What does a diamond sign signify in UML class diagrams?
Design PatternsUmlClass DiagramDesign Patterns Problem Overview
Consider the below two diagrams. The top one contains a plain arrow at the right end, and the bottom one contains an arrow with a diamond at the left end and a plain arrow at the right end. The great book GoF has usages of both of these two kinds of diagrams.
I want to understand what does the diamond signify in the second case, and which one of these two gives a stronger association between Customer and Order.
Is the Customer class responsible for the lifecycle of Order class in either of the two cases?
Design Patterns Solutions
Solution 1 - Design Patterns
If there is no diamond, then we have a simple association.
If the diamond is left empty, it signifies it is an aggregation. This relation is stronger than a simple association. In this case a Customer aggregates Orders.
If the diamond is black, this means it is a composition, which is even stronger than an aggregation because the composite class cannot be composite by other classes. Its "life" depends on the container.
I think it's explained a little bit more clearly, at:
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Ezra, Aviad (May 28, 2009) "UML Class Diagram: Association, Aggregation and Composition"
Solution 2 - Design Patterns
The customer and order is a "has-a" composition relationship because if the customer ceases to exist so does his/her order. Therefore, there is a life cycle dependency between the container (customer) and the instances within that container (order). When the container ceases to exist so does all its instances.
On the other hand, if we say a customer has a book. In this case, we have a "has-a" aggregation relationship. Because if the customer ceases to exist, the book is still available.
Solution 3 - Design Patterns
It signifies aggregation. From wikipedia:
> In UML, it is graphically represented as a hollow diamond shape on the > containing class end of the tree with a single line that connects the > contained class to the containing class.The aggregate is semantically > an extended object that is treated as a unit in many > operations,although physically it is made of several lesser objects.