Object (computer science)

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In computer science, an object can be a variable, a data structure, a function, or a method, and as such, is a value in memory referenced by an identifier.


In the object-oriented programming paradigm, object can be a combination of variables, functions, and data structures; in particular in class-based variations of the paradigm it refers to a particular instance of a class.

In the relational model of database management, an object can be a table or column, or an association between data and a database entity (such as relating a person's age to a specific person). [1]

Object-based languages

An important distinction in programming languages is the difference between an object-oriented language and an object-based language. A language is usually considered object-based if it includes the basic capabilities for an object: identity, properties, and attributes. A language is considered object-oriented if it is object-based and also has the capability of polymorphism, inheritance, encapsulation, and, possibly, composition. Polymorphism refers to the ability to overload the name of a function with multiple behaviors based on which object(s) are passed to it. Conventional message passing discriminates only on the first object and considers that to be "sending a message" to that object. However, some OOP languages such as Flavors and the Common Lisp Object System (CLOS) enable discriminating on more than the first parameter of the function. [2] Inheritance is the ability to subclass an object class, to create a new class that is a subclass of an existing one and inherits all the data constraints and behaviors of its parents but also adds new and/or changes one or more of them. [3] [4]

Object-oriented programming

Object-oriented programming is an approach to designing modular reusable software systems. The object-oriented approach is an evolution of good design practices that go back to the very beginning of computer programming. Object-orientation is simply the logical extension of older techniques such as structured programming and abstract data types. An object is an abstract data type with the addition of polymorphism and inheritance.

Rather than structure programs as code and data, an object-oriented system integrates the two using the concept of an "object". An object has state (data) and behavior (code). Objects can correspond to things found in the real world. So for example, a graphics program will have objects such as circle, square, menu. An online shopping system will have objects such as shopping cart, customer, product. The shopping system will support behaviors such as place order, make payment, and offer discount. The objects are designed as class hierarchies. So for example with the shopping system there might be high level classes such as electronics product, kitchen product, and book. There may be further refinements for example under electronic products: CD Player, DVD player, etc. These classes and subclasses correspond to sets and subsets in mathematical logic. [5] [6]

Specialized objects

An important concept for objects is the design pattern. A design pattern provides a reusable template to address a common problem. The following object descriptions are examples of some of the most common design patterns for objects. [7]

Distributed objects

The object-oriented approach is not just a programming model. It can be used equally well as an interface definition language for distributed systems. The objects in a distributed computing model tend to be larger grained, longer lasting, and more service-oriented than programming objects.

A standard method to package distributed objects is via an Interface Definition Language (IDL). An IDL shields the client of all of the details of the distributed server object. Details such as which computer the object resides on, what programming language it uses, what operating system, and other platform-specific issues. The IDL is also usually part of a distributed environment that provides services such as transactions and persistence to all objects in a uniform manner. Two of the most popular standards for distributed objects are the Object Management Group's CORBA standard and Microsoft's DCOM. [8]

In addition to distributed objects, a number of other extensions to the basic concept of an object have been proposed to enable distributed computing:

Some of these extensions, such as distributed objects and protocol objects, are domain-specific terms for special types of "ordinary" objects used in a certain context (such as remote method invocation or protocol composition). Others, such as replicated objects and live distributed objects, are more non-standard, in that they abandon the usual case that an object resides in a single location at a time, and apply the concept to groups of entities (replicas) that might span across multiple locations, might have only weakly consistent state, and whose membership might dynamically change.

The Semantic Web

The Semantic Web is essentially a distributed-objects framework. Two key technologies in the Semantic Web are the Web Ontology Language (OWL) and the Resource Description Framework (RDF). RDF provides the capability to define basic objects—names, properties, attributes, relations—that are accessible via the Internet. OWL adds a richer object model, based on set theory, that provides additional modeling capabilities such as multiple inheritance.

OWL objects are not like standard large-grained distributed objects accessed via an Interface Definition Language. Such an approach would not be appropriate for the Internet because the Internet is constantly evolving and standardization on one set of interfaces is difficult to achieve. OWL objects tend to be similar to the kinds of objects used to define application domain models in programming languages such as Java and C++.

However, there are important distinctions between OWL objects and traditional object-oriented programming objects. Traditional objects get compiled into static hierarchies usually with single inheritance, but OWL objects are dynamic. An OWL object can change its structure at run time and can become an instance of new or different classes.

Another critical difference is the way the model treats information that is currently not in the system. Programming objects and most database systems use the "closed-world assumption". If a fact is not known to the system that fact is assumed to be false. Semantic Web objects use the open-world assumption, a statement is only considered false if there is actual relevant information that it is false, otherwise it is assumed to be unknown, neither true nor false.

OWL objects are actually most like objects in artificial intelligence frame languages such as KL-ONE and Loom.

The following table contrasts traditional objects from Object-Oriented programming languages such as Java or C++ with Semantic Web Objects: [10] [11]

OOP ObjectsSemantic Web Objects
Classes are regarded as types for instances.Classes are regarded as sets of individuals.
Instances can not change their type at runtime.Class membership may change at runtime.
The list of classes is fully known at compile-time and cannot change after that.Classes can be created and changed at runtime.
Compilers are used at build-time. Compile-time errors indicate problems.Reasoners can be used for classification and consistency checking at runtime or build-time.
Classes encode much of their meaning and behavior through imperative functions and methods.Classes make their meaning explicit in terms of OWL statements. No imperative code can be attached.
Instances are anonymous insofar that they cannot easily be addressed from outside of an executing program.All named RDF and OWL resources have a unique URI under which they can be referenced.
Closed world: If there is not enough information to prove a statement true, then it is assumed to be false.Open world: If there is not enough information to prove a statement true, then it may be true or false. [12]

See also

Related Research Articles

In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state and implementations of behavior. In many languages, the class name is used as the name for the class, the name for the default constructor of the class, and as the type of objects generated by instantiating the class; these distinct concepts are easily conflated. Although, to the point of conflation, one could argue that is a feature inherent in a language because of its polymorphic nature and why these languages are so powerful, dynamic and adaptable for use compared to languages without polymorphism present. Thus they can model dynamic systems more easily.

The Common Object Request Broker Architecture (CORBA) is a standard defined by the Object Management Group (OMG) designed to facilitate the communication of systems that are deployed on diverse platforms. CORBA enables collaboration between systems on different operating systems, programming languages, and computing hardware. CORBA uses an object-oriented model although the systems that use the CORBA do not have to be object-oriented. CORBA is an example of the distributed object paradigm.

In software engineering and computer science, abstraction is:

An interface description language or interface definition language (IDL), is a generic term for a language that lets a program or object written in one language communicate with another program written in an unknown language. IDLs describe an interface in a language-independent way, enabling communication between software components that do not share one language, for example, between those written in C++ and those written in Java.

Common Lisp Object System

The Common Lisp Object System (CLOS) is the facility for object-oriented programming which is part of ANSI Common Lisp. CLOS is a powerful dynamic object system which differs radically from the OOP facilities found in more static languages such as C++ or Java. CLOS was inspired by earlier Lisp object systems such as MIT Flavors and CommonLoops, although it is more general than either. Originally proposed as an add-on, CLOS was adopted as part of the ANSI standard for Common Lisp and has been adapted into other Lisp dialects such as EuLisp or Emacs Lisp.

In programming languages and type theory, polymorphism is the provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types.

In computer science, a metaobject is an object that manipulates, creates, describes, or implements objects. The object that the metaobject pertains to is called the base object. Some information that a metaobject might define includes the base object's type, interface, class, methods, attributes, parse tree, etc. Metaobjects are examples of the computer science concept of reflection, where a system has access to its own internal structure. Reflection enables a system to essentially rewrite itself on the fly, to alter its own implementation as it executes.

Class-based programming, or more commonly class-orientation, is a style of object-oriented programming (OOP) in which inheritance occurs via defining classes of objects, instead of inheritance occurring via the objects alone.

In computing and systems design a loosely coupled system is one in which each of its components has, or makes use of, little or no knowledge of the definitions of other separate components. Subareas include the coupling of classes, interfaces, data, and services. Loose coupling is the opposite of tight coupling.

The object–relational impedance mismatch is a set of conceptual and technical difficulties that are often encountered when a relational database management system (RDBMS) is being served by an application program written in an object-oriented programming language or style, particularly because objects or class definitions must be mapped to database tables defined by a relational schema.

In object-oriented programming, inheritance is the mechanism of basing an object or class upon another object or class, retaining similar implementation. Also defined as deriving new classes from existing ones such as super class or base class and then forming them into a hierarchy of classes. In most class-based object-oriented languages, an object created through inheritance, a "child object", acquires all the properties and behaviors of the "parent object", with the exception of: constructors, destructor, overloaded operators and friend functions of the base class. Inheritance allows programmers to create classes that are built upon existing classes, to specify a new implementation while maintaining the same behaviors, to reuse code and to independently extend original software via public classes and interfaces. The relationships of objects or classes through inheritance give rise to a directed graph.

Component-based software engineering

Component-based software engineering (CBSE), also called components-based development (CBD), is a branch of software engineering that emphasizes the separation of concerns with respect to the wide-ranging functionality available throughout a given software system. It is a reuse-based approach to defining, implementing and composing loosely coupled independent components into systems. This practice aims to bring about an equally wide-ranging degree of benefits in both the short-term and the long-term for the software itself and for organizations that sponsor such 'software.'

In mathematical logic and computer science, some type theories and type systems include a top type that is commonly denoted with top or the symbol ⊤. The top type is sometimes called also universal type, or universal supertype as all other types in the type system of interest are subtypes of it, and in most cases, it contains every possible object of the type system. It is in contrast with the bottom type, or the universal subtype, which every other type is supertype of and it is often that the type contains no members at all.

In computing, object model has two related but distinct meanings:

  1. The properties of objects in general in a specific computer programming language, technology, notation or methodology that uses them. Examples are the object models of Java, the Component Object Model (COM), or Object-Modeling Technique (OMT). Such object models are usually defined using concepts such as class, generic function, message, inheritance, polymorphism, and encapsulation. There is an extensive literature on formalized object models as a subset of the formal semantics of programming languages.
  2. A collection of objects or classes through which a program can examine and manipulate some specific parts of its world. In other words, the object-oriented interface to some service or system. Such an interface is said to be the object model of the represented service or system. For example, the Document Object Model (DOM) is a collection of objects that represent a page in a web browser, used by script programs to examine and dynamically change the page. There is a Microsoft Excel object model for controlling Microsoft Excel from another program, and the ASCOM Telescope Driver is an object model for controlling an astronomical telescope.

Object-oriented design is the process of planning a system of interacting objects for the purpose of solving a software problem. It is one approach to software design.

Frames are an artificial intelligence data structure used to divide knowledge into substructures by representing "stereotyped situations". They were proposed by Marvin Minsky in his 1974 article "A Framework for Representing Knowledge". Frames are the primary data structure used in artificial intelligence frame language; they are stored as ontologies of sets.

In type theory, bounded quantification refers to universal or existential quantifiers which are restricted ("bounded") to range only over the subtypes of a particular type. Bounded quantification is an interaction of parametric polymorphism with subtyping. Bounded quantification has traditionally been studied in the functional setting of System F<:, but is available in modern object-oriented languages supporting parametric polymorphism (generics) such as Java, C# and Scala.

Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code: data in the form of fields, and code, in the form of procedures.

In computing, an application programming interface (API) is an interface that defines interactions between multiple software applications or mixed hardware-software intermediaries. It defines the kinds of calls or requests that can be made, how to make them, the data formats that should be used, the conventions to follow, etc. It can also provide extension mechanisms so that users can extend existing functionality in various ways and to varying degrees. An API can be entirely custom, specific to a component, or designed based on an industry-standard to ensure interoperability. Through information hiding, APIs enable modular programming, allowing users to use the interface independently of the implementation.


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  12. Table excerpted from tables in: http://www.w3.org/2001/sw/BestPractices/SE/ODSD/