Keep in mind that both aggregation and acquaintance deal with composition. On a different topic, whole and part may be pivotal, but here the focus is on the relationships between participants. Acquaintance and aggregation constitute relationships between the participants. So that’s where we need to focus and try and understand the difference between the two. Unfortunately, there is a difference between them, but it is subtle and based on intent as much as specific details. That means there is a right and wrong insofar as using one or the other.

One thing that has helped me is to compare the relationship between a Client class and its acquaintance and an Aggregator class and the Aggregatee. I cannot think of a single design pattern where the Client has an aggregate relationship with a design pattern, but you will find lots of patterns where you’ll find aggregate relations between the pattern participants. If you look at the way in which those relations are implemented, you’ll begin to see how they are different.

As for me…Nenhum descanso para o mau.

Kindest regards,
Bill

I would like to share a concept that helps me better understand aggregation: It’s the “whole to its parts” idea. For instance, if you want to give emphasis that the “whole” (Context class) needs its “parts” (the State classes) to do what’s suppose to do. Sometimes classes are associated but the idea of ” whole to its parts” doesn’t apply so the aggregation symbols are not necessary, an Acquaintance would be just fine. I would even say there is no right or wrong, just a matter of giving more detail to the diagram.

Nice post! How can I rate it?

Tot ziens!

This post was mentioned on Twitter by AS3DP: ActionsScript 3.0 Design Pattern Relations Part II: Aggregation #asdp http://www.as3dp.com/?p=2476…

A composition is a collected object made up of other objects, such as the Context compositing the States creating an aggregate object. The aggreateinstance in Figure 1 is an object created by the Aggregator with properties of the Agregatee. (I use the term properties broadly here to include both properties and methods.) So a composition has no child classes as do classes related by inheritance. Instead, a composition is an object made up of other objects without inheritance. I don’t mean to sound picky, but because a key principle is to favor composition over inheritance, we need to separate one from the other.

I hope that helps nudge the concept of aggregation (as a type of composition) closer to the light. As always, it’s great to hear from you!

Kindest regards,
Bill