Aggregation vs Composition in UML

In UML (Unified Modeling Language), aggregation and composition are two types of relationships between classes or objects. Understanding the difference between aggregation and composition is important for designing a clear and efficient UML diagram.

Aggregation

Aggregation represents a relationship where one class (the whole) contains another class (the part), but the part can exist independently of the whole. In UML diagrams, aggregation is denoted by an empty diamond shape on the whole class pointing towards the part class.

For example, a car and its wheels have an aggregation relationship. The wheels can exist independently of the car.

Composition

Composition is a stronger form of aggregation where the part class is completely dependent on the whole class. In composition, if the whole class is destroyed, the part class is also destroyed. In UML diagrams, composition is denoted by a filled diamond shape on the whole class pointing towards the part class.

For example, a car and its engine have a composition relationship. If the car is destroyed, the engine is also destroyed as it is a part of the car.

Understanding the distinctions between aggregation and composition in UML is important for accurately representing the relationships between classes and objects in a system.