Question

I still have issues reading UML diagrams.

Just looking at the Builder pattern

http://en.wikipedia.org/wiki/Builder_pattern

I don’t understand why the relationship between the Director and the Builder is an aggregate relationship?

Answer 1

The director sends a sequence of build commands to the builder. Therefore, the director has to know about the builder.

We need some kind of relationship going from the director to the builder. It could either be a composition relationship or an aggregate relationship.

If it was a composition relationship, the director would be responsible for creating and destroying the builder.

But the point of this pattern is that the director doesn’t know or care what the builder is. The calling framework creates the builder and passes this dependency to the director. The builder exists independently of the director. That’s why it’s an aggregate relationship.