Aggregation is the typical whole/part relationship. This is exactly the same as an association with the exception that instances cannot have cyclic aggregation relationships.

Sample class Person is shown below to demonstrate Aggregation relationship with Address.Person class

Aggregation follows the Has-A model. This creates a parent-child relationship between two classes, with one class owning the object of another.

So, what makes aggregation unique?

Independent lifetimes

In aggregation, the lifetime of the owned object does not depend on the lifetime of the owner.

The owner object could get deleted, but the owned object can continue to exist in the program. In aggregation, the parent only contains a reference to the child, which removes the child’s dependency.

Aggregation

You can probably guess from the illustration above that we will need object references to implement aggregation.

Example

Let’s take the example of people and their country of origin. Each person is associated with a country, but the country can exist without that person.