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Lesson 3Review: use case diagram notation
ObjectiveReview the Modeling Elements of the Use Case Diagram.

Use Case Diagram Notation

The first step in developing the use case model is the creation of a use case diagram.
The diagram consists of four fundamental model elements: the system, the actors, use cases, and associations.

Review of Use Case Diagram Notation

Use case diagram
Use case diagram

  1. Actor: People, systems, and devices that have a stake in the successful operation of the system.
  2. System: Sets the context of the system in relation to the actors who use it and the features it must provide.
  3. Use case: Identifies the key features of the system. Without these features, the system will not fulfill the user/actor requirements. Each use case expresses a goal that the system must achieve.
  4. Association: Identifies an interaction between elements.

Use Case Diagrams

A use case diagram shows the relationship among use cases within a system or other semantic entity and their actors.

Use Case Diagram Semantics

Use case diagrams show actors and use cases together with their relationships. The use cases represent functionality of a system or a classifier, like a subsystem or a class, as manifested to external interactors with the system or the classifier.


A use case diagram is a graph of actors, a set of use cases, possibly some interfaces, and the relationships between these elements. The relationships are associations between the actors and the use cases, generalizations between the actors, and generalizations, extends, and includes among the use cases. The use cases may optionally be enclosed by a rectangle that represents the boundary of the containing system or classifier.

Not all tools implement the System icon because it is always the same, that is, a single rectangle for the system being described.
For all practical purposes, it is implied by the diagram itself.


Actors may be represented in the use case diagram with different icons for people, systems, and devices, or even subtypes of each of these three classifications The icons used to represent actors are not standard. The UML recommends, but does not dictate, the icons that represent model elements.


An association identifies a dialog between an actor and a use case or between use cases. An actor may initiate a dialog with a use case, as when a customer chooses the deposit function on an automated teller machine. The machine and customer continue the dialog until either an error condition occurs or the dialog is successfully completed. Focus your attention first on the associations between actors and use cases. Alternatively, a use case may initiate the dialog in the form of a notification. The notification may or may not require a response from the user.
Dialogs are covered more extensively later in this module. For now, it is sufficient for you to know that some form of dialog is required in your model and to know who is involved.

Use Case Diagram Elements

The diagram below matches the model element labels to the visual elements of the use case diagram.

Use Case System

  1. complex whole formed from related parts: a combination of related parts organized into a complex whole
  2. set of principles: a scheme of ideas or principles by which something is organized
  3. way of proceeding: a method or set of procedures for achieving something

Use case diagram consisting of 1) System, 2) Actor, 3) Association, 4) Use Case Association