Object-oriented programming (OOP) is a programming paradigm that uses "objects" – data structures consisting of datafields and methods – 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 has roots that can be traced 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, and discrete event simulation) as a superset of Algol. Simula also used automatic garbage collection which had been invented earlier for the functional programming language Lisp. 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. Experimentation with various extensions to Lisp (like LOOPS and Flavors introducing multiple inheritance and mixins), eventually led to the Common Lisp Object System (CLOS, a part of the first standardized object-oriented programming language, ANSI Common Lisp), which integrates functional programming and object-oriented programming and allows extension via a Meta-object protocol. 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 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.
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.
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).
The term OOPS, which refers to an object-oriented programming system, was common in early development of object-oriented programming.
 Fundamental concepts and features
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:
- 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, operations or features). One might say that a class is a blueprint or factory that describes the nature of something. For example, the class
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.
- A pattern (exemplar) of a class. The class of
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.
- One can have an instance of a class or a particular object. The instance is the actual object created at runtime. In programmer jargon, the
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.
- An object's abilities. In language, methods (sometimes referred to as "functions") are verbs.
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.
- Message passing
- "The process by which an object sends data to another object or asks the other object to invoke a method."  Also known to some programming languages as interfacing. For example, the object called
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.
- "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
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.
- Each subclass can alter its inherited traits. For example, the
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
- 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
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.
- 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,
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.
Abstraction is also achieved through Composition. For example, a class
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.
- Encapsulation conceals the functional details of a class from objects that send messages to it.
- For example, the
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 the 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.
- (Subtype) polymorphism
- Polymorphism allows the programmer to treat derived class members just like their parent class' 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
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[dubious ] that can be used with many different types of objects.
- Decoupling allows for the separation of object interactions from classes and inheritance into distinct layers of abstraction. A common use of decoupling is to polymorphically decouple the encapsulation, which is the practice of using reusable code to prevent discrete code modules from interacting with each other. However, in practice decoupling often involves trade-offs with regard to which patterns of change to favor. The science of measuring these trade-offs in respect to actual change in an objective way is still in its infancy.
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.
 Main features
The attempt to distill OOP to a minimal set of features is considered futile by programming language researcher Benjamin C. Pierce. Nevertheless, he identifies the following as fundamental features that are found in most object-oriented languages and that, in concert, support the OOP programming style:
- Dynamic dispatch -- when a method is invoked on an object, the object itself determines what code gets executed by looking up the method at run time in a table associated with the object. This feature distinguishes an object from an abstract data type (or module), which has a fixed (static) implementation of the operations for all instances
- Encapsulation (or multi-methods, in which case the state is kept separate)
- Subtype polymorphism
- Class inheritance (or delegation)
- Open recursion -- a special variable (syntactically it may be a keyword), usually called
self, that allows a method body to invoke another method body of the same object. This variable is late-bound; it allows a method defined in one class to invoke another method that is defined later, in some subclass thereof.
Similarly, in his 2003 book, Concepts in programming languages, John C. Mitchell identifies four main features: dynamic dispatch, abstraction, subtype polymorphism, and inheritance. Michael Lee Scott in Programming Language Pragmatics considers only encapsulation, inheritance and dynamic 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[dubious ]
- abstract data types (which have existential types) allow the definition of modules but these do not support dynamic dispatch
- recursive types
- records are basis for understanding objects if function literals can be stored in fields (like in functional programming languages), but the actual calculi need be considerably more complex to incorporate essential features of OOP. Several extensions of System F<: that deal with mutable objects have been studied; these allow both subtype polymorphism and parametric polymorphism (generics)
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").
 OOP languages
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Simula (1967) is generally accepted as the first language to have the primary features of an object-oriented language. It was created for making simulation programs, in which what came to be called objects were the most important information representation. Smalltalk (1972 to 1980) is arguably the canonical example, and the one with which much of the theory of object-oriented programming was developed.
- Languages called "pure" OO languages, because everything in them is treated consistently as an object, from primitives such as characters and punctuation, all the way up to whole classes, prototypes, blocks, modules, etc. They were designed specifically to facilitate, even enforce, OO methods. Examples: Smalltalk, Eiffel, Ruby, JADE
- Languages designed mainly for OO programming, but with some procedural elements. Examples: C++, Java, Python.
- Languages that are historically procedural languages, but have been extended with some OO features. Examples: Fortran 2003, Perl, COBOL 2002.
- Languages with most of the features of objects (classes, methods, inheritance, reusability), but in a distinctly original form. Examples: Oberon (Oberon-1 or Oberon-2).
- Languages with abstract data type support, but not all features of object-orientation, sometimes called object-based languages. Examples: Modula-2 (with excellent encapsulation and information hiding), Pliant, CLU.
 OOP in scripting
In recent years, object-oriented programming has become especially popular in scripting programming languages. Python, Ruby and Groovy 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.
 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.
 Inheritance and behavioral subtyping
It is intuitive to assume that inheritance creates a semantic "is a" relationship, and thus to infer that objects instantiated from subclasses can always be safely used instead of those instantiated from the superclass. This intuition is unfortunately false in most OOP languages, in particular in all those that allow mutable objects. Subtype polymorphism as enforced by the type checker in OOP languages (with mutable objects) cannot guarantee behavioral subtyping in a any context. Behavioral subtyping is undecidable in general, so it cannot be implemented by a program (compiler). Class or object hierarchies need to be carefully designed considering possible incorrect uses that cannot be detected syntactically. This issue is known as the Liskov substitution principle.
 Gang of Four 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:
- Creational patterns (5): Factory Pattern, Abstract Factory Pattern, Singleton Pattern, Builder Pattern, Prototype Pattern
- Structural patterns (7): Adapter Pattern, Bridge Pattern, Composite Pattern, Decorator Pattern, Facade Pattern, Flyweight Pattern, Proxy Pattern
- Behavioral patterns (11): Chain of Responsibility Pattern, Command Pattern, Interpreter Pattern, Iterator Pattern, Mediator Pattern, Memento Pattern, Observer Pattern, State pattern, Strategy Pattern, Template Pattern, Visitor Pattern
 Object-orientation and databases
Both object-oriented programming and relational database management systems (RDBMSs) are extremely common in software today[update]. 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  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."
A number of well-known researchers and programmers have criticized OOP. Here is a necessarily incomplete list:
- Luca Cardelli wrote a paper titled 'Bad Engineering Properties of Object-Oriented Languages'.
- 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."
- A study by Potok et al.  has shown no significant difference in productivity between OOP and procedural approaches.
- 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.. In , a theoretical foundation on OOP is proposed.
- 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).
- Paul Graham has suggested that the purpose of OOP is to act as a "herding mechanism" which keeps mediocre programmers in mediocre organizations from "doing too much damage". This is at the expense of slowing down productive programmers who know how to use more powerful and more compact techniques. 
- Joe Armstrong, the principal inventor of Erlang, is quoted as saying "The problem with object-oriented languages is they’ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle." 
 See also
- Aspect-oriented programming
- Circle-ellipse problem
- Constructor overloading
- Design by contract
- Dot notation
- Interface description language
- Lepus3 an Object-Oriented design description language
- Object association
- Object database
- Object-oriented analysis and design
- Object-relational impedance mismatch (and The Third Manifesto)
- Object-relational mapping
- Procedural programming
- Software componentry
- ↑ Kay, Alan. "The Early History of Smalltalk". http://gagne.homedns.org/~tgagne/contrib/EarlyHistoryST.html. Retrieved 2007-09-13.
- ↑ 2.0 2.1 Armstrong, The Quarks of Object-Oriented Development. In descending order of popularity, the "quarks" are: Inheritance, Object, Class, Encapsulation, Method, Message Passing, Polymorphism, Abstraction
- ↑ B. Stroustrup, The C++ Programming Language, 3rd-ed., p. 158
- ↑ Pierce, Benjamin (2002). Types and Programming Languages. MIT Press. ISBN 0-262-16209-1. , section 18.1 "What is Object-Oriented Programming?"
- ↑ John C. Mitchell, Concepts in programming languages, Cambridge University Press, 2003, ISBN 0521780985, p.278
- ↑ Michael Lee Scott, Programming language pragmatics, Edition 2, Morgan Kaufmann, 2006, ISBN 0126339511, p. 470
- ↑ 7.0 7.1 A Theory of Objects, Martin Abadi and Luca Cardelli
- ↑ Meyer, Second Edition, p. 230
- ↑ M.Trofimov, OOOP - The Third "O" Solution: Open OOP. First Class, OMG, 1993, Vol. 3, issue 3, p.14.
- ↑ "Mode inheritance, cloning, hooks & OOP (Google Groups Discussion)". http://groups.google.com/group/comp.emacs.xemacs/browse_thread/thread/d0af257a2837640c/37f251537fafbb03?lnk=st&q=%22Richard+Stallman%22+oop&rnum=5&hl=en#37f251537fafbb03.
- ↑ http://www.csm.ornl.gov/~v8q/Homepage/Papers%20Old/spetep-%20printable.pdf
- ↑ C. J. Date, Introduction to Database Systems, 6th-ed., Page 650
- ↑ C. J. Date, Hugh Darwen, Foundation for Future Database Systems: The Third Manifesto (2nd Edition)
- ↑ The AI Effect
- ↑ STLport: An Interview with A. Stepanov
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 Further reading
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- Schach, Stephen (2006). Object-Oriented and Classical Software Engineering, Seventh Edition. McGraw-Hill. ISBN 0-073-19126-4.
- Abadi, Martin; Luca Cardelli (1998). A Theory of Objects. Springer-Verlag. ISBN 0-387-94775-2.
- Abelson, Harold; Gerald Jay Sussman, (1997). Structure and Interpretation of Computer Programs. The MIT Press. ISBN 0-262-01153-0.
- Armstrong, Deborah J. (February 2006). "The Quarks of Object-Oriented Development". Communications of the ACM 49 (2): 123–128. doi:10.1145/1113034.1113040. ISSN 0001-0782. http://portal.acm.org/citation.cfm?id=1113040. Retrieved 2006-08-08.
- Booch, Grady (1997). Object-Oriented Analysis and Design with Applications. Addison-Wesley. ISBN 0-8053-5340-2.
- Eeles, Peter; Oliver Sims (1998). Building Business Objects. John Wiley & Sons. ISBN 0-471-19176-0.
- Gamma, Erich; Richard Helm, Ralph Johnson, John Vlissides (1995). Design Patterns: Elements of Reusable Object Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
- Harmon, Paul; William Morrissey (1996). The Object Technology Casebook - Lessons from Award-Winning Business Applications. John Wiley & Sons. ISBN 0-471-14717-6.
- Jacobson, Ivar (1992). Object-Oriented Software Engineering: A Use Case-Driven Approach. Addison-Wesley. ISBN 0-201-54435-0.
- Kay, Alan. The Early History of Smalltalk. http://gagne.homedns.org/%7etgagne/contrib/EarlyHistoryST.html.
- Meyer, Bertrand (1997). Object-Oriented Software Construction. Prentice Hall. ISBN 0-13-629155-4.
- Rumbaugh, James; Michael Blaha, William Premerlani, Frederick Eddy, William Lorensen (1991). Object-Oriented Modeling and Design. Prentice Hall. ISBN 0-13-629841-9.
- Taylor, David A. (1992). Object-Oriented Information Systems - Planning and Implementation. John Wiley & Sons. ISBN 0-471-54364-0.
- Schreiner, Axel-Tobias (1993). Object oriented programming with ANSI-C. Hanser. ISBN 3-446-17426-5. http://hdl.handle.net/1850/8544.
- Programming Styles: Procedural, OOP, and AOP
- What is OOP? (Tutorial on OOP in PHP)
- Discussion about the flaws of OOD