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L4.2 — UML: Class Diagrams covers the most common UML diagram type — class diagrams. These show the structure of a system: its classes, attributes, operations, and the relationships between them. Covers class notation (visibility, attributes, operations), parameter kinds (in, out, inout), association, aggregation (weak "has-a"), composition (strong "contains-a"), generalisation ("is-a" / inheritance), and how to distinguish aggregation from composition in Java code.

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At-a-glance Checklist

• [ ] Explain that class diagrams are structural diagrams showing classes, attributes, operations, and relationships
• [ ] Know the three rows of a class box: name, attributes, operations
• [ ] Use visibility symbols: - (private), + (public), # (protected)
• [ ] Write attribute syntax: visibility name [multiplicity] : type = initialValue
• [ ] Write operation syntax: visibility name(kind param: type = default): returnType
• [ ] Distinguish parameter kinds: in (read only), out (produce only), inout (read and modify)
• [ ] Explain why in is preferred over inout for display/read operations (clean design)
• [ ] Draw an association (solid line with optional label, arrow, multiplicity)
• [ ] Draw aggregation (empty diamond ◇) — weak "has-a", part can exist independently
• [ ] Draw composition (filled diamond ◆) — strong "contains-a", part cannot exist without the whole
• [ ] Distinguish aggregation from composition by asking: "does the part exist without the container?"
• [ ] Draw generalisation (hollow triangle arrow) — "is-a" / inheritance
• [ ] Identify where objects are instantiated in Java to determine aggregation vs composition
• [ ] Model a system as a class diagram from a use case description

1. What is a Class Diagram?

• A structural diagram showing the classes in a system, their attributes, operations, and relationships