CSCI 370 Lec 18: Use Case, Sequence, and State Diagrams in UML

Thursday, April 24

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Use Case Diagrams: Modeling System Interactions

Use case diagrams provide a high-level overview of a system’s functional requirements and how different users (actors) interact with the system.

Core Elements

1. Actors: Stick figures outside the system box representing users or external systems.
2. System Boundary: A large rectangle labeled with the system name (e.g., “Online Shopping System”).
3. Use Cases: Ovals inside the system boundary that represent activities or services the system offers.

4. Relationships:

   • Association: A solid line from an actor to a use case indicating interaction.
   • «include»: Indicates that one use case includes behavior from another.
   • «extend» (not shown here): Represents optional or conditional behavior.

Example Breakdown

In the diagram titled Online Shopping System:

• Actors include: Customer, Authentication, Identity Provider, Credit Payment Service, and PayPal.

• Use Cases include:

  • Log In
  • View Items
  • Make Purchase
  • Complete Checkout
• The customer interacts with all use cases, while external services (e.g., PayPal) connect only to specific processes such as checkout.

[Insert image: insert__use_case_diagram__here]

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Context Diagrams: System Ecosystem Overview

A context diagram offers a simplified, high-level view of the system and its interactions with external entities.

Characteristics:

• The system is represented as a red circle or box in the center.
• All external actors or systems are positioned around it.
• Arrows indicate the flow of data or interaction.

This diagram is useful for visualizing what systems or users communicate with your application, without going into implementation details.

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Activity Diagrams with Swimlanes

Activity diagrams visualize workflows within a system, often structured using swimlanes to denote different roles or components.

Swimlane Explanation:

• Each lane corresponds to a different participant (e.g., Staffing Dept, HR Dept, General Manager).
• Activities are placed in lanes based on who performs them.
• Diamonds represent decision points.

• Bars (forks/joins):

  • Horizontal bars with multiple arrows splitting = parallel execution.
  • Multiple incoming arrows joined into one = wait for all tasks to complete before proceeding.

[Insert image: insert__activity_diagram_with_swimlanes__here]

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Sequence Diagrams: Timeline of Interactions

Sequence diagrams illustrate how objects interact over time.

Key Concepts:

• Actors and Objects are placed horizontally.
• Lifelines (dotted vertical lines) track each participant’s timeline.
• Activation Bars (solid rectangles) show when an object is active.

• Messages:

  • Solid arrow: method call or action
  • Dotted arrow: return or response
• Read from top to bottom to follow the sequence of events.

Example:

• User requests money via ATM
• ATM sends request to bank
• Bank checks balance and replies
• ATM displays result to user

Sequence diagrams are ideal for modeling real-time system behavior.

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State Diagrams: Tracking Object State Transitions

State diagrams model the lifecycle of a single object as it transitions between various states.

Components:

• States: Represented as labeled rounded rectangles.
• Transitions: Arrows showing how and why the system changes from one state to another.
• Initial state: Shown with a filled black circle.
• Final state: Shown with a bullseye target symbol.

Example:

For a car engine and transmission system:

• Engine: States include Off and On
• Transmission: States include Park, Drive, Reverse, etc.
• Valid transitions depend on the engine being on.

This diagram helps reason about allowable and disallowed system conditions and transitions.

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Summary of Diagram Types:

Diagram Type	Purpose	Notable Features
Use Case	Shows user interaction with system	Actors, use cases, «include» links
Context	High-level external system relationships	System as a circle; external actors
Activity	Workflow and task order	Swimlanes, decisions, forks/joins
Sequence	Time-ordered object interactions	Lifelines, activation, messages
State	Object state transitions	Initial/final states, transitions

Each diagram plays a distinct role in software modeling, helping developers and stakeholders visualize and analyze different system aspects.