Class Diagrams

Class diagrams model object-oriented designs and domain models. They show entities (classes), their attributes/methods, and relationships.

Basic Syntax

classDiagram
    ClassName

Defining Classes with Members

classDiagram
    class BankAccount {
        +String owner
        +Decimal balance
        -String accountNumber
        +deposit(amount)
        +withdraw(amount)
        +getBalance() Decimal
    }

Visibility modifiers:

• + Public
• - Private
• # Protected
• ~ Package/Internal

Member syntax:

• +type attribute - Attribute with type
• +method(params) ReturnType - Method with parameters and return type

Relationships

Association (--)

Loose relationship where entities use each other but exist independently.

classDiagram
    Title -- Genre

Composition (*--)

Strong ownership - child cannot exist without parent. When parent is deleted, children are deleted.

classDiagram
    Order *-- LineItem
    House *-- Room

Aggregation (o--)

Weaker ownership - child can exist independently. Represents "has-a" relationship.

classDiagram
    Department o-- Employee
    Playlist o-- Song

Inheritance (<|--)

"Is-a" relationship. Child class inherits from parent class.

classDiagram
    Animal <|-- Dog
    Animal <|-- Cat
    
    class Animal {
        +String name
        +makeSound()
    }
    
    class Dog {
        +bark()
    }

Dependency (<..)

One class depends on another, often as a parameter or local variable.

classDiagram
    OrderProcessor <.. PaymentGateway

Realization/Implementation (<|..)

Class implements an interface.

classDiagram
    class Drawable {
        <<interface>>
        +draw()
    }
    Drawable <|.. Circle
    Drawable <|.. Rectangle

Multiplicity

Show how many instances participate in a relationship:

classDiagram
    Customer "1" --> "0..*" Order : places
    Order "1" *-- "1..*" LineItem : contains
    Author "1..*" -- "1..*" Book : writes

Common multiplicities:

• 1 - Exactly one
• 0..1 - Zero or one
• 0..* or * - Zero or many
• 1..* - One or many
• m..n - Between m and n

Relationship Labels

classDiagram
    Customer --> Order : places
    Order --> Product : contains
    Driver --> Vehicle : drives

Class Stereotypes

Mark special class types:

classDiagram
    class IRepository {
        <<interface>>
        +save(entity)
        +findById(id)
    }
    
    class UserService {
        <<service>>
        +createUser()
    }
    
    class UserDTO {
        <<dataclass>>
        +String name
        +String email
    }

Abstract Classes and Methods

classDiagram
    class Shape {
        <<abstract>>
        +int x
        +int y
        +draw()* abstract
        +move(x, y)
    }
    
    Shape <|-- Circle
    Shape <|-- Rectangle

Generic Classes

classDiagram
    class List~T~ {
        +add(item: T)
        +get(index: int) T
    }
    
    List~String~ <-- StringProcessor

Comprehensive Example: E-Commerce Domain

classDiagram
    %% Core entities
    class Customer {
        +UUID id
        +String email
        +String name
        +Address shippingAddress
        +placeOrder(cart: Cart) Order
        +getOrderHistory() List~Order~
    }
    
    class Order {
        +UUID id
        +DateTime orderDate
        +OrderStatus status
        +Decimal total
        +calculateTotal() Decimal
        +ship()
        +cancel()
    }
    
    class LineItem {
        +int quantity
        +Decimal pricePerUnit
        +getSubtotal() Decimal
    }
    
    class Product {
        +UUID id
        +String name
        +String description
        +Decimal price
        +int stockQuantity
        +reduceStock(quantity: int)
        +isAvailable() bool
    }
    
    class Category {
        +String name
        +String description
    }
    
    class Cart {
        +addItem(product: Product, quantity: int)
        +removeItem(product: Product)
        +getTotal() Decimal
        +clear()
    }
    
    %% Relationships
    Customer "1" --> "0..*" Order : places
    Customer "1" --> "1" Cart : has
    Order "1" *-- "1..*" LineItem : contains
    LineItem "1" --> "1" Product : references
    Product "0..*" --> "1" Category : belongs to
    Cart "1" o-- "0..*" Product : contains
    
    %% Enums
    class OrderStatus {
        <<enumeration>>
        PENDING
        PAID
        SHIPPED
        DELIVERED
        CANCELLED
    }
    
    Order --> OrderStatus

Domain-Driven Design Patterns

Entities

classDiagram
    class User {
        <<entity>>
        -UUID id
        +String email
        +String name
    }

Value Objects

classDiagram
    class Money {
        <<value object>>
        +Decimal amount
        +String currency
        +add(other: Money) Money
    }
    
    class Address {
        <<value object>>
        +String street
        +String city
        +String postalCode
    }

Aggregates

classDiagram
    class Order {
        <<aggregate root>>
        -UUID id
        +addLineItem(item)
        +removeLineItem(item)
    }
    
    Order *-- LineItem

Tips for Effective Class Diagrams

1. Start with core entities - Add attributes and methods incrementally
2. Show only relevant details - Omit obvious getters/setters unless important
3. Use appropriate relationships - Choose between association, aggregation, and composition carefully
4. Add multiplicity - Clarifies how many instances participate
5. Group related classes - Use notes or visual proximity
6. Document invariants - Use notes to explain business rules

Common Patterns

Repository Pattern

classDiagram
    class IRepository~T~ {
        <<interface>>
        +save(entity: T)
        +findById(id: UUID) T
        +delete(entity: T)
    }
    
    class UserRepository {
        +findByEmail(email: String) User
    }
    
    IRepository~User~ <|.. UserRepository

Factory Pattern

classDiagram
    class ShapeFactory {
        +createShape(type: String) Shape
    }
    
    class Shape {
        <<abstract>>
        +draw()*
    }
    
    ShapeFactory ..> Shape : creates
    Shape <|-- Circle
    Shape <|-- Rectangle

Strategy Pattern

classDiagram
    class PaymentProcessor {
        -PaymentStrategy strategy
        +setStrategy(strategy: PaymentStrategy)
        +processPayment(amount: Decimal)
    }
    
    class PaymentStrategy {
        <<interface>>
        +pay(amount: Decimal)*
    }
    
    PaymentStrategy <|.. CreditCardPayment
    PaymentStrategy <|.. PayPalPayment
    PaymentProcessor --> PaymentStrategy