The Unified Modeling Language User Guide SECOND EDITION [Electronic resources] نسخه متنی

Terms and Concepts

A use case diagram is a diagram that shows a set of use cases and actors and their relationships.

Common Properties

A use case diagram is just a special kind of diagram and shares the same common properties as do all other diagramsa name and graphical contents that are a projection into a model. What distinguishes a use case diagram from all other kinds of diagrams is its particular content.

Contents

Use case diagrams commonly contain

• Subject

• Use cases

• Actors

• Dependency, generalization, and association relationships

Like all other diagrams, use case diagrams may contain notes and constraints.

Use case diagrams may also contain packages, which are used to group elements of your model into larger chunks. Occasionally, you'll want to place instances of use cases in your diagrams as well, especially when you want to visualize a specific executing system.

Notation

The subject is shown as a rectangle containing a set of use case ellipses. The name of the subject is placed within the rectangle. The actors are shown as stick figures placed outside the rectangle; their names are placed under them. Lines connect actor icons to the use case ellipses with which they communicate. Relationships among use cases (such as extend and include) are drawn inside the rectangle.

Common Uses

You apply use case diagrams to model the use case view of a subject, such as a system. This view primarily models the external behavior of a subjectthe outwardly visible services that the subject provides in the context of its environment.

When you model the use case view of a subject, you'll typically apply use case diagrams in one of two ways.

1. To model the context of a subject

Modeling the context of a subject involves drawing a line around the whole system and asserting which actors lie outside the subject and interact with it. Here, you'll apply use case diagrams to specify the actors and the meaning of their roles.

2. To model the requirements of a subject

Modeling the requirements of a subject involves specifying what that subject should do (from a point of view of outside the subject) independent of how that subject should do it. Here, you'll apply use case diagrams to specify the desired behavior of the subject. In this manner, a use case diagram lets you view the whole subject as a black box; you can see what's outside the subject and you can see how that subject reacts to the things outside, but you can't see how that subject works on the inside.