Unified Modeling Language (UML) is a non-proprietary, third generation modeling language. The Unified Modelling Language is an open method used to specify, visualise, construct and document the artifacts of an object-oriented software-intensive system under development. The UML represents a compilation of "best engineering practices" which have proven successful in modelling large, complex systems, especially at the architectural level. See software architecture.

UML integrates the concepts of Booch, OMT and OOSE by fusing them into a single, common and widely usable modelling language. UML aims to be a standard modelling language which can model concurrent and distributed systems.

UML is not an industry standard, but is taking shape under the auspices of the Object Management Group (OMG). OMG has called for information on object-oriented methodologies, that might create a rigorous software modeling language. Many industry leaders have responded in earnest to help create the standard.

There are three prominent models of the UML system development:

  • Functional Model: Showcases the functionality of the system from the User's Point of View. Includes Use Cases Diagrams.

  • Dynamic Model: Showcases the internal behaviour of the system. Includes Sequence Diagrams, Activity Diagrams, Statechart Diagrams.

  • Object Model: Showcases the structure and substructure of the system using objects, attributes, operations, and associations. Includes Class Diagrams

It is important to distinguish between a UML model, and a UML diagram, or set of diagrams, including Use Case Diagram, Collaboration Diagram, Activity Diagram, Sequence Diagram, Deployment Diagram, Component Diagram, Class Diagram, StateChart Diagram -- a UML diagram is a graphical representation of the information in the model, but the model exists independently. XMI in its current version provides interchange for the model, but not for the diagrams.

UML uses a graphical notation which has text equivalents in Java and other Object-oriented languages, and also ontological equivalents which are high-level enough to merit articles in Wikipedia. To show the degree of development of this language, it is possible to state concepts such as political processes in UML notation. Thus, it is possible to translate these schemas into executable programming languages.

Table of contents
1 UML Use Cases Diagram
2 UML Class Diagram
3 UML Sequence Diagram

UML Use Cases Diagram

This diagram describes the functionality of the (simple) Restaurant System. The Food Critic actor can Eat Food, Pay for Food, or Drink Wine. Only the Chef Actor can Cook Food. Use Cases are in oval and Actors are stick figures. The box defines the Use Cases location within the Restaurant System.

The OMG defines a graphical notation for use cases; but it refrains for defining any written format for describing use cases in detail. Many people thus suffer under the misapprehension that a use case is its graphical notation; when in fact it is the written description of scenarios to provide a business task which provides the true value of a use case.

UML Class Diagram

This diagram describes the structure of a simple Restaurant System. UML shows is_a relationships with a triangle; and containers with diamond shape. Additionally, the role of the relationship may be specified as well as the cardinality (in the diagram above, the roles are accidentally on the wrong end of the lines --- for example, seats should be at the association end that terminates on Patrons --- see Fowler and Scott, UML Distilled, 2nd Edition, page 56 or Mellor and Balcer, Executable UML, page 214). The Restaurant System has any number of Food dishes, and one Kitchen, Dining Area, and any number of staff. All of these objects are associated to one Restaurant. (This model did not include a washroom.)

UML Sequence Diagram

This diagram describes the sequences of messages of the (simple) Restaurant System. This diagram represents a Patron ordering food, eating the food, drinking wine, then paying for the food. The dotted lines extending downwards indicate the timeline. The arrows represent messages (stimuli) from an actor or object to other objects. For example, the Patron sends message 'pay' to the Cashier.

  • Actor
  • Activity
  • Interface
  • Package
  • Class
  • Statechart,
  • Sequence
  • Event

  • IS-A
  • HAS
  • USES

  • There is a cardinality notation which corresponds to Database Modelling cardinality , eg: 1:0..1, 1:*

  • There are specialized classes for common uses, such as Role, Use Case
  • The concept of a stereotype is built-in to the language

See also: Open Source UML programs:
Article based on Unified Modeling Language at FOLDOC, used with permission.
Message board for UML at [1]
''A free UML tutorial