Java Design Concepts

So many times I am asked by developers what the difference is between Aggregation and Composition. I will try and explain the difference in the following section by making use of examples and UML diagrams. Aggregation and Composition are both types of associations between objects.

Aggregation

Aggregation is a kind of association that specifies a whole/part relationship between the aggregate (whole) and component part. This relationship between the aggregate and component is a weak “has a” relationship as the component may survive the aggregate object. The component object may be accessed through other objects without going through the aggregate object. The aggregate object does not take part in the lifecycle of the component object, meaning the component object may outlive the aggregate object. The state of the component object still forms part of the aggregate object.

An example of aggregation is a History-Class object contains zero or more of Student objects. The state of each Student object has an influence on the state of the History-Class object. If the History-Class object is destroyed, the Student objects may continue to exist.

The UML diagram above illustrates the aggregation association line with an open diamond on the aggregate side.

Composition

Composition is a kind of association very similar to aggregation except where the composite object has sole responsibility for the disposition of the component parts. The relationship between the composite and the component is a strong “has a” relationship, as the composite object takes ownership of the component. This means the composite is responsible for the creation and destruction of the component parts. An object may only be part of one composite. If the composite object is destroyed, all the component parts must be destroyed, or the reference and responsibility of the component part must be handed over to another object. Composition enforces encapsulation as the component parts usually are members of the composite object.

An example of composition is a House object contains zero or more Room objects. The state of each Room object has an influence on the House object. If the House object is destroyed, the Room objects will also be destroyed.

The UML diagram above illustrates the composition association line with a solid diamond on the composite side.