UML Use Case Diagrams

Before we start

Why this matters

Outline · Lab VII

Topics in this chapter

Select a topic to jump to the slide that explains it.

1. Use cases
2. Notation
3. Relationships
4. The ATM example
5. Building your diagram
6. Common mistakes

Concept 1 of 12

What a use case diagram shows

Concept 2 of 12

The notation

Concept 2 of 12

The notation (cont.)

Notation

Actors: roles, not people

Concept 4 of 12

Include, extend and generalization

Concept 4 of 12

Include, extend and generalization (cont.)

Interactive

Include or extend?

Concept 6 of 12

The ATM use case diagram

Concept 6 of 12

The ATM use case diagram (cont.)

• Customer (primary) starts all four transactions.
• Bank server (secondary) is involved in each, because the ATM must ask the bank for account data.
• Every transaction «include»s Authenticate: it always happens first.
• Handle excess amount «extend»s Withdraw cash at the extension point “excess amount” — only when the amount is too large.

Redrawn from the manual’s Figure VII.14 (Request Cash, Request Balance, Request Statement, Request Balance Transfer, User Identify, Process Excess Amount).

The ATM example

Describing a use case in text

The diagram is only an index. Each use case also needs a use case description (specification):

Interactive

Walk through the scenarios

Concept 9 of 12

Finding actors and use cases

Building your diagram

Drawing it step by step

1. Draw the system boundary and name it.
2. Place primary actors on the left, secondary actors on the right.
3. Add use cases inside the box, named verb + object, in a sensible order.
4. Connect each actor to its use cases with associations.
5. Look for steps shared by several use cases → «include».
6. Look for optional or exceptional behaviour → «extend» (name the extension point).
7. Use generalization only for true “is a kind of” relations.
8. Give use cases IDs (UC-01…) and write a description for each.

In Visual Paradigm: Diagram Navigator → Use Case Diagram → New; pick System, Actor and Use Case from the toolbar; drag from a shape’s resource icon to create connected use cases, «include» and «extend»; use Package to group use cases; set ID formats under Tools → Options → Diagramming.

Building your diagram

Drawing it step by step (cont.)

PlantUML source

@startuml
left to right direction
actor Customer
actor "Bank server" as Bank
rectangle "ATM system" {
  usecase "Withdraw cash" as W
  usecase "Authenticate" as A
  usecase "Print receipt" as P
}
Customer -- W
W -- Bank
W ..> A : <<include>>
P ..> W : <<extend>>
@enduml

Output · the diagram PlantUML draws

The same idea written in PlantUML, a text-to-diagram tool: you type the text on top and it draws the diagram below.

Concept 11 of 12

Common mistakes

Your turn

Your turn: the Library Management System

Worked example 1 of 3

Worked example 1

Worked example 2 of 3

Worked example 2

Worked example 3 of 3

Worked example 3

Practice

Practice (1 of 2)

Try each one on paper first, then check your answer.

Practice

Practice (2 of 2)

Try each one on paper first, then check your answer.

Check yourself

Quiz

Resources

Further learning

Hand-picked material to go deeper. Links open in a new tab.

Resources

Further learning (cont.)

Summary

Summary

• A use case is a user goal plus its scenarios; the diagram shows actors, use cases and the system boundary.
• Actors are roles outside the system: primary actors start use cases, secondary actors support them.
• «include»: always, base → included. «extend»: optional, extension → base.
• Generalization means “is a kind of” for actors or use cases.
• Every use case needs a description with main and alternative flows.
• Avoid steps as use cases, order arrows, and internal parts drawn as actors.