In software engineering, both composition and aggregation are used to establish relationships between objects, but they represent different ways of constructing complex objects from simpler ones.
Composition is a strong form of aggregation where a complex object is constructed by creating a group of related objects and treating them as a single unit. In composition, the lifetime of the child objects is controlled by the parent object. In other words, when the parent object is destroyed, all the child objects are also destroyed.
On the other hand, aggregation is a weaker form of composition where a complex object is constructed by creating a group of related objects and treating them as a collection. In aggregation, the lifetime of the child objects is not controlled by the parent object. In other words, when the parent object is destroyed, the child objects may continue to exist.
One way to differentiate between composition and aggregation is to look at the "has-a" relationship. Composition implies a strong "has-a" relationship, where the parts are an integral part of the whole, while aggregation implies a weak "has-a" relationship, where the parts are just a collection.
In terms of design patterns, the Composite pattern is an example of composition where objects are combined into a tree-like structure to represent part-whole hierarchies. The Decorator pattern is another example of composition where objects are wrapped in other objects to provide additional behavior or responsibilities.
On the other hand, the Factory pattern is an example of aggregation where objects are created by a separate factory object and passed to the client object. The Observer pattern is another example of aggregation where objects register with a subject object to receive notifications when the subject's state changes.
In summary, both composition and aggregation are important concepts in object-oriented programming, and choosing one over the other depends on the specific requirements and constraints of the problem at hand. Composition is useful when you need to create a strong part-whole relationship between objects, while aggregation is useful when you need to represent a weak collection of related objects.