Agenda

1. Quick recap
2. What are class diagrams?
3. Core elements of class diagrams
4. Associations, multiplicity & composition
5. Advanced concepts (generalization, interfaces, dependencies)
6. Interactive exercise 1: Identify model elements
7. Design heuristics and good practices
8. Interactive exercise 2: Build a small class model
9. Wrap-up & next steps

Recap from Last Class

• Why modeling is crucial
• How abstraction helps communication
• First exposure to diagrams (morning routine exercise)

Today: move from informal to formal models.

What is a Class Diagram?

• Purpose: describes the static structure of a system
• Main elements: classes, attributes, operations, relationships
• Used for:
  • Domain modeling
  • Design-level documentation
  • Communication between stakeholders

Sample class diagram

Order processing

Core Elements

Attribute ::= visibility name: type multiplicity = default {props}

Operation ::= visibility name(parameter-list): return-type {props}

Parameter ::= direction name: type = default_value

Associations

• Represent relationships between classes
• Can have:
  • Roles: names describing relationship ends
  • Multiplicity: number of instances
  • Navigability: direction of the relationship

Properties vs Associations:

Properties

Associations

Bidirectional Associations

• Pair of properties which are linked together as inverses

A bidirectional association

If I start with a car, take its owner, then take the cars property of the owner, then I should find the original car among those cars.

Another way to show a bidirectional association

Aggregation vs Composition

• Aggregation (◇): “has-a” relationship, but parts can exist independently

  

• Composition (⬧): “owns-a” relationship, parts die with the whole

  

Generalization and Interfaces

• Generalization: inheritance between classes

  

• Implementation of an interface

  

Dependencies

• A dependency exists between two elements if
  • changes to the definition of one element (the supplier or target)
  • may cause changes to the other (the client or source).
• Indicated with a dashed arrow

Example – Interfaces and abstract classes in Java

Interfaces and abstract classes in Java – expanded view

Interfaces and abstract classes in Java – ball-and-socket view

Interactive Exercise 1: Spot the Elements

Class: Library
 - name: String
 - books: List<Book>
 + addBook(b: Book)
 + findBook(title: String): Book

Class: Book
 - title: String
 - author: String
 + borrow()
 + return()

Task (5 minutes): Identify classes, attributes, operations, and their relationships.

Design Heuristics & Good Practices

• Favor composition over inheritance
• Keep diagrams readable (<20 classes per diagram)
• Use consistent naming conventions
• Model only what’s necessary
• Document assumptions and constraints

Example – over-complicated diagram

Example – Simplified (conceptual) diagram

Interactive Exercise 2: Build a Class Diagram

Scenario: Online food delivery system.

• Entities: Customer, Restaurant, Order, MenuItem, DeliveryDriver
• Operations: place order, assign driver, calculate total

Task (15 minutes):

• Form small groups
• Sketch a class diagram
• Show associations, multiplicities, and at least one inheritance relationship

Then: Present and discuss different design choices.

Wrap-Up

Today’s takeaways:

• Class diagrams model the structure of systems
• Associations and multiplicities matter
• Composition, inheritance, and dependencies define relationships
• Modeling requires balance: detail vs. clarity

Next class: requirements analysis