Mastering UML: The Six Core Relationships Every Developer Should Know

UML’s Six Major Relationships

In UML class diagrams, the common relationships are Generalization, Realization, Association, Aggregation, Composition, and Dependency.

1.1 Generalization (Inheritance) Generalization denotes that a subclass (or sub‑interface) inherits the features of a superclass (or super‑interface) and can add new functionality. In Java it is expressed with the extends keyword. The arrow is a solid line with a closed triangular head pointing to the parent class.

1.2 Realization Realization describes a class implementing one or more interfaces. In Java this relationship is marked with the implements keyword. The arrow is a dashed line with a closed triangular head pointing to the interface.

1.3 Dependency A dependency is a weak, temporary usage relationship where one class (A) uses another class (B). Changes in B may affect A. In code this appears as method parameters, local variables, or static method calls. The arrow is a dashed line with an open arrowhead pointing to the used class.

1.4 Association Association is a stronger, long‑term relationship where two classes know each other’s attributes and methods. It can be unidirectional or bidirectional and is represented in code by member variables. Bidirectional association uses two arrows or a line without arrows; unidirectional uses a single arrow.

1.4.1 Bidirectional Association Both parties are aware of each other and can invoke each other’s public members.

1.4.2 Self‑Association A class may reference itself, which is useful in recursive structures.

1.5 Aggregation Aggregation is a special form of association representing a whole‑part relationship where parts can exist independently of the whole (has‑a). It is shown with a hollow diamond at the whole side.

1.6 Composition Composition is a stronger whole‑part relationship where parts cannot exist without the whole (strong aggregation). It is depicted with a filled diamond at the whole side.

Comprehensive Example The final diagram summarizes the relative strength of the relationships: Generalization = Realization > Composition > Aggregation > Association > Dependency.