# UML Class Diagrams — Object-Oriented Programming (OOP)

Source: https://www.skillbyai.com/en/oop/r-uml

> Read and draw UML class diagrams with visibility, multiplicity and relationship arrows.

## A shared picture of the design

**UML class diagrams** are the standard way to sketch object-oriented designs. A class is a box with three compartments: **name**, **attributes** and **operations**. **Visibility** markers: `+` public, `-` private, `#` protected, `~` package. Abstract classes and methods are shown in *italics* (or with `{abstract}`), interfaces with `«interface»`, and static members underlined. Relationships use distinct arrows: **inheritance (generalisation)** is a solid line with a **hollow triangle** pointing to the parent; **interface realisation** is a **dashed** line with a hollow triangle; **association** is a plain solid line, optionally with an arrow for direction; **aggregation** has a **hollow diamond** at the whole's end; **composition** a **filled diamond**; **dependency** a dashed arrow. **Multiplicity** labels the ends: `1`, `0..1`, `*` or `1..*`. You do not need every detail; use UML to communicate the important structure quickly in design discussions, documentation and exams. Text-based tools such as **PlantUML** and **Mermaid** generate diagrams from code-like descriptions, which fit nicely in version control.

## A library system in PlantUML

Text that renders as a class diagram with the standard notation.

```text
@startuml
interface Borrowable {
  +borrow(member: Member): Loan
}
abstract class Item {
  -id: String
  -title: String
  +{abstract} loanDays(): int
}
class Book extends Item {
  -isbn: String
  +loanDays(): int
}
class Dvd extends Item {
  +loanDays(): int
}
class Member {
  -name: String
}
class Loan {
  -due: LocalDate
}
class Library

Item ..|> Borrowable
Library "1" *-- "*" Item : owns >
Library "1" o-- "*" Member : registers >
Member "1" -- "0..*" Loan
Loan "*" --> "1" Item
@enduml
```

## Diamonds sit on the whole

In aggregation and composition, the diamond is drawn at the end of the **whole** (the department, the order), not the part. Exams often check this detail.

**Quiz:** In UML, how is composition shown?

- [ ] A dashed line with a hollow triangle
- [ ] A solid line with a hollow triangle
- [x] A solid line with a filled diamond at the whole's end
- [ ] A dashed arrow

*Answer:* A solid line with a filled diamond at the whole's end. Composition uses a filled diamond; aggregation uses a hollow one.