UML class diagram

Draft

Definition

UML Class diagrams are one kind of official diagram types of the Unified modeling language (UML). They describe the structure of a system.

“If someone were to come up to you in a dark alley and sy, "Psst, wanna see a UML diagram?" that diagram would probably be a class diagram.”(Fowler, 1993:35).

“A class diagram describes the types of objects in the system and the various kinds of static relationships taht exist amoung them. Class diagrams also show the properties and oprations of a class and the constraints that apply to the way objects are connected. The UML uses the term feature' as a general term that covers properties and oprations of a class.” (Fowler, 1993:35).

Architecture

Class diagrams are made with quite a complex "language". Here are some elements (I know I have to go over this - Daniel K. Schneider 16:45, 5 June 2007 (MEST))

Classes

• Classes are represented with boxes
• Represented by: a rectangle with one, two or three "fields": classname, properties (attributes) and operations.
• Properties are structural elements of class. Now it get's a little bit complicated. You can describe a property either with an attribute (i.e. within the class box) or with an association (see below).

Association

• A relationship between two classes or properties
• Represented by: a solid line, or if directional with an arrow directed from the source object to the target object, or if bidirectional with an arrow on both ends.

Aggregation

• A part-of relationship
• Represented by: a solid line, with an emtpy lozange at aggregation end and arror at member end
• Example: a learning object is part of an environment

Generalization

• A is-a relationship
• Represented by: A solid line with a triangular arrow from specialized class to class
• Example: Learner is a role

Composition

• Like aggregation but you can add more constraints.
• An instance of a class can be potentially a component of several classes, but can only be owned by one.
• Represented by: a solid line, with an filled lozange at aggregation end and arror at member end

Multiplicity

• of an association end is the number of possible instances of the class associated with a single instance of the other end.
• Represented on both ends by the following syntax

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

More

• There is more (sorry)...

Examples

IMS Learning Design

Here is the official UML Diagram that summarizes the IMS Learning Design educational modeling language:

UML diagram of IMS Learning Design

Here is an annotated version. Blue boxes (roughly) give an idea of the relationships involved.

UML diagram of IMS Learning Design annotated with UML relationships

References

On line tutorials

• IMS Learning Design Information Model, Version 1.0 Final Specification, HTML, retrieved 16:45, 5 June 2007 (MEST).

• Randy Miller, Practical UML: A Hands-On Introduction for Developers, Borland Developer network. (Short and excellent general UML tutorial).

• Donald Bell, UML basics: An introduction to the Unified Modeling Language, IBM Developper Works / Rational Rose. (IBM has a lot of UML and use case tutorials, needs some searching skills)

Books

• Fowler Martin (1993). UML Distilled: A Brief Guide to the Standard Object Modeling Language, Third Edition. Addison-Wesley Professional. ISBN 0321193687 (good for quick reference and good enough if you just want to understand how to read UML diagrams).

• Pilone, Dan and Neil Pitman (2005). UML 2.0 in a Nutshell, O'Reilly Media, ISBN 0596007957 (this is the best book Daniel K. Schneider bought, better than the first edition).