object-oriented programming

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object-oriented programming (OOP) A programming technique that combines data abstraction, inheritance, and dynamic type binding. The central feature is the object, which comprises a data structure definition and its defined procedures in a single structure. Objects are instances of a class, each instance having its own private instance variables. The class definition defines the properties of the objects in a class. Hierarchical class structures are possible in which objects in a class inherit the properties of the parent class in addition to properties explicitly defined for the class. This facilitates sharing of code, since users can inherit objects from system collections of code.

The procedures of an object (often called methods) are activated by messages sent to the object by another object. Thus in an object-oriented programming system the basic control structure is message passing. The programmer identifies the real-world objects of the problem and the processing requirements of those objects, encapsulating these in class definitions, and the communications between objects. The program is then essentially a simulation of the real world in which objects pass messages to other objects to initiate actions.

The most complete realization of an object-oriented programming system is Smalltalk; the concepts also appear in combination with conventional languages, for example C++ and CLOS.

object-oriented programming a modular approach to computer program (software) design. Each module, or object, combines data and procedures (sequences of instructions) that act on the data; in traditional, or procedural, programming the data are separated from the instructions. A group of objects that have properties, operations, and behaviors in common is called a class. By reusing classes developed for previous applications, new applications can be developed faster with improved reliability and consistency of design. The first object-oriented programs, written in the language Simula 67, were used extensively for modeling and simulation, primarily in Europe during the late 1960s and early 1970s. The technique was popularized in the United States during the following decade using the language SmallTalk and achieved its greatest prominence with the development of the object-oriented language C++ during the late 1980s and 1990s.

Bibliography: See P. W. Oman and T. G. Lewis, Milestones in Software Evolution (1990); T. Budd, An Introduction to Object-Oriented Programming (1991); P. Varhol, Object-Oriented Programming: The Software Development Revolution (1993); P. Coad and J. Nicola, OOP, Object-Oriented Programming (1993).

object-oriented programming language A programming language that provides facilities for the definition and manipulation of objects, an object being a collection of data which can be sent messages that change the values of its components. For example, a typical object in a stock control system is a product which is stored in a warehouse. A product object contains data that is relevant to the use which is made of it, for example its name, the quantity in stock, and the level below which it has to be reordered. Typical messages which such an object would receive include a message which reduces its stock level, increases its stock level, and which returns with the name of the supplier of the product. Object-oriented languages are not new: they date from the late 1960s. However, in the 1990s they have achieved increasing prominence: first through the use of C++ in conventional applications and then through the use of JAVA in Internet applications. If current trends continue, Java and C++, together with the programming language PERL, will dominate software development for the Internet in the coming years.

object-oriented database A term not well defined but applied to software products that provide persistence for applications written in object-oriented languages with features such as rollback and recovery. Thus ONTOS provides persistence for C++ and GemStone for a variant of Smalltalk-80. There is no generally accepted object-oriented data model on which products can be based as with the relational model, although various proposals have been and are being made. It remains unclear whether all the concepts of object-oriented programming are relevant in the database context.