Class Diagrams

# Class Diagrams Class diagrams model object-oriented designs and domain models. They show entities (classes), their attributes/methods, and relationships. ## Basic Syntax ```mermaid classDiagram ClassName ``` ## Defining Classes with Members ```mermaid 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. ```mermaid classDiagram Title -- Genre ``` ### Composition (`*--`) Strong ownership - child cannot exist without parent. When parent is deleted, children are deleted. ```mermaid classDiagram Order *-- LineItem House *-- Room ``` ### Aggregation (`o--`) Weaker ownership - child can exist independently. Represents "has-a" relationship. ```mermaid classDiagram Department o-- Employee Playlist o-- Song ``` ### Inheritance (`<|--`) "Is-a" relationship. Child class inherits from parent class. ```mermaid 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. ```mermaid classDiagram OrderProcessor <.. PaymentGateway ``` ### Realization/Implementation (`<|..`) Class implements an interface. ```mermaid classDiagram class Drawable { <> +draw() } Drawable <|.. Circle Drawable "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 ```mermaid classDiagram Customer --> Order : places Order --> Product : contains Driver --> Vehicle : drives ``` ## Class Stereotypes Mark special class types: ```mermaid classDiagram class IRepository { <> +save(entity) +findById(id) } class UserService { <> +createUser() } class UserDTO { <> +String name +String email } ``` ## Abstract Classes and Methods ```mermaid classDiagram class Shape { <> +int x +int y +draw()* abstract +move(x, y) } Shape <|-- Circle Shape <|-- Rectangle ``` ## Generic Classes ```mermaid classDiagram class List~T~ { +add(item: T) +get(index: int) T } List~String~ "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 { <> PENDING PAID SHIPPED DELIVERED CANCELLED } Order --> OrderStatus ``` ## Domain-Driven Design Patterns ### Entities ```mermaid classDiagram class User { <> -UUID id +String email +String name } ``` ### Value Objects ```mermaid classDiagram class Money { <> +Decimal amount +String currency +add(other: Money) Money } class Address { <> +String street +String city +String postalCode } ``` ### Aggregates ```mermaid classDiagram class Order { <> -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 ```mermaid classDiagram class IRepository~T~ { <> +save(entity: T) +findById(id: UUID) T +delete(entity: T) } class UserRepository { +findByEmail(email: String) User } IRepository~User~ <|.. UserRepository ``` ### Factory Pattern ```mermaid classDiagram class ShapeFactory { +createShape(type: String) Shape } class Shape { <> +draw()* } ShapeFactory ..> Shape : creates Shape <|-- Circle Shape <|-- Rectangle ``` ### Strategy Pattern ```mermaid classDiagram class PaymentProcessor { -PaymentStrategy strategy +setStrategy(strategy: PaymentStrategy) +processPayment(amount: Decimal) } class PaymentStrategy { <> +pay(amount: Decimal)* } PaymentStrategy <|.. CreditCardPayment PaymentStrategy PaymentStrategy ```