Class Diagrams

A class diagram is classes, what they hold and what they do, and how they relate. It starts with class and its title.

class Shapes on a canvas
Shape [abstract]
  + name: string
  + area(): number
  + draw()
Circle
  - radius: number
  + area(): number
Square
  - side: number
  + area(): number
Canvas
  + shapes: Shape[]
  + render()
Shape <|- Circle
Shape <|- Square
Canvas o- Shape: holds

The text above: Canvas holding Shapes, and Circle and Square inheriting from the abstract Shape

Settings

Setting	Also	Value
`direction`	`flow`, `orientation`, `layout`, `rankdir`	`down` (the default), `right`, `up` or `left`

Classes and members

A class is named on a line of its own - class Name is the same - with its members indented under it. A member with brackets is an operation and goes in the class's second compartment; any other member is a field.

A member starts with its visibility, as a symbol or a word:

Symbol	Word	Visibility
`+`	`public`	public
`-`	`private`	private
`#`	`protected`	protected - written as the word, since a line starting with `#` is a comment
`~`	`internal`, `package`	internal

static before a member underlines it and abstract sets it in italic: static count: number, abstract deposit(amount). A member that belongs to no class - written at the top rather than under one - is said.

Stereotypes

A word in brackets after a class that is not a colour, a role or muted is the class's stereotype, shown above its name: [interface], [abstract], [enum], [record], [service] - any word you need.

Relationships

The plain arrows draw associations and dependencies - Order -> Customer, Report ..> Printer - and UML's ends, written with a space on each side, draw the rest:

<|-, <|--, -|>, --|> - inheritance: a hollow triangle at the more general class.
<|.., ..|> - realisation: a dashed line with a hollow triangle at the interface.
*-, *--, -*, --* - composition: a filled diamond at the whole.
o-, o--, -o, --o - aggregation: a hollow diamond at the whole.

The more general end of each relationship - the base class, the interface, the whole - is laid out above the other. A label goes after a colon: Canvas o- Shape: holds.

A line with a count at each end writes multiplicities: Bank 1-* Account. Each end is 1, 0..1, 1..* (also 1..n) or * (also 0..*, n, many).

class Accounts
Account [abstract]
  protected balance: number
  static count: number
  abstract deposit(amount)
Saving
  - rate: number
  + addInterest()
IAudited [interface]
  + audit()
Bank
  + accounts: Account[]
Account <|- Saving
Account ..|> IAudited
Bank 1-* Account: holds

The text above: a protected field, a static one underlined, an abstract operation in italic, an interface realised by a dashed line, and Bank holding many Accounts

Packages and notes

package Name - also namespace or group - frames the classes indented under it. note Class: what to say puts a note beside a class.