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Definitions

Having a common glossary of terms is important for accurate, precise, and concise communication among team members. The following definitions are ChiMu’s distilling and reconciliation of the many great concepts and work that have been contributed to OO. Many of the sources for these terms are mentioned in the References section of this document. A few particularly notable sources are:

The Dictionary of Object Technology [Firesmith+E 95].
UML: The Unified Modeling Language [Rational 98]
Design Patterns, especially [Gamma+HJV 96]

Categorized

The following sections categorize definitions by starting with the core OO concepts (Objects) and growing to more domain-specific definitions.

Object

The core concept to OO is Objects. Everything else in OO is to make understanding, managing, or implementing Objects easier. The definition of Object is the foundation for all other definitions.

Definitions

Object	An identifiable, encapsulated entity that can only be interacted with by sending messages.
Message	A stimulus sent to an object with a name and any parameters (as Objects) that the message requires. A message will cause the receiver Object to return an answer or nothing. In most Object Languages, the sender has to wait for the answer before continuing.

Identity	The ability to tell one object from another object independently of whether their appearance (behavior) is identical.
Behavior	The response of an object to a stimulus. An object’s behavior is the answers it gives to messages both now and in the future. (See State).
State	An abstraction to describe and simplify understanding an object’s behavior. An object’s behavior can be described as the answers it gives to current messages (which are determined by the current state) and the changes to its state caused by these messages.

More Information

The best source for the concept of Object is Alan Kay’s writings [Kay 95] and Smalltalk itself [Goldberg+R 83, Squeak]. You could also look at Simula (the progenitor of Smalltalk), but it has other concepts mixed into the language besides Objects. Some methodologies focus on Objects more than others [Wirfs-Brock+WW 90, Wilkinson 95], but all of the main OO methodology books (e.g. [Booch 95, Rumbaugh+BPEL 91, Jacobson+CJO 92]) have objects at the core.

Type

Types allow you to think about the commonality of objects’ exteriors. They are the first conceptual abstraction above Objects and immediately provide an enormous amount of ability to reasoning about Objects. The amount of abstraction and formality associated with Types can depend on of the project or the current perspective.

Definitions

Type	Describes a common exterior (public behavior) of a set of Objects. Can also be used to conceptually group and understand objects by their similar behavior.
Operation	A description of the ability for an object to respond to a particular message and the contract/requirement for that message.
Interface	A description of a Type focused on the Operations that the objects can respond to.
Is-A	An object is a Type if an Object supports all the exterior requirements of that Type.

Extend	To define a new Type (called a Subtype) in terms of an existing type (called a Supertype). The new Subtype will have the same contract (operations) as the Supertype but can add new functionality: as either new operations or enhancement in capability to existing operations.
Subtype	A Type that extends another Type (called the Supertype).
Supertype	A Type that has been extended by another Type.

More Information

Many methodologies and authors do not make a clear distinction between Type and Class (usually referring to both a "Class" and the context determines what was really meant), so this can cause confusion. The following discuss Types independently of Classes [Kilov+R 94, Fowler 97, Jacobson+CJO 92, Cook+D 94], but all of the main methodology books do focus on Types during analysis [Booch 94, Fowler 97, Rumbaugh+BPEL 91]. The description of contracts and interfaces in [Meyer 97] is the standard and an excellent reference on formal Types and their integration into OOP (but you have to be willing to accept many differences in terminology). Java supports the separation of Type from Class through interfaces and classes, see [Coad+M 96, Gosling+JS 96].

Class

Classes provide a common implementation for similar objects. They describe the interior of objects so it is easier to work with many objects of many different Types. It is important to conceptually separate Classes from Types even though it is a common (bad) practice to use Class for both ideas.

Definitions

Class	Provides a common implementation for a set of objects.
Method	An implementation of an operation for a particular object/class.
Instance Variable	A way to store encapsulated state information for a particular object. Instance variables are completely hidden within the Object, but they enable two objects of the same Class to have different external Behavior.
Instance	An object is an Instance of a Type if an object supports all the exterior requirements of that type (see "Is-A"). An object is an instance of a Class if it is implemented by that class.
Extent	The collection of all instances of a Type or Class.
Inherit	To define a new Class in terms of an Existing Class (the Superclass) by starting with the Superclass’s implementation and overiding or adding to it.

More Information

All the methodologies focus on Classes, so any of them would be good for more information. Many are bad at separating the concept of Class from Type (see above under "Type"). Class is the most common term in OO Design and Analysis

Relationship Modeling

The above Type and Class concepts are just the beginning of the tools that can be used to reason about and implement objects. Relationship Modeling adds on the ability to describe relationships among Objects and Types of Objects.

Definitions

Link	A connection between two objects which allows one or both to know about the other object. By default links are assumed to be bidirectional in analysis, but they can be defined to be only traversable in one direction.
Traverse	To move from one object to another by a Link. If a Link is traversable from an Object than that Object can get to (knows about) the other Object.
Association₁	A relationship between two Types that allows or requires Objects of those Types to be Linked.
Role	The name of the "position" within a relationship an Object or Type holds. For example, a binary association has two roles that distinguish the two participants in the relationship.

Association₂	An Association₁, but must be between Types which have Objects with Identity. See Attribute and ValueObject.
ValueObject	An object that does not have identity independent of its value. A ValueObject is immutable and should be considered identical to anything that it is equal to. Primitive data types in Smalltalk (most numbers, Symbols) are ValueObjects. Java Strings are very close to ValueObjects except they are not guaranteed to be identical for the same value (they would be if they did an automatic "intern()"). Java primitive types are not Objects.
Immutable	Can not be changed after being created. Immutable objects can not be changed after they are created and fully initialized.
Attribute₁	A public property of an object that shows an aspect of the state of the object. Frequently there is a minimal collection of attributes that uniquely determine the state of the object. See also Property.
Attribute₂	See BasicAttribute.
Attribute₃	See Instance Variable.
BasicAttribute	An Attribute₁ that takes its value from ValueObjects. This is as opposed to associations, which connect two or more objects with identity. A BasicAttribute is traversable only from the Object to the ValueObject.
Property	Synonym for Attribute₁ and sometimes for Attribute₂.
Feature	The Eiffel term for Operation where Operation includes both methods and attributes₁.

More Information

UML [Fowler 97, Rational 98] is now the primary source for the base concepts of Relationship Modeling in OO. More detailed modeling is done in other methods (for example, see [Kilov+R 95]). Many sources provide concepts for and examples of relationship modeling in particular domain areas (see the Patterns references). The biggest problem with Relationship modeling terminology is the overloaded and fuzzy meaning of "attribute", so be careful to consider what meaning was intended in a given context.

Patterns

The following list has definitions for terms that were codified into OO language through well-known Design Patterns. There are many more Patterns than the following definitions, but these are among the most common and accepted terms.

Definitions

Adapter	An object that can convert an Interface of one Class to the interface another Object expects.
Factory	An object that can create other objects.
Functor	An object that models an operation.
Observable	An object that can be Observed. See Observer.
Observer	An object that "looks at" another object (the Observable) and can respond to events in the Observable without the Observable being knowledgeable about the Observer.
Prototype	An object that is used as a template for creating other Objects.
Proxy	An object that stands in for another object (the RealObject) and manages the client interaction with the RealObject.
Singleton	An object that is the only instance of a Class.
Strategy	An object that encapsulates an algorithm to be used with an Object.
Visitor	An object that represents an operation that can be performed on the elements of an Object structure (frequently a hierarchy or sequence).

More Information

See [Gamma+HJV 96] for the full patterns to most of these definitions. Some definitions may also be covered in other Pattern resources and Patterns are frequently the source of new terminology.

Pattern: Proxy

The Proxy pattern is important to Information Systems, so the following provides some more details.

Definitions

Proxy	An object that stands in for another object (the RealObject) and manages the client interaction with the RealObject.
Forwarder	A proxy which immediately forwards messages, possibly over process and machine boundaries, to the RealSubject.
Replicate	A proxy which holds local state and performs local operations which are later propagated to the RealSubject
Stub	A proxy which acts as a placeholder for the RealObject and must become another type of proxy (for example, forwarder or replicate) when interacted with by a client.
RealIdentity	The identity of the RealObject that a proxy represents instead of the proxy’s independent identity. For proxies we are rarely interested in their own identity, we just want to know the identity of the RealObject on the server.
IdentityKey	A value that defines the RealIdentity of a Proxy.
Binding	Associating a client object to a database object, which turns the client object into a Proxy

More Information

See [Gamma+HJV 96] for the basic proxy concept. See information on distributed processing (e.g. [GemStone 95]) and Object-Relational Mapping (e.g. [Fussell 96, Fussell 97a]) for details on the Proxy variations.

Pattern: Functor

The Functor pattern is common to all types of OO Systems. The following provide some more definitions.

Definitions

Functor	"An object that models an operation" [Firesmith+E 95]. For a Java implementation, a basic functor is an Interface with a single, generic, operation.
Procedure_f	A functor that does not return a value
Function_f	A functor that returns an Object
Predicate_f	A functor that returns a boolean

Getter_f	A functor that is designed to retrieve a value from an object (the first parameter)
Setter_f	A functor that stores into an object (the first parameter) a value (the second parameter)

More Information

Note that almost all of the definitions above also have a more general meaning, so to be specific you would need to append "Functor" to the end to be precise (e.g. "a Predicate Functor is a functor that returns a boolean")

Architecture

The following terms are related to system architecture.

Definitions

Architecture	A system’s concepts, structures, and interactions. Also, the description of a system’s desired architecture before construction.
Framework	A strong partition of generalized functionality common to many parts of an application. Also, more formally, a collection of interacting classes that describes most of the behavior a client requires and can be subclassed and parameterized to customize and complete the functionality.
Layer	A logical, horizontal division of a system that provides a particular system abstraction to the client above the layer.
Module	A base level subsystem: one which does not contain any other subsystems
Partition	A vertical division of a system into areas of related functionality.
Subsystem	A division of a system into a cohesive unit of functionality (tightly related classes and internal subsystems) with a public interface and a private implementation.
Tier	A level on a hierarchy of processes over which a system is divided.

More Information

See the resources under Architecture (e.g. [Shaw+G 96, Fussell 96]) and also some of the large-picture methodology and project-management books (e.g. [Booch 96, Jacobson+CJO 92]).

Object-Oriented Information Systems

Object-Oriented Information Systems use Objects (called DomainObjects, BusinessObjects, or EntityObjects) to capture the knowledge, operations, and rules about a business within a computer.

Definitions

ObjectBase	.An ObjectBase captures the knowledge, operations, and rules required to usefully represent a particular part of the world in a computer. An ObjectBase contains all the objects that represent a particular state of your DomainModel and all the knowledge contained therein. Also called an ObjectSpace.
DomainModel	All the static rules, constraints, and operations that apply to DomainObjects. The DomainModel can either be conceptual to help understand the behavior of DomainObjects or it can be implemented as DomainClasses.
DomainObject	An object which captures knowledge about a domain. DomainObjects allow a computer to inspect, imply, modify, and "reason" about that information in either very simple ways (the facts) or more complex ways (the rules and implications).

More Information

See [Fussell 96].

Other Terms

The following are some currently unclassified terms.

Definitions

Registry	An object that remembers other objects and can search through and retrieve them through one or more properties. Usually the objects within a Registry are all of the same type.
Container	A Registry but without the implication of a primary registration property (e.g. a "key").
ExtentRegistry	A Registry that contains all the objects of a Type (the Extent of the Type). An ExtentRegistry is a close equivalent to a RelVar or Table in the context of Objects.
ObjectShadow	The information needed to see that an object exists without any true representation of the real object. Relational databases could be considered to work with ObjectShadows: they record the information about an object but never have a real object to interact with.

Java Terms

The following terms are very common in Java and not mentioned elsewhere.

Definitions

Bean

An Object that knows about its own properties (can introspect) and has several other capabilities. Any Java Object can be a Bean but some Objects have more Bean functionality.

More Information

See the Java JDKs and information

Alphabetical

The following table contains all the previous mentioned definitions combined and sorted alphabetically.

Adapter	An object that can convert an Interface of one Class to the interface another Object expects.
Architecture	A system’s concepts, structures, and interactions. Also, the description of a system’s desired architecture before construction.
Association₁	A relationship between two Types that allows or requires Objects of those Types to be Linked.
Association₂	An Association₁, but must be between Types which have Objects with Identity. See Attribute and ValueObject.
Attribute₁	A public property of an object that shows an aspect of the state of the object. Frequently there is a minimal collection of attributes that uniquely determine the state of the object. See also Property.
Attribute₂	See BasicAttribute.
Attribute₃	See Instance Variable.
BasicAttribute	An Attribute₁ that takes its value from ValueObjects. This is as opposed to associations, which connect two or more objects with identity. A BasicAttribute is traversable only from the Object to the ValueObject.
Bean	An Object that knows about its own properties (can introspect) and has several other capabilities. Any Java Object can be a Bean but some Objects have more Bean functionality.
Behavior	The response of an object to a stimulus. An object’s behavior is the answers it gives to messages both now and in the future. (See State).
Binding	Associating a client object to a database object, which turns the client object into a Proxy
Class	Provides a common implementation for a set of objects.
Container	A Registry but without the implication of a primary registration property (e.g. a "key").
DomainModel	All the static rules, constraints, and operations that apply to DomainObjects. The DomainModel can either be conceptual to help understand the behavior of DomainObjects or it can be implemented as DomainClasses.
DomainObject	An object which captures knowledge about a domain. DomainObjects allow a computer to inspect, imply, modify, and "reason" about that information in either very simple ways (the facts) or more complex ways (the rules and implications).
Extend	To define a new Type (called a Subtype) in terms of an existing type (called a Supertype). The new Subtype will have the same contract (operations) as the Supertype but can add new functionality: as either new operations or enhancement in capability to existing operations.
Extent	The collection of all instances of a Type or Class.
ExtentRegistry	A Registry that contains all the objects of a Type (the Extent of the Type). An ExtentRegistry is a close equivalent to a RelVar or Table in the context of Objects.
Factory	An object that can create other objects.
Feature	The Eiffel term for Operation where Operation includes both methods and attributes₁.
Forwarder	A proxy which immediately forwards messages, possibly over process and machine boundaries, to the RealSubject.
Framework	A strong partition of generalized functionality common to many parts of an application. Also, more formally, a collection of interacting classes that describes most of the behavior a client requires and can be subclassed and parameterized to customize and complete the functionality.
Function_f	A functor that returns an Object
Functor	An object that models an operation.
Functor	"An object that models an operation" [Firesmith+E 95]. For a Java implementation, a basic functor is an Interface with a single, generic, operation.
Getter_f	A functor that is designed to retrieve a value from an object (the first parameter)
Identity	The ability to tell one object from another object independently of whether their appearance (behavior) is identical.
IdentityKey	A value that defines the RealIdentity of a Proxy.
Immutable	Can not be changed after being created. Immutable objects can not be changed after they are created and fully initialized.
Inherit	To define a new Class in terms of an Existing Class (the Superclass) by starting with the Superclass’s implementation and overiding or adding to it.
Instance	An object is an Instance of a Type if an object supports all the exterior requirements of that type (see "Is-A"). An object is an instance of a Class if it is implemented by that class.
Instance Variable	A way to store encapsulated state information for a particular object. Instance variables are completely hidden within the Object, but they enable two objects of the same Class to have different external Behavior.
Interface	A description of a Type focused on the Operations that the objects can respond to.
Is-A	An object is a Type if an Object supports all the exterior requirements of that Type.
Layer	A logical, horizontal division of a system that provides a particular system abstraction to the client above the layer.
Link	A connection between two objects which allows one or both to know about the other object. By default links are assumed to be bidirectional in analysis, but they can be defined to be only traversable in one direction.
Message	A stimulus sent to an object with a name and any parameters (as Objects) that the message requires. A message will cause the receiver Object to return an answer or nothing. In most Object Languages, the sender has to wait for the answer before continuing.
Method	An implementation of an operation for a particular object/class.
Module	A base level subsystem: one which does not contain any other subsystems
Object	An identifiable, encapsulated entity that can only be interacted with by sending messages.
ObjectBase	.An ObjectBase captures the knowledge, operations, and rules required to usefully represent a particular part of the world in a computer. An ObjectBase contains all the objects that represent a particular state of your DomainModel and all the knowledge contained therein. Also called an ObjectSpace.
ObjectShadow	The information needed to see that an object exists without any true representation of the real object. Relational databases could be considered to work with ObjectShadows: they record the information about an object but never have a real object to interact with.
Observable	An object that can be Observed. See Observer.
Observer	An object that "looks at" another object (the Observable) and can respond to events in the Observable without the Observable being knowledgeable about the Observer.
Operation	A description of the ability for an object to respond to a particular message and the contract/requirement for that message.
Partition	A vertical division of a system into areas of related functionality.
Predicate_f	A functor that returns a boolean
Procedure_f	A functor that does not return a value
Property	Synonym for Attribute₁ and sometimes for Attribute₂.
Prototype	An object that is used as a template for creating other Objects.
Proxy	An object that stands in for another object (the RealObject) and manages the client interaction with the RealObject.
Proxy	An object that stands in for another object (the RealObject) and manages the client interaction with the RealObject.
RealIdentity	The identity of the RealObject that a proxy represents instead of the proxy’s independent identity. For proxies we are rarely interested in their own identity, we just want to know the identity of the RealObject on the server.
Registry	An object that remembers other objects and can search through and retrieve them through one or more properties. Usually the objects within a Registry are all of the same type.
Replicate	A proxy which holds local state and performs local operations which are later propagated to the RealSubject
Role	The name of the "position" within a relationship an Object or Type holds. For example, a binary association has two roles that distinguish the two participants in the relationship.
Setter_f	A functor that stores into an object (the first parameter) a value (the second parameter)
Singleton	An object that is the only instance of a Class.
State	An abstraction to describe and simplify understanding an object’s behavior. An object’s behavior can be described as the answers it gives to current messages (which are determined by the current state) and the changes to its state caused by these messages.
Strategy	An object that encapsulates an algorithm to be used with an Object.
Stub	A proxy which acts as a placeholder for the RealObject and must become another type of proxy (for example, forwarder or replicate) when interacted with by a client.
Subsystem	A division of a system into a cohesive unit of functionality (tightly related classes and internal subsystems) with a public interface and a private implementation.
Subtype	A Type that extends another Type (called the Supertype).
Supertype	A Type that has been extended by another Type.
Tier	A level on a hierarchy of processes over which a system is divided.
Traverse	To move from one object to another by a Link. If a Link is traversable from an Object than that Object can get to (knows about) the other Object.
Type	Describes a common exterior (public behavior) of a set of Objects. Can also be used to conceptually group and understand objects by their similar behavior.
ValueObject	An object that does not have identity independent of its value. A ValueObject is immutable and should be considered identical to anything that it is equal to. Primitive data types in Smalltalk (most numbers, Symbols) are ValueObjects. Java Strings are very close to ValueObjects except they are not guaranteed to be identical for the same value (they would be if they did an automatic "intern()"). Java primitive types are not Objects.
Visitor	An object that represents an operation that can be performed on the elements of an Object structure (frequently a hierarchy or sequence).

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