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Object-oriented programming (OOP) is a computer programming paradigm that emphasizes the following aspects:
It is in fact a set of ideas which mostly existed before. They have been brought together, with associated terminology, to create a programming framework. Together the ideas behind OO are said to be so powerful they create a Paradigm shift in programming.
The exact definitions of these have some variation depending on point of view.
Notes: Abstraction is important to but not unique to OOP. Reusability is a benefit often attributed to OOP.
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 changing the way that programmers and software engineers think about software.
The fundamental principle of object-oriented programming is that a computer program is composed of a collection of individual units, or objects which can function like sub-programs. To make the overall computation happen, each object is capable of receiving messages, processing data, and sending messages to other objects. In short, the objects can interact through their own functions (or methods) and their own data.
In this way, messages can be handled, as appropriate, by one chunk of code or by many in a seamless way. It is claimed that this gives more flexibility over simple step-by-step programming, called imperative programming or structured programming in the field of computer science.
Proponents of OOP also claim that OOP is more intuitive and that it is easier to learn, for those new to computer programming, than previous approaches. In OOP, objects are simple, self contained and easily identifiable. This modularity allows the program parts to correspond to real aspects of the problem and thereby to model the real world. Object-oriented programming often begins from a written statement of the problem situation. Then by a process of inserting objects or variables for nouns, methods for verbs and attributes for adjectives, a good start is made on a framework for a program that models, and deals with, that situation. This allows one to learn how to program in object-oriented languages.
However, it is recognized that OOP does not necessarily mean lack of complexity. Meta class programming (see Meta class) for example is a demanding skill, and OOP programs can have a complex web of shared or distinct responsibilities, attributes and methods. It can be challenging to distribute responsibility over objects, or classes—one of many popular implementation schemes.
Despite this, there is general recognition that the OOP approach is often simpler to develop and to maintain, lending itself to more intuitive analysis, coding, and understanding of complex situations and procedures.
Objective verification of such benefits beyond narrow niches is elusive. It is often heavily debated whether the benefits of OOP are based on universal truths or based on a fit to human psychology. "Would aliens prefer OOP?" is sometimes used as a thought-provoking question.
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.
The definitions of OOP are disputed. In the most general sense, object-oriented programming refers to the practice of viewing software primarily in terms of the "things" (objects) it manipulates, rather than the actions it performs. Other paradigms such as functional and procedural programming focus primarily on the actions, with the objects being secondary considerations; in OOP, the situation is reversed.
One distinguishing feature of OOP is the handling of subtypes of data types. Objects' data is generally required to satisfy programmer-defined constraints (e.g. class invariants). These constraints are then both relied on and preserved by the actions (methods) defined for the data. These constraints may either be explicitly declared or implicitly assumed by the programmer. Object-oriented languages provide mechanisms for ensuring that such assumptions are local to one part of the program. They are usually part of 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 have a specific view to OOP that is less general in certain aspects from the more general definition.
Widely-used terminology distinguishes object-oriented programming from object-based. The former is held to include inheritance (described below), while the latter does not.
The most popular and developed model of OOP is a class-based model, as opposed to an object-based model. In this model, objects are entities that combine state (i.e., data), behavior (i.e., procedures, or methods) and identity (unique existence among all other objects). The structure and behavior of an object are defined by a class, which is a definition, or blueprint, of all objects of a specific type. An object must be explicitly created based on a class and an object thus created is considered to be an instance of that class. An object is similar to a structure, with the addition of method pointers, member access control, and an implicit data member which locates instances of the class (i.e. actual objects of that class) in the class hierarchy (essential for runtime inheritance features).
One object's data and/or functionality may be based on those of other objects, from which the former object is said to inherit. This allows commonalities among different kinds of objects to be expressed once and reused multiple times. Inheritance is also commonly held to include subtyping, whereby one type of object is defined to be a more specialised version of another type (see Liskov substitution principle), though non-subtyping inheritance is also possible. Inheritance is typically expressed by describing classes of objects arranged in an inheritance hierarchy reflecting common behavior.
Class-based languages, or, to be more precise, typed languages, where subclassing is the only way of subtyping, have been criticized for mixing up implementations and interfaces—the essential principle in object-oriented programming. It says one might create a bag class that stores a collection of objects, then extends it to make a new class called a set class where the duplication of objects is eliminated. Now, a function that takes a bag class may expect that adding two objects increases the size of a bag by two, yet if one passes an object of a set class, then adding two objects may or may not increase the size of a bag by two. The problem arises precisely because subclassing implies subtyping even in the instances where the principle of subtyping, known as the Liskov Substitution Principle, does not hold.
Also, another common example is that a person object created from a child class cannot become an object of parent class because a child class and a parent class inherit a person class but class-based languages mostly do not allow to change the kind of class of the object at runtime.
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. SELF, a programming language developed by Sun Microsystems is an instance of a language that uses prototyping for behavior sharing rather than classification. NewtonScript, Act1, IoLanguage and DELEGATION are other examples. Hybrid and Exemplars use both prototyping and classification. 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.
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. Some people regard OBP is not OOP.
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.
Hierarchical taxonomies often do not match the real world and real-world changes according to some critics, and should be avoided. However, many OOP proponents also suggest avoiding hierarchies, instead using OO techniques such as "composition". A simple way of avoiding over-specification of hierarchies when modelling the real world is to consider the most specific types of objects and model relationships between those.
While OOP is popular for managing access to lower-level or external services, it has proven more difficult and inconsistent for "business modeling". Also, many feel that OOP runs counter to the philosophy of relational modeling and relational databases, returning to the navigational database arrangements of the 1960's. It's not clear that this is the fault of OOP, since database modelling is based fundamentally on different premises than object-oriented modelling. In any case, relational database tables map to associations in object-oriented models, and the differences seem to be purely due to differences in focus. There is a history of misinterpretation of the relationship between object-oriented and relational modelling, which may muddy this issue. Also, there are variances in opinions about the roles and definitions of each. For example, some feel that OOP unnecessarily will copy noun relationship information from the database, when "once and only once" (no duplication) mantra dictates that such is bad practice. Others, in contrast, feel that OOP does not require this duplication, even though some existing OOP-to-relational database products mistakenly take this view, confusing object's data with relationship data. These people would also argue that strict distinctions should be made between data associated with the modelled objects, data associated with the roles and data associated with associations; in particular, object's data should not be (directly) stored in databases by this view, because databases are not a suitable storage for objects, the object already has some mechanism for storing its private information, and storage in database would require unnecessary replication between the object's image in its own storage and the database. The impedance mismatch between databases and OOP is caused by difference of scale between operations performed by objects and databases; database transactions, the smallest unit of work performed by databases, are much larger than any operations provided by OOP objects. Instead, by this view, databases are good for storing relationships between objects and the references to objects that are associated with roles that those relationships are built on; objects' data could only be stored in databases after collecting and summarising data from groups of objects. Object's private representation details have no place in databases.
Needless to say, the "proper" relationship between OOP and databases is a complex and contentious topic which currently has no consensus solution.
While it is claimed that OOP is better for "large applications", others feel that large applications should instead be reduced to many small applications, such as event-driven procedures that "feed" off of a database and declarative programming-based user interface frameworks.
The bottom line of the conflict seems to be that OOP is mostly a behaviorist view of software design which conflicts with the data-centric, declarative view. In the first, the "interfaces" are primarily behaviors, and data is grouped into objects. In the second the interfaces are primarily data (declarations) and behaviours are grouped into functions, such as "tasks", or "events". The tradeoffs of each approach are complex and often delve deep into human psychology theories. Sometimes both are used, such that OOP is used to build platform facilities and functional or declarative method is used to build applications for the platform.
Some feel that past criticisms leveled against procedural techniques are based upon poor languages, poor coding practices, or lack of knowledge about how to properly use databases instead of code to manage state and "noun models".
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