Use Case Diagrams

Use Case diagrams describe the high-level functional requirements of a system. They show what a system does, not how it does it.

1. Key Elements

• Actors: External entities that interact with the system (stick figures). Can be humans or other systems.
• Use Cases: Specific tasks or functions the system performs (ovals).
• System Boundary: A box defining the scope of the system.
• Associations: Lines connecting actors to use cases.

2. Advanced Relationships

Use cases can relate to each other:

• <<include>>: One use case must include another to complete its task.
  • Example: “Withdraw Cash” <<include>> “Authenticate User”.
• <<extend>>: One use case optionally adds behavior to another under specific conditions.
  • Example: “Calculate Tax” <<extend>> “Place Order” (only if tax applies).

3. Decisions: Actor or Use Case?

• Actor: Is it outside the system boundary? Does it initiate or receive something?
• Use Case: Is it a goal the actor wants to achieve?

4. Example

PlantUML Code:

left to right direction
actor "Customer" as c
actor "Bank Clerk" as b

rectangle "ATM System" {
  c -- (Withdraw Cash)
  c -- (Check Balance)
  (Withdraw Cash) ..> (Authenticate) : <<include>>
  (Check Balance) ..> (Authenticate) : <<include>>
  (Withdraw Cash) <.. (Print Receipt) : <<extend>>
  b -- (Maintenance)
}