Dependency, association, aggregation, realization

Relationships connect classes so that they can collaborate. UML defines four primary relationships between classes.

1. Dependency

A using relationship: a change in one class (the supplier) may affect another (the client). It is temporary, e.g. a class uses another as a method parameter, local variable or return type. Notation: dashed line with open arrowhead pointing to the supplier, optionally with a stereotype <<use>>.

 +---------+  - - - - - - ->  +---------+
 | Printer |                  | Report  |
 +---------+                  +---------+
 Printer.print(Report r)

2. Association

A structural relationship: objects of one class are linked to objects of another for a longer time. It can have a name, role names, multiplicity and navigation arrow. Notation: solid line.

 +---------+ 1     0..* +---------+
 | Teacher |------------| Course  |
 +---------+   teaches  +---------+

3. Aggregation (and composition)

A special association for a whole-part ("has-a") relationship. In aggregation the part can live independently of the whole (a Department has Teachers). In composition the part's life depends on the whole (a Building has Rooms). Notation: aggregation = hollow diamond at the whole; composition = filled diamond.

 +------------+ <>-------- +---------+   aggregation
 | Department |            | Teacher |
 +------------+            +---------+
 +------------+ <#>------- +---------+   composition
 |  Building  |            |  Room   |
 +------------+            +---------+

4. Realization

A relationship between a specification (interface) and the class that carries it out. The class guarantees to provide the operations declared by the interface. Notation: dashed line with a hollow triangle pointing to the interface.

 +------------+
 | <<interface>>
 |  Payment   |
 +------------+
       ^
       : (dashed, hollow triangle)
 +------------+
 | CardPayment|
 +------------+

(Generalization, the "is-a" relation, is the fifth common relationship and is drawn as a solid line with a hollow triangle.)