What type of relationship does aggregation describe?

Aggregation describes a "has-a" relationship, which means that one object (the whole) can contain references to other objects (the parts). In this context, the whole can exist independently of its parts. For example, consider a Library and its Books. A library has books, but the books can also exist independently of the library; they may exist in other libraries or be owned by individuals.

This type of relationship emphasizes the ownership and lifecycle of the contained objects as being separate yet still related to the container object. The relationship is often depicted in UML diagrams with an empty diamond at the whole end, indicating that while one object contains another, they are not strictly tied together.

In contrast, other relationship types such as "belongs-to," "uses-a," and "part-of" convey different meanings in object-oriented design. "Belongs-to" suggests a more dependent relationship, typically used in a stronger form of association. "Uses-a" implies that one class utilizes another without owning it, indicating a more transient relationship. "Part-of" indicates a composition relationship where the lifecycle of the parts is tied to the whole, meaning if the whole is destroyed, the parts are also destroyed. Thus, aggregation specifically aligns with the concept of