What does a diamond sign signify in UML class diagrams?

design-patterns uml class-diagram
Geek picture Geek · Feb 28, 2013 · Viewed 78.9k times · Source

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?

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Answer

C.Champagne picture C.Champagne · Feb 28, 2013

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 aggregated class cannot be aggregated by other classes. Its "life" depends on the container.

I think it's explained a little bit more clearly, at:

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