An aggregation is a relationship between two classes where one class contains an instance of another class. For example, when an object A contains a reference to another object B or we can say Object A has a HAS-A relationship with Object B, then it is termed as Aggregation in Java Programming. Aggregation in Java helps in reusing the code.

An aggregation is a form of association where the relation of Association can be considered the containing class 'owning' the contained class. The lifetime of that relationship cannot be defined. 'Owning' can be determined as a single-direction Association.

Aggregation is a relationship that comes under object-oriented programming, classifying an instance of a class as "has a." It's a form of association with weaker relationship strength, whereby the lifetime of the contained object (part) is not controlled based on the lifetime of the container object (whole).

In Java, we can model aggregation with a plain old reference: class Car { List<Wheel> wheels; The member can be any type of class, except a non-static inner class. In the code snippet above both classes have their separate source file. However, we can also use a static inner class: List<Wheel> wheels; static class Wheel {}