Glossary

This glossary provides brief definitions of the principal terms of object technology, discussed in detail in the book Object-Oriented Software Construction, 2nd Edition and used in this website. Italics font in a definition marks a term or phrase, other than the ubiquitous "class", that is itself the subject of another definition.

Abstract class

See deferred class.

Abstract data type (ADT)

A set of mathematical elements specified by listing the functions applicable to all these elements and the formal properties of these functions.

Abstract object

An element of an abstract data type (ADT).

Ancestor (of a class)

The class itself, or one of its direct or indirect parents.

Assertion

A formal condition describing the semantic properties of software elements, especially routines and loops. Used in expressing contracts. Assertions include in particular preconditions, postconditions, class invariants and loop invariants.

Assignment attempt

An operation that conditionally attaches an object to a reference, only if the object’s type conforms to the type declared for the corresponding entity.

Asynchronous call

A call which lets its caller proceed before it completes. Antonym: synchronous call.

Attribute

The description of a field present in all the instances of a class. Along with the routine, one of the two forms of feature.

Behavior class

A class, usually deferred, describing a set of adaptable behaviors through effective routines relying on some components (usually deferred features) that may be redeclared to capture specific variants of the general behaviors.

Class

A partially or totally implemented abstract data type. Serves both as a module and as a type (or type pattern if the class is generic.)

Class invariant

An assertion which must be satisfied on creation of every instance of a class, and preserved by every exported routine of the class, so that it will be satisfied by all instances of the class whenever they are externally observable.

Client

A class that uses the features of another, its supplier, on the basis of the supplier's interface specification (contract).

Cluster

A group of related classes or, recursively, of related clusters.

Component

See reusable software component.

Concurrent

Able to use two or more processors. Antonym: sequential.

Conformance

A relation between types. A type conforms to another if it is derived from it by inheritance.

Constrained genericity

A form of genericity where a formal generic parameter represents not an arbitrary type, but one that is required to conform to a certain type, known as the constraint. See constrained genericity.

Container data structure

An object whose primary use is to provide access to a number of other objects. Examples include lists, queues, stacks, arrays.

Contract

The set of precise conditions that govern the relations between a supplier class and its clients. The contract for a class includes individual contracts for the exported routines of the class, represented by preconditions and postconditions, and the global class properties, represented by the class invariant. See also Design by Contract.

Contravariance

The policy allowing a feature redeclaration to change the signature so that a new result type will conform to the original but the original argument types conform to the new. See also: covariance, novariance.

Covariance

The policy allowing a feature redeclaration to change the signature so that the new types of both arguments and result conform to the originals. See also: contravariance, novariance.

Current object (or: current instance)

During the execution of an object-oriented software system, the target of the most recently started routine call.

Defensive programming

A technique of fighting potential errors by making every module check for many possible consistency conditions, even if this causes redundancy of checks performed by clients and suppliers. Contradicts Design by Contract.

Deferred class

A class which has at least one deferred feature. Antonym: effective class.

Deferred feature

A feature which, in a certain class, has a specification but no implementation. May be declared as deferred in the class itself, or inherited as deferred and not effected in the class. Antonym: effective feature.

Descendant (of a class)

The class itself, or one of its direct or indirect heirs.

Design by Contract

A method of software construction that designs the components of a system so that they will cooperate on the basis of precisely defined contracts. See also: defensive programming.

Direct instance (of a class)

An object built according to the mold defined by the class.

Dynamic

Occurring during the execution of a system. See also run time. Antonym: static.

Dynamic binding

The guarantee that every execution of an operation will select the correct version of the operation, based on the type of the operation's target.

Dynamic typing

The policy whereby applicability of operations to their target objects is only checked at run time, prior to executing each operation.

Effect

A class effects a feature if it inherits it in deferred form and provides an effecting for that feature.

Effecting

A redeclaration which provides an implementation (as attribute or routine) of a feature inherited in deferred form.

Effective class

A class which only has effective features (that is to say, does not introduce any deferred feature, and, if it inherits any deferred feature, effects it). Antonym: deferred class.

Effective feature

A feature declared with an implementation - either as a routine which is not deferred, or as an attribute. Antonym: deferred feature.

Encapsulation

See information hiding.

Entity

A name in the software text that denotes a run-time value (object or reference).

Event-driven computation

A style of software construction where developers define the control structure by listing possible external events and the system's response to each of them, rather than by specifying a pre-ordained sequence of steps.

Exception

The inability of a routine to achieve its contract through one of its possible strategies. May result in particular from a failure of a routine called by the original routine. Will be treated as resumption, organized panic or false alarm.

Exporting a feature

Making the feature available to clients. Exports may be selective (to specified classes only) or general.

Extendibility

The ability of a software system to be changed easily in response to different choices of requirements, architecture, algorithms or data structures.

Failure

The inability of a routine's execution to fulfill the routine's contract. Must trigger an exception.

False alarm

Along with resumption and organized panic, one of the three possible responses to an exception; resumes the execution of the current strategy, possibly after taking some corrective action.

Feature

The attributes and routines of a class.

Feature renaming

The attribution, by a class, of a new name to an inherited feature, not changing any other property. See also redeclaration.

Field

One of the values making up an object.

Function

A routine which returns a result. (The other form of routine is the procedure.)

Garbage collection

A facility provided by the runtime to recycle the memory space used by objects that have become useless. Garbage collection is automatic, that is to say does not require any change to the text of the systems whose objects are being recycled.

Generalization

The process of turning specialized program elements into general-purpose, reusable software components.

Generating class

Same as generator.

Generator (of an object)

The class of which the object is a direct instance.

Generic class

A class having formal parameters representing types. Such a class will yield a type only through generic derivation.

Generic derivation

The process of providing a type for each formal generic parameter of a generic class, yielding a type as a result.

Genericity

The support, by a software notation, for type-parameterized modules'; specifically, in an O-O notation, for generic classes. Can be unconstrained or constrained.

Heir (of a class)

A class that inherits from the given class. Antonym: parent.

Identity

See object identity.

Information hiding

The ability to prevent certain aspects of a class from being accessible to its clients, through an explicit exporting policy and through reliance on the short form as the primary vehicle for class documentation.

Inheritance

A mechanism whereby a class is defined in reference to others, adding all their features to its own.

Instance (of a class)

An object built according to the mold defined by the class or any one of its proper descendants. See also direct instance, proper descendant, generator.

Instance variable

Smalltalk term for attribute.

Interface (of a class)

See contract, abstract data type.

Invariant

See class invariant, loop invariant.

Iterator

A control structure describing preordained sequencing of some actions but not defining the actions themselves. Iterators often apply to data structures, such as an iterator describing the traversal of a list or a tree.

Loop invariant

An assertion which must be satisfied prior to the first execution of a loop, and preserved by every iteration, so that it will hold on loop termination.

Loop variant

An integer expression which must be non-negative prior to the first execution of a loop, and decreased by every iteration, so that it will garantee loop termination.

Message

Routine call.

Metaclass

A class whose instances are classes themselves.

Method

Smalltalk term for routine.

Module

A unit of software decomposition. In the object-oriented approach, classes provide the basic form of module.

Multiple inheritance

The unrestricted form of inheritance, whereby a class may have any number of parents. Antonym: single inheritance.

Non-separate

Antonym of separate.

Novariance

The policy prohibiting any feature redeclaration from changing the signature. See also: contravariance, covariance.

Object

A run-time data structure made of zero or more values, called fields, and serving as the computer representation of an abstract object. Every object is an instance of some class.

Object identity

A property that uniquely identifies an object independently of its current contents (fields).

Object-oriented

Built from classes, assertions, genericity, inheritance, polymorphism and dynamic binding.

Object-oriented analysis

The application of object-oriented concepts to the modeling of problems and systems from both software and non-software domains.

Object-oriented database

A repository of persistent objects, permitting their storage and retrieval on the basis of object-oriented concepts, and supporting database properties such as concurrent access, locking and transactions.

Object-oriented design

The process of building the architecture of systems through object-oriented concepts.

Object-oriented implementation

The process of building executable software systems through object-oriented concepts. Differs from object-oriented design primarily by the level of abstraction.

Organized panic

Along with resumption and false alarm, one of the three possible responses to an exception; abandons the execution of the current strategy, triggering an exception in the caller, after restoring the class invariant for the current object.

Overloading

The ability to let a feature name denote two or more operations.

Package

A module of non-object-oriented languages such as Ada, providing encapsulation of a set of variables and routines.

Parallel

See concurrent.

Parameterized class

See generic class.

Parent (of a class)

A class from which the given class inherits. Antonym: heir.

Persistence

The ability of a software development environment or language to make objects persistent and support the retrieval of persistent objects for use by systems.

Persistent object

An object that (through storage in a file or database or transmission across a network) survives executions of systems that create or manipulate it. Antonym: transient object.

Polymorphic data structure

A container data structure hosting objects of two or more possible types.

Polymorphism

The ability for an element of the software text to denote, at run time, objects of two or more possible types.

Postcondition

An assertion attached to a routine, which must be guaranteed by the routine's body on return from any call to the routine if the precondition was satisfied on entry. Part of the contract governing the routine.

Precondition

An assertion attached to a routine, which must be guaranteed by every client prior to any call to the routine. Part of the contract governing the routine.

Predicate

See assertion.

Procedure

A routine which does not return a result. (The other form of routine is the function.)

Processor

A mechanism providing a single thread of computation. May be a physical device, such as the CPU of a computer, or a software device, such as a task or thread of an operating system.

Program

See system.

Proper ancestor (of a class)

A direct or indirect parent of the class.

Proper descendant (of a class)

A direct or indirect heir of the class.

Redeclaration

A feature declaration which, instead of introducing a new feature, adapts some properties (such as the signature, precondition, postcondition, implementation, deferred/effective status, but not the name) of a feature inherited from a parent. A redeclaration may be a redefinition or an effecting. See also feature renaming.

Redefinition

A redeclaration which is not an effecting, that is to say, changes some properties of a feature inherited as effective, or changes the specification of a feature inherited as deferred while leaving it deferred.

Reference

A run-time value that uniquely identifies an object.

Renaming

See feature renaming.

Representation

The physical layout of data in RAM (or other storage), and the choices of what data is stored and what data is computed at run time, in order to represent the abstract data type in question.

Representation Independence

The ability of a class to present an unchanging interface to its clients, and implement alternate representations of the underlying object without the clients needing to know or care about it. In the object-oriented method, dynamic binding and polymorphism are major contributors to making this possible.

Resumption

See retrying.

Retrying

Along with organized panic and false alarm, one of the three possible responses to an exception; tries a new strategy for achieving the routine's contract.

Reusability

The ability of a software development method to yield software elements that can be used in many different applications, and to support a software development process relying on pre-existing reusable software components.

Reusable software component

An element of software that can be used by many different applications.

Reversible development

A software development process that lets insights gained in later phases affect the results obtained in earlier phases. Normally part of a seamless development process.

Root class

The generator of a system's root object. Executing the system means creating an instance of the root class (the root object), and calling a creation procedure on that instance.

Root object

The first object created in the execution of a system.

Routine

A computation defined in a class, and applicable to the instances of that class. Along with the attribute, one of the two forms of feature.

Runtime (noun, one word)

Any set of facilities supporting the execution of systems. See run time.

Run time (noun, two words)

The time when a system is being executed. Also used as an adjective, with a hyphen, as in "the run-time value of an entity". See also dynamic and runtime.

Schema evolution

Change to one or more classes of which some persistent instances exist.

Seamless development

A software development process which uses a uniform method and notation throughout all activities, such as problem modeling and analysis, design, implementation and maintenance. See also reversible development.

Selective export

See exporting a feature.

Separate

Handled by a different processor. Antonym: non-separate.

Sequential

Running on only one processor. Antonym: concurrent.

Short form (of a class)

A form of class documentation generated from the class text, showing only interface properties of the class. The short form documents the contract attached to the class and the underlying abstract data type.

Signature (of a feature)

The type part of the feature's specification. For an attribute or a function, includes the result type; for a routine, includes the number of arguments and the type of each.

Single inheritance

A restricted form of inheritance whereby each class may have at most one parent. Antonym: multiple inheritance.

Software component

See reusable software component.

Specification (of a class)

The short form of the class.

Specification (of a feature)

The properties of a feature that are relevant to a client. Includes the name, signature, header comment and contract of the feature.

Subcontract

The ability of a class to let some proper descendant handle some of its feature calls, thanks to redeclaration and dynamic binding.

Supplier

A class that provides another, its client, with features to be used through an interface specification (contract).

Static

Applying to the text of a system, not to a particular execution. Antonym: dynamic.

Static binding

The premature choice of operation variant, resulting in possibly wrong results and (in favorable cases) run-time system crash.

Static typing

The ability to check, on the basis of the software text alone, that no execution of a system will ever try to apply to an object an operation that is not applicable to that object.

Synchronous call

A call which forces the caller to wait until it completes. Antonym: asynchronous call.

System

A set of classes that can be assembled to produce an executable result.

Template

C++ term for generic class (for unconstrained genericity only).

Traitor

A reference to a separate object, associated in the software text with an entity that is declared as non-separate.

Transient object

An object that exists only during the execution of the system that creates it. Antonym: persistent object.

Type

The description of a set of objects equipped with certain operations. In the object-oriented approach every type is based on a class.

Type checking, typing

See static typing, dynamic typing.

Unconstrained genericity

A form of genericity where a formal generic parameter represents an arbitrary type. See constrained genericity.

Variant

See loop variant.