UML aggregation vs association

Question

Here I am, with another question about aggregation and association. I wanted to learn some basics of UML, so I started reading \"UML distilled\" by Martin Fowler. I read bot

Answer 1

In UML aggregation is under-defined and since they haven't got any clearly defined semantic. A valid use-case of an aggregation is the encapsulation of a several classes, as stated in "Domain Driven Design" by Eric Evans.

E.g. a car has four wheels. You might want to calculate the total amount of meters each wheel has driven, for each car. This calculation is done by the car-entity, since it knows which wheels it has and you don't care which wheels belong to which car.

The car is the aggregation-root for all it's parts, like wheels, and you can't access the parts of a car from outside the aggregation, just the root.

So basically an aggregation encapsulates a set of classes which belong to each other.