Associations

Associations define the link between two entities in the use case diagram. These links can be between an actor and a use_case, two actors, or two use_cases.

Declaration

To declare associations in use case diagrams use characters to draw the connection between the associated entities. The default association is defined by a double hyphen (--) between an actor and a use_case.

Example: Association Declaration

@startuml
'Example: Association Declaration

'Create an association between Zarek and Stock.
:Zarek: -- (Stock)

@enduml

Properties

Associations have eight properties. Entity_1, entity_2, and association_type are required. The properties generally follow the below order. Direction is placed inside of the association type.

• entity_1 - an actor or use_case

• association_type - defined by a character drawing as a line or arrow

• direction - determines the direction that the association line or arrow is drawn

• entity_2 - an actor or use_case

• line_color - defines the color of the association line or arrow

• line_type - defines the type of line drawn for the association

• text_color - determines the color of the text

• text - can be used to explain the association

Entity_1 & Entity_2

The entity fields contain either an actor name or use_case description. If either has an alias, use the alias.

Example: Association Entities

@startuml
'Example: Association Entities

'Create an actor.
:Zarek:

'Create a use_case. 
(Stock)

'Create an association between the two entities.
Zarek -- Stock

@enduml

Association Type

Association_type is determined by a character drawing of a line or arrow. The three association types are shown below.

• -- draws a line between the two entities

• -- > draws an arrow between the two entities

• --|> draws an open arrow between the two entities

Example: Association Type

@startuml
'Example: Association Type

'Create a single line association between two entities.
:Zarek: -- (Stock)

'Create an arrow association between two entities.
:Ivy: --> (Bag)

'Create an open arrow association between two entities.
(Cleaning) --|> (Mop)

@enduml

Direction

There are three methods you can use to determine the direction of association lines.

• direction_parameter - a setting used to determine association direction

• character_drawing - drawing the direction of the association with characters

• directional_word - determines the direction of the arrow with words

Note: Mixing direction_parameter with character_drawing or directional_word will result in the association line pointing the wrong direction.

Direction Parameter

The default direction for a use case diagram is top to bottom. The direction can be changed with the direction_parameter.

Example: Direction Parameter

@startuml
'Example: Use Case Direction Parameter

'Change the default association direction  to left to right.
left to right direction

'Create a single line association between two entities.
:Zarek: -- (Stock)

'Create an arrow association between two entities.
:Ivy: --> (Bag)

'Create an open arrow association between two entities.
(Cleaning) --|> (Mop)

@enduml

Character Drawings

Direction can be determined by the number of dashes in the drawing as well as placement of the greater than or less than sign.

• -- > draws a downward arrow

• - > draws an arrow to the right

• < -- draws an upward arrow

• < - draws an arrow to the left

Example: Direction Character Drawings

@startuml
'Example: Use Case Character Drawings

'Create an association with an arrow pointing right.
:Zarek: -> (Right)

'Create an association with an arrow pointing left.
(Left) <- :Ivy:

'Create an association with an arrow pointing down.
:Sean: --> (Down)

'Create an association with an arrow pointing up.
(Up) <-- :Maria:

@enduml

Directional Words

Direction can be determined by adding directional words to the middle of the association_type between the hyphens.

Example: Directional Words

@startuml
'Example: Use Case Directional Words

'Create an association with an arrow pointing right.
:Zarek: -Right-> (Right)

'Create an association with an arrow pointing left.
:Ivy:   -Left-> (Left)

'Create an association with an arrow pointing down.
:Sean:  -Down-> (Down)

'Create an association with an arrow pointing up.
:Maria: -Up-> (Up)

@enduml

Line Color

The line_color is defined by a standard color name or hex code. If you use this property alone it must come after a hash (#) sign and touch the hash sign. If any other properties follow this place a semicolon (;) between them. No spaces are needed.

Example: Line Color

@startuml
'Example: Association Line Color

'Create an association with a colored line.
:Zarek: -- (Stock) #Blue

@enduml

Line Type

The line_type defines the texture of the exterior line of the actor. If you use this property alone it must come after a hash (#) sign and touch the hash sign. If any other properties follow this place a semicolon (;) between them. No spaces are needed.

The line_types are as follows.

• line.bold

• line.dashed

• line.dotted

Example: Line Type

@startuml
'Example: Association Line Type

'Create an association with a bold line.
:Zarek: -- (Bold) #line.bold

'Create an association with a dashed line.
:Zarek: -- (Dashed) #line.dashed

'Create an association with a dotted line.
:Zarek: -- (Dotted) #line.dotted

@enduml

Text Color

The text_color determines the color of the text and is defined by a standard color name or hex code. If you use this property alone it must come after a hash (#) sign and touch the hash sign. If any other properties follow this place a semicolon (;) between them. No spaces are needed.

Example: Text Color

@startuml
'Example: Association Text Color

'Create an association with colored text.
:Zarek: -- (Stock) #text:blue : Text

@enduml

Text

The text field can be used to describe the association between the two entities. Text supports creole for emphasis.

Example: Text

@startuml
'Example: Association Text

'Create an association with multiline text.
'Use creole for emphasis.
:Zarek: -- (Stock) : Text\n**Stuff**

@enduml

With All Properties

Example: Association With All Properties

@startuml
'Example: Association With All Properties

'Create an association with all properties.
:Zarek: -left- (Stock) #blue;line.dashed;text:green : Text\n**Stuff**

@enduml