对Qualified Associations的总结

Qualified Associations

A qualified association is the UML equivalent of a programming concept variously known as associative arrays, maps, hashes, and dictionaries. Figure 5.10 shows a way that uses a qualifier to represent the association between the Order and Order Line classes. The qualifier says that in connection with an Order, there may be one Order Line for each instance of Product.

From a software perspective, this qualified association would imply an interface along the lines of

class Order ...
  public OrderLine getLineItem(Product aProduct);
  public void addLineItem(Number amount, Product forProduct);

Thus, all access to a given Order Line requires a Product as an argument, suggesting an implementation using a key and value data structure.

It's common for people to get confused about the multiplicities of a qualified association. In Figure 5.10, an Order may have many Line Items, but the multiplicity of the qualified association is the multiplicity in the context of the qualifier. So the diagram says that an Order has 0..1 Line Items per Product. A multiplicity of 1 would indicate that Order would have to have a Line Item for every instance of Product. A * would indicate that you would have multiple Line Items per Product but that access to the Line Items is indexed by Product.

In conceptual modeling, I use the qualifier construct only to show constraints along the lines of "single Order Line per Product on Order."