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Software Architecture Guide

UML Association

Explanation

Section titled “Explanation”

An association is a line in a UML class diagram (see UML Class Diagram). It connects two classes. It says that at least one of the two classes refers to the other.

Drawing it

A binary association is a solid line between two classes. A reflexive (self) association is a binary association that connects a class to itself.

Names and role names

The line may carry a name. Each end may carry a role name. Officially, a role name is the association end name. It indicates the attribute name used in code.

Multiplicity

An end may show a multiplicity such as * or 0..1. Multiplicity states how many objects take part in the association. You read it from the perspective of the other end.

Navigability

An open arrowhead on one end documents that the class at that end can be reached efficiently. A cross on an end marks it as not navigable. A navigability arrow points from the source object to the target object.

Association or attribute

An association line and an attribute show the same relationship. They are semantically equal. The line gives more visual emphasis. Showing both is therefore redundant, so pick one.

Example

Section titled “Example”

A diagram has two classes, Customer and Order. A solid line joins them. The end at Order is labeled orders with multiplicity *. The end at Customer is labeled buyer with multiplicity 1. An open arrow points to Order. The diagram says a customer can reach its orders. In code, Customer would hold an attribute named orders. The diagram does not also list orders inside the Customer box. That would show the same fact twice.

Trade-offs

Section titled “Trade-offs”

An association line gives a relationship visual weight. An attribute keeps the diagram compact. Both mean the same thing, so choose by how much emphasis the relationship deserves. Do not use both for the same relationship.

An association with a name, role names, multiplicities and a navigability arrow.

Diagram source 
@startuml
class Customer
class Order
Customer "buyer 1" --> "orders *" Order : places
@enduml

Checks

Section titled “Checks”
  • Each relationship between two classes is shown either as an association line or as an attribute, not both.
  • Association ends that matter carry a multiplicity such as * or 0..1.
  • Role names on association ends match the attribute names used in code.
  • Where reachability matters, navigability is shown with an open arrowhead or a cross.
Section titled “Related”

Sources

Section titled “Sources”