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L4.1 — UML: Use Case Diagrams introduces the software engineering big picture (waterfall vs agile), abstraction and models, the Unified Modelling Language (UML), and use case diagrams — the first tool for modelling a system from the user's perspective. Covers use case descriptions, use case scenarios, actors, use cases, system boundaries, and the include/extend relationships.

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At-a-glance Checklist

• [ ] Explain the stages of the software development process (requirements → design → implementation → testing → deployment)
• [ ] Compare waterfall vs agile approaches — when to use each
• [ ] Define abstraction and explain why models are useful
• [ ] Explain the purpose of UML and why a unified language is needed
• [ ] Write a use case description (name, system, actors, stakeholders, preconditions, main flow, extensions)
• [ ] Write a use case scenario from a problem description
• [ ] Identify actors by extracting nouns from a scenario
• [ ] Identify use cases by extracting verbs from a scenario
• [ ] Know when to merge or split use cases for the right level of detail
• [ ] Draw a use case diagram with actors, use cases, and a system boundary
• [ ] Use include relationships (always happens — dashed arrow with <<include>>)
• [ ] Use extend relationships (sometimes happens — dashed arrow with <<extend>>)
• [ ] Know the difference in arrow direction between include and extend
• [ ] Identify extension points in a use case diagram

1. The Software Engineering Big Picture

The software development process follows these stages: