Object (computer science)

In computer science, an object is a programming element that has state, has associated operations and is accessed via an identifier.

Object is a categorization into which many programming concepts fall. For example, an object can be a programming language variable, data structure, function, or method.

In object-oriented programming (OOP), an object is an abstract data type with the addition of polymorphism and inheritance. An object-oriented system integrates code and data as objects that each have state (data) and behavior (code). Any language that supports a construct with state and behavior is classified as object-based. In a class-based language, an object is an also known as an instance of a class.

In the relational model of database management, aspects such as table and column may act as objects.^[1]

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.^[2]

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

Protocol objects are components of a protocol stack that enclose network communication within an object-oriented interface.
Replicated objects are groups of distributed objects (called replicas) that run a distributed multi-party protocol to achieve high consistency between their internal states, and that respond to requests in a coordinated way. Examples include fault-tolerant CORBA objects.
Live distributed objects (or simply live objects)^[3] generalize the replicated object concept to groups of replicas that might internally use any distributed protocol, perhaps resulting in only a weak consistency between their local states.

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 (WOL) 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:^[4]^[5]

OOP Objects	Semantic 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.^[6]

References

^ Oppel, Andy (2005). SQL Demystified. McGraw Hill. p. 7. ISBN 0-07-226224-9.
^ Orfali, Robert (1996). The Essential Client/Server Survival Guide. New York: Wiley Computer Publishing. pp. 399–403. ISBN 0-471-15325-7.
^ Ostrowski, K., Birman, K., Dolev, D., and Ahnn, J. (2008). "Programming with Live Distributed Objects", Proceedings of the 22nd European Conference on Object-Oriented Programming, Paphos, Cyprus, July 07–11, 2008, J. Vitek, Ed., Lecture Notes in Computer Science, vol. 5142, Springer-Verlag, Berlin, Heidelberg, 463-489, http://portal.acm.org/citation.cfm?id=1428508.1428536.
^ Berners-Lee, Tim; James Hendler; Ora Lassila (May 17, 2001). "The Semantic Web A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities". Scientific American. 284: 34–43. doi:10.1038/scientificamerican0501-34. Archived from the original on April 24, 2013.
^ Knublauch, Holger; Oberle, Daniel; Tetlow, Phil; Wallace, Evan (2006-03-09). "A Semantic Web Primer for Object-Oriented Software Developers". W3C. Retrieved 2008-07-30.
^ Table excerpted from tables in: http://www.w3.org/2001/sw/BestPractices/SE/ODSD/