Aggregation
With aggregation, an object will always be referenced by other objects. This can be shown by an open diamond (a diamond without any fill color) in the UML. Previously we described aggregation and composition using the Vehicle class example. We will use the same example for the UML diagrams.
As you can see, a car can have multiple Passengers and one Driver within. Using the blank diamond, we can demonstrate that the lives of the Passengers and Drivers are not bound to the car.
