepts and design classes
12. Confusion between interfaces and implementations ^[5]
13. Confusion between levels of detail

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^[6] 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".

OOP as a new paradigm, point of view, and marketing term

OOP is subject to much contention as to its precise definition and its principal ideas.

In the most general terms, OOP is the practice of writing program text that is decomposed into modules that encapsulate the representation of one data type per module, instead of into collections of functions that call each other, or clauses that trigger each other. OOP concepts and practices have been brought together, with associated terminology, to create a new programming framework. Together, the ideas behind OOP are said to be so powerful that they create a paradigm shift in programming. (Other programming paradigms, such as functional and procedural programming, focus primarily on actions -- or, in logical programming, on assertions -- that trigger execution of program code.)

OOP arose independently out of research into simulation system oriented languages, with SIMULA 67, and out of research into highly secure system architectures, with capability-based OS and CPU architectures.

Some experts say that the original definition of object-orientation came from the object in grammar. The requirements for software are always subject-oriented. However, since the requirements for the subject are often complicated, subject-oriented programs often become complicated and monolithic. Therefore, as an alternative, some researchers started thinking in an object-oriented way. This represented a paradigm shift from the usual or previous subject-oriented mode of thinking.

According to object-oriented principles, the verb in a program statement is always attached to the object, and the logic associated with a requirement is likewise handled in the object. The following are some examples of the ways by which a subject-oriented requirement is translated into object-oriented thinking:

• Subject-oriented: The Sales Application saves the Transaction
• Object-oriented: The Transaction saves itself upon receiving a message from the Sales Application

• Subject-oriented: The Sales Application prints the Receipt
• Object-oriented: The Receipt prints itself upon receiving a message from the Sales Application

One distinguishing feature of OOP is the handling of subtypes of data types.

Objects' data are generally required to satisfy programmer-defined constraints (i.e., class invariants). A datatype restricted by such a constraint constitutes a subtype of the same datatype without the constraint. These constraints are then both relied upon and preserved by the actions (methods) that are defined for the data.

Data constraints may be either explicitly declared or implicitly assumed by the programmer. In either case, object-oriented languages provide mechanisms for ensuring that such assumptions or constraints remain local to one part of the program. Constraints on and assumptions about data are usually included in the documentation of object-oriented programs.

OOP itself has been used to market many products and services, and the actual definitions and benefits attributed to OOP have often been colored by commercial marketing goals. Similarly, many programming languages reflect a specific view or philosophy of OOP that is narrower and, in certain respects, less general than that embodied in the more general or standard definition.

As noted above, at the end of the previous section, widely-used terminology distinguishes object-oriented programming from object-based programming. The former is held to include inheritance (described below), while the latter does not.

The exact definitions of some of these terms show some variation, depending on point of view. In particular, languages with static typing often reflect and embody slightly different views of OO from those reflected by and embodied in languages with dynamic typing, due to a focus on the compile-time rather than the run-time properties of programs.

Note: Abstraction is important, but not unique, to OOP. Other programming paradigms employ it as well.

Reusability is the benefit most often claimed for OOP. However, opponents argue that reuse of software is as old as the invention of the subroutine, reputedly prior to 1950. In fact, reuse is disputed by opponents as being a primary, or even a large, benefit. The ease of translation to and from the target environment, the (improved) ability to maintain a program once written, the ability to do localized debugging, and the (improved) ability to do much larger parallel development efforts, are all cited as more significant reasons to use an OOP language. Supporters of OO methods believe such doubts result from misunderstanding of the method and misuse of OO in software where reusability is not required. The reusability requirement is often offered as reason for use of various design practices associated to OOP, such as strong cohesion and low coupling of classes, orthogonality of interfaces in APIs, or primitiveness of methods.

OOP is often called a paradigm rather than a style or type of programming, to emphasize the point that OOP can change the way software is developed by actually changing the way in which programmers and software engineers think about software. As a paradigm, OOP is about overall system design as much as it is about programming. A system is designed by defining the objects that will exist and interact within the system. Due to encapsulation, the code that actually does the work is irrelevant to an object, and to the people using the object. The challenge in OOP, therefore, is of designing a sane object system.

There are distinct parallels between the object-oriented paradigm and systems theory. OOP focuses on objects as units in a system, whereas systems theory focuses on the system itself. In between, one may find software design patterns or other techniques that use classes and objects as building blocks for larger components. Such components can be seen as an intermediate step from the object-oriented paradigm towards the more "real-life oriented" models of systems theory.

Actor model

OOP is a decomposition paradigm for program code, not a model for computation.

OOP is often confused with the Actor model of computation. In response to a message that it receives, an Actor can make local decisions, create more Actors, send more messages, and determine how to respond to the next message received.

Almost all OOP languages and systems, including all the major ones, such as SIMULA, Smalltalk, Eiffel, C++, Java, Ruby, Python, Delphi, VB .NET, and C#, have message passing programming capabilities.

See Actor model implementations for a discussion on implementations of the Actor model.

In OOP the emphasis is not on how computation is organized, but on how program text is decomposed into modules, because it is this decomposition that matters as to the program text's comprehensibility and maintainability.

OOP is based on the assumption that the program text's comprehensibility and maintainability are improved by decomposing the text into modules, and that the best way to decompose it into modules is to minimize dependencies among modules and maximize the cohesion of functions inside each module, and that this is best achieved by encapsulating the representation of a data type in each module.

There are several distinct styles of object-oriented programming. The distinctions between different styles occur because different programming languages emphasize different aspects of object-oriented facilities and combine with other constructs in different ways.

OOP with procedural languages

In procedural languages, OOP often appears as a form where data types are extended to behave like a type of an object in OOP, very similar to an abstract data type with an extension such as inheritance. Each method is actually a subprogram which is syntactically bound to a class.

Static typing with the object-oriented paradigm

Many object-oriented programming languages, such as C++ and Java, have a static type system that can be used to check and enforce constraints of object-oriented design to some extent at compile-time, i.e. statically. Object-oriented facilities combine with static typing in various ways. Classes are types of objects. Many object-oriented languages provide mechanisms for statically checking the type of method parameters, types of private and public data members, types of object references and check the correctness of inheritance and subtyping relationships. Static type checking can also check API compatibility, enforce data constraints on the users of libraries created with object-oriented methods and reduce the number of type checks performed at run-time for various forms of method dispatch.

Some object-oriented languages, such as Eiffel, supplement the type system with assertions specifying and documenting invariants of classes and the contracts of methods, though current Eiffel compilers only check these at run-time, i.e. dynamically.

See Class-based OOP.

Prototype-based model

Other than using classes, prototyping is another, less popular, means of achieving object-oriented behavior sharing. After an object is defined, another similar object will be defined by referring to the original one as a template, then listing the new object's differences from the original. Perhaps the most popular prototype-based language is JavaScript, which is an implementation of ECMAScript. In prototyping systems, objects themselves are the templates, while classification systems use classes as templates for objects.

The classification approach is so predominant in OOP that many people would define objects as encapsulations that share data by classification and inheritance. However, the more generic term "behavior sharing" acknowledges alternate techniques such as prototyping.

See Prototype-based programming.

Object-based model

Object-based programming is centered around the creation of objects and their interactions, but may not have some of the key features of the class-based object-oriented paradigm such as inheritance. Such object-based systems are usually not regarded as object-oriented, because inheritance (viewing delegation as a form of inheritance) is typically identified as the core feature of OOP.

Multimethod model

In this model, the "receiver" argument to a message is not given special status in message dispatch. Instead, the runtime values of all arguments to message are consulted to determine which method should be executed at runtime. This is related to double or multimethod dispatch.

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

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").

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[1], 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 exasperated by the fact that different developers may process thoughts differently such 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 [2] applications that are publicly available for analysis. In this view, anecdotes are not reliable evidence.

See also

• Object-oriented programming language
• Aspect-oriented programming
• Software componentry
• Interface description language
• List of object-oriented programming terms
• Refactoring
• CORBA
• DCOM
• Object-relational mapping
• Object-Relational impedance mismatch
• Object database
• Object-oriented image classification

Further reading

• ; Luca Cardelli. A Theory of Objects. Springer-Verlag. ISBN 0-387-94775-2.
• Abelson, Harold; Gerald Jay Sussman, . Structure and Interpretation of Computer Programs. The MIT Press. ISBN 0-262-01153-0.
• Booch, Grady. Object-Oriented Analysis and Design with Applications. Addison-Wesley. ISBN 0-8053-5340-2.
• ; . Building Business Objects. John Wiley & Sons. ISBN 0-471-19176-0.
• Gamma, Erich; Richard Helm, Ralph Johnson, John Vlissides. Design Patterns: Elements of Reusable Object Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
• Harmon, Paul; . The Object Technology Casebook - Lessons from Award-Winning Business Applications. John Wiley & Sons. ISBN 0-471-14717-6.
• Jacobson, Ivar. Object-Oriented Software Engineering: A Use Case-Driven Approach. Addison-Wesley. ISBN 0-201-54435-0.
• Kay, Alan. The Early History of Smalltalk.
• Rumbaugh, James; , , , . Object-Oriented Modeling and Design. Prentice Hall. ISBN 0-13-629841-9.
• . Object-Oriented Information Systems - Planning and Implementation. John Wiley & Sons. ISBN 0-471-54364-0.

Notes

1. ^ Meyer, chapter 3
2. ^ ^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
3. ^ On the representation of roles in Object-oriented and conceptual modeling
4. ^ Meyers: "Effective C++", second edition, Item 40
5. ^ Meyers: "Effective C++", second edition, Item 36
6. ^ Meyer, Second Edition, p. 230

References

• Armstrong, Deborah J. (February 2006). "The Quarks of Object-Oriented Development". Communications of the ACM 49 (2): 123-128. ISSN 0001-0782. Retrieved on 2006-08-08.

• Meyer, Bertrand (1997). Object-Oriented Software Construction. Prentice Hall. ISBN 0-13-629155-4.

External links

• Object-oriented programming FAQ
• Example of the subtyping problem
• What are OOP's Jargons and Complexities
• The report "Object Orientation Redefined" outlines how OO has been considered a thinking tool, i.e. "we view the world in objects", and how such an approach may further our understanding of object-oriented formalisation.
• Modeling Roles : A practical series of Analysis Patterns How to easily identify a Role problem and precisely determine which Role analysis pattern is best to model it.

Criticism

• Objects Have Failed
• Arguments Against OOP (C2 wiki discussion)
• OOP Better in Theory than in Practice
• Why Paul Graham is Creating a New Language Not Especially Object-Oriented
• OOP Oversold - Large website devoted to cataloging OOP criticism