Class Diagrams

Source: Embarcadero: Class Diagrams

Class diagrams

A Class diagram gives an overview of a system by showing its classes and the relationships among them. Class diagrams are static — they display what interacts but not what happens when they do interact.

The class diagram below models a customer order from a retail catalog. The central class is the Order. Associated with it are the Customer making the purchase and the Payment. A Payment is one of three kinds: Cash, Check, or Credit. The order contains OrderDetails (line items), each with its associated Item.


UML class notation is a rectangle divided into three parts: class name, attributes, and operations. Names of abstract classes, such as Payment, are in italics. Relationships between classes are the connecting links.

Our class diagram has three kinds of relationships.

association — a relationship between instances of the two classes. There is an association between two classes if an instance of one class must know about the other in order to perform its work. In a diagram, an association is a link connecting two classes.

aggregation — an association in which one class belongs to a collection. An aggregation has a diamond end pointing to the part containing the whole. In our diagram, Order has a collection of OrderDetails.

generalization — an inheritance link indicating one class is a superclass of the other. A generalization has a triangle pointing to the superclass. Payment is a superclass of Cash, Check, and Credit.

An association has two ends. An end may have a role name to clarify the nature of the association. For example, an OrderDetail is a line item of each Order.

A navigability arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item, but not the other way around. The arrow also lets you know who "owns" the association's implementation; in this case, OrderDetail has an Item. Associations with no navigability arrows are bi-directional.

The multiplicity of an association end is the number of possible instances of the class associated with a single instance of the other end. Multiplicities are single numbers or ranges of numbers. In our example, there can be only one Customer for each Order, but a Customer can have any number of Orders.

This table gives the most common multiplicities.

Multiplicities Meaning
0..1 zero or one instance. The notation n . . m indicates n to m instances.
0..* or * no limit on the number of instances (including none).
1 exactly one instance
1..* at least one instance

Every class diagram has classes, associations, and multiplicities. Navigability and roles are optional items placed in a diagram to provide clarity.

Source: Wikipedia: Class Diagram

Class diagram
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Hierarchy of UML 2.0 Diagrams, shown as a class diagram. The individual classes are represented just with one section, but they often contain up to three sections.

In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, and the relationships between the classes.

* 1 Overview
* 2 Members
o 2.1 Visibility
o 2.2 Scope
* 3 Relationships
o 3.1 Instance Level Relationships
+ 3.1.1 External links
+ 3.1.2 Association
+ 3.1.3 Aggregation
+ 3.1.4 Composition
+ 3.1.5 Differences between Composition and Aggregation
o 3.2 Class Level Relationships
+ 3.2.1 Generalization
+ 3.2.2 Realization
o 3.3 General Relationship
+ 3.3.1 Dependency
o 3.4 Multiplicity
* 4 Analysis Stereotypes
o 4.1 Boundaries
o 4.2 Entities
o 4.3 Controls
* 5 See also
* 6 References
* 7 External links

[edit] Overview

The class diagram is the main building block in object oriented modelling. They are being used both for general conceptual modelling of the systematics of the application, and for detailed modelling translating the models into programming code. The classes in a class diagram represent both the main objects and or interactions in the application and the objects to be programmed. In the class diagram these classes are represented with boxes which contain three parts: [1]
A class with three sections.

* The upper part holds the name of the class
* The middle part contains the attributes of the class
* The bottom part gives the methods or operations the class can take or undertake

In the conceptual design of a system, a number of classes are identified and grouped together in a class diagram which helps to determine the statical relations between those objects. With detailed modeling, the classes of the conceptual design are often split in a number of subclasses.

In order to further describe the behavior of systems, these class diagrams can be complemented by state diagram or UML state machine. Also instead of class diagrams Object role modeling can be used if you just want to model the classes and their relationships.[1]
[edit] Members

UML provides mechanisms to represent class members, such as attributes and methods, and additional information about them.
[edit] Visibility

To specify the visibility of a class member (i.e., any attribute or method) there are the following notations that must be placed before the member's name.[2]:

public visible to all elements that can access the contents of the namespace that owns it.

  1. protected visible to elements that have a generalization relationship to the namespace that owns it.

– private only visible inside the namespace that owns it.
~ package owned by a namespace that is not a package, and is visible to elements that are in the same package as its owning namespace. Only named elements that are not owned by packages can be marked as having package visibility. Any element marked as having package visibility is visible to all elements within the nearest enclosing package (given that other owning elements have proper visibility). Outside the nearest enclosing package, an element marked as having package visibility is not visible.
[edit] Scope

The UML specifies two types of scope for members: instance and classifier.[2] In the case of instance members, the scope is a specific instance. For attributes, it means that its value can vary between instances. For methods, it means that its invocation affects the instance state, in other words, affects the instance attributes. Otherwise, in the classifier member, the scope is the class. For attributes, it means that its value is equal for all instances. For methods, it means that its invocation do not affect the instance state. Classifier members are commonly recognized as "static" in many programming languages. To indicate that a member has the classifier scope, its name must be underlined. Otherwise, as default, the instance scope is considered.
[edit] Relationships

A relationship is a general term covering the specific types of logical connections found on class and object diagrams. UML shows the following relationships:
[edit] Instance Level Relationships
[edit] External links

A Link is the basic relationship among objects. It is represented as a line connecting two or more object boxes. It can be shown on an object diagram or class diagram. A link is an instance of an association. In other words, it creates a relationship between two classes.
[edit] Association
Class diagram example of association between two classes

An Association represents a family of links. Binary associations (with two ends) are normally represented as a line, with each end connected to a class box. Higher order associations can be drawn with more than two ends. In such cases, the ends are connected to a central diamond.

An association can be named, and the ends of an association can be adorned with role names, ownership indicators, multiplicity, visibility, and other properties. There are five different types of association. Bi-directional and uni-directional associations are the most common ones. For instance, a flight class is associated with a plane class bi-directionally. Associations can only be shown on class diagrams. Association represents the static relationship shared among the object of two classes.

Example: "department offers courses", is an association relation.
[edit] Aggregation
Class diagram showing Aggregation between two classes

Aggregation is a variant of the "has a" or association relationship; aggregation is more specific than association. It is an association that represents a part-whole or part-of relationship. As a type of association, an aggregation can be named and have the same adornments that an association can. However, an aggregation may not involve more than two classes.

Aggregation can occur when a class is a collection or container of other classes, but where the contained classes do not have a strong life cycle dependency on the container—essentially, if the container is destroyed, its contents are not.

In UML, it is graphically represented as a hollow diamond shape on the containing class end of the tree of lines that connect contained class(es) to the containing class.
[edit] Composition
Class diagram showing Composition between two classes at top and Aggregation between two classes at bottom

Composition is a stronger variant of the "owns a" or association relationship; composition is more specific than aggregation. It is represented with a solid diamond shape.

Composition usually has a strong life cycle dependency between instances of the container class and instances of the contained class(es): If the container is destroyed, normally every instance that it contains is destroyed as well. Note that a part can (where allowed) be removed from a composite before the composite is deleted, and thus not be deleted as part of the composite.

The UML graphical representation of a composition relationship is a filled diamond shape on the containing class end of the tree of lines that connect contained class(es) to the containing class.
[edit] Differences between Composition and Aggregation

When attempting to represent real-world whole-part relationships, e.g., an engine is part of a car, the composition relationship is most appropriate. However, when representing a software or database relationship, e.g., car model engine ENG01 is part of a car model CM01, an aggregation relationship is best, as the engine, ENG01 may be also part of a different car model. Thus the aggregation relationship is often called "catlog" containment to distinguish it from composition's "physical" containment.

The whole of a composition must have a multiplicity of 0..1 or 1, indicating that a part must belong to only one whole; the part may have any multiplicity. For example, consider University and Department classes. A department belongs to only one university, so University has multiplicity 1 in the relationship. A university can (and will likely) have multiple departments, so Department has multiplicity 1..*.
[edit] Class Level Relationships
[edit] Generalization
Class diagram showing generalization between one superclass and two subclasses

The Generalization relationship indicates that one of the two related classes (the subtype) is considered to be a specialized form of the other (the super type) and supertype is considered as 'Generalization' of subtype. In practice, this means that any instance of the subtype is also an instance of the supertype. An exemplary tree of generalizations of this form is found in binomial nomenclature: human beings are a subtype of simian, which are a subtype of mammal, and so on. The relationship is most easily understood by the phrase 'A is a B' (a human is a mammal, a mammal is an animal).

The UML graphical representation of a Generalization is a hollow triangle shape on the supertype end of the line (or tree of lines) that connects it to one or more subtypes.

The generalization relationship is also known as the inheritance or "is a" relationship.

The supertype in the generalization relationship is also known as the "parent", superclass, base class, or base type.

The subtype in the specialization relationship is also known as the "child", subclass, derived class, derived type, inheriting class, or inheriting type.

Note that this relationship bears no resemblance to the biological parent/child relationship: the use of these terms is extremely common, but can be misleading.

* Generalization-Specialization relationship

A is a type of B
E. g. "an oak is a type of tree", "an automobile is a type of vehicle"

Generalization can only be shown on class diagrams and on Use case diagrams.
[edit] Realization

In UML modeling, a realization relationship is a relationship between two model elements, in which one model element (the client) realizes the behavior that the other model element (the supplier) specifies. A realization is indicated by a dashed line with a unfilled arrowhead towards the supplier.

Realizations can only be shown on class or component diagrams.

A realization is a relationship between classes, interfaces, components, and packages that connects a client element with a supplier element. A realization relationship between classes and interfaces and between components and interfaces shows that the class realizes the operations offered by the interface.
[edit] General Relationship
Class diagram showing dependency between "Car" class and "Wheel" class
[edit] Dependency

Dependency is a weaker form of relationship which indicates that one class depends on another because it uses it at some point of time. Dependency exists if a class is a parameter variable or local variable of a method of another class.
[edit] Multiplicity

The association relationship indicates that (at least) one of the two related classes makes reference to the other. In contrast with the generalization relationship, this is most easily understood through the phrase 'A has a B' (a mother cat has kittens, kittens have a mother cat).

The UML representation of an association is a line with an optional arrowhead indicating the role of the object(s) in the relationship, and an optional notation at each end indicating the multiplicity of instances of that entity (the number of objects that participate in the association).

Common multiplicities are:…
0..1 No instances, or one instance (optional, may)
1 Exactly one instance
0..* or * Zero or more instances
1..* One or more instances (at least one)
1..3, 6, 9..* Mixed instances, valid syntax (albeit silly)
[edit] Analysis Stereotypes

In the early stages of a project's technical analysis, class diagrams can be used to produce early conceptual models of the system. Classes at this stage often take the form of boundaries, controls and entities and rarely survive into the design without heavy changes.
[edit] Boundaries

Boundary classes handle the communication between actors and the system's internal components. They might be user interfaces, system interfaces or device interfaces (for example). They are typically identified by each actor–use-case pair on the system's use-case diagram.

They are drawn as circles with a short line to the left attached to a vertical line the same height as the circle (as though it is attached to the side of the use-case system boundary). Alternatively, they can be drawn as normal classes with the «boundary» stereotype notation above the class name.
[edit] Entities

Entity classes model the information handled by the system, and sometimes the behaviour associated with the information. They should not be identified as database tables or other data-stores.

They are drawn as circles with a short line attached to the bottom of the circle. Alternatively, they can be drawn as normal classes with the «entity» stereotype notation above the class name.
[edit] Controls

Control classes handle the flow of control for a use-case and can therefore be seen as co-ordinating classes. These do not do everything in the use case, but co-ordinate with other classes that can do the work for them.

They are drawn as circles with a stick arrow-head pointing to the left at the top of the circle. Alternatively, they can be drawn as normal classes with the «control» stereotype notation above the class name.
[edit] See also
Search Wikimedia Commons Wikimedia Commons has media related to: Class diagram

* Executable UML
* List of UML tools

Related diagrams

* Domain model
* Entity-relationship model
* Object diagram

[edit] References

1. ^ a b Scott W. Ambler (2009) UML 2 Class Diagrams. Webdoc 2003-2009. Accessed Dec 2, 2009
2. ^ a b OMG Unified Modeling Language (OMG UML) Superstructure[dead link], Version 2.2 Beta: May 2008. Retrieved 16 November 2008.

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[edit] External links

* Introduction to UML 2 Class Diagrams
* UML 2 Class Diagram Guidelines
* IBM Class diagram Introduction
* OMG UML 2.2 specification documents

v • d • e
Unified Modeling Language
Organizations: Object Management Group • UML Partners • Persons: Grady Booch • Ivar Jacobson • James Rumbaugh

Object oriented: Object-oriented programming • Object-oriented analysis and design

Structure: Actor • Attribute • Artifact • Class • Component • Interface • Object • Package

Behavior: Activity • Event • Message • Method • State • Use case

Relationships: Aggregation • Association • Composition • Dependency • Generalization (or Inheritance)
Extensibility: Profile • Stereotype • Other concepts: Multiplicity
Structure diagrams
Class diagram • Component diagram • Composite structure diagram • Deployment diagram • Object diagram • Package diagram
Behavior diagrams
Activity diagram • State Machine diagram • Use case diagram
Interaction diagrams
Communication diagram • Sequence diagram • Interaction overview diagram • Timing diagram
Other topics
Glossary of UML terms • Rational Unified Process • Comparison of UML tools • Systems Modeling Language • UML colors • XMI
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Categories: UML diagrams
Hidden categories: All articles with dead external links | Articles with dead external links from May 2010 | Articles needing additional references from February 2009 | All articles needing additional references
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