UML class diagrams and sequence diagrams — Low-Level Design (LLD) — Object Design, UML and Classic Problems in C#

2. UML class diagrams and sequence diagrams

Basic★ Must knowLesson 2 of 8

Class diagram notation:

Symbol	Meaning	Example
`+` / `-` / `#`	public / private / protected	`+Park(vehicle): Ticket`
Solid line + hollow triangle	Inheritance (is-a)	`Car ──▷ Vehicle`
Dashed line + hollow triangle	Implements interface	`UpiPayment ┈┈▷ IPayment`
Filled diamond	Composition — part can't exist without the whole	`Floor ◆── Spot`
Hollow diamond	Aggregation — part can exist independently	`Team ◇── Player`
Plain arrow	Association / dependency	`Ticket ──> Spot`
`1`, `0..1`, ``, `1..`	Multiplicity	`ParkingLot 1 ── * Floor`

mermaid

classDiagram
    class ParkingLot { +Park(Vehicle) Ticket +Unpark(Ticket) Receipt }
    class Floor { +FindSpot(VehicleType) Spot }
    class Spot { +SpotType Type +bool IsFree +Occupy(Vehicle) +Release() }
    class Vehicle { <<abstract>> +string Number +VehicleType Type }
    ParkingLot "1" *-- "*" Floor
    Floor "1" *-- "*" Spot
    Vehicle <|-- Car
    Vehicle <|-- Bike

Sequence diagram — the order of calls between objects for one use case:

mermaid

sequenceDiagram
    participant G as EntryGate
    participant L as ParkingLot
    participant F as Floor
    participant T as TicketService
    G->>L: Park(car)
    L->>F: FindSpot(Car)
    F-->>L: spot A-12
    L->>T: Issue(car, spot)
    T-->>G: Ticket #881