Aggregation

How to read it

The hollow diamond marks shared aggregation. The class at the diamond end is the whole, the aggregate, and the class at the plain end is the part. Robert C. Martin's example is a Window that contains many Shapes. UML 2.5.1 describes aggregation as one instance grouping together a set of others, and then leaves its meaning open: "Precise semantics of shared aggregation varies by application area and modeler." Course notes give the usual reading, that the parts can exist without the whole and may be shared. Kansas State University's textbook compares it to a shopping cart: the groceries existed before they went into the cart and still exist after they come out. Virginia Tech's OpenDSA notes have a department with an aggregation of professors, any of whom could work in more than one department.

Also searched as: aggregation symbol, hollow diamond uml, uml aggregation.

Aggregation or composition?

The two diamonds differ in fill, and in how much the standard says about them. A filled diamond is Composition, and UML 2.5.1 gives it firm rules: a part belongs to at most one composite at a time, and is deleted along with it. For the hollow diamond the specification gives no rule at all.

Martin Fowler wrote in 2003 that aggregation has no semantics beyond those of a regular Association, and quoted James Rumbaugh, one of UML's three original authors, calling it "a modeling placebo". Fowler's advice to anyone who meets one is to ask what the diagram's author meant by it. Robert C. Martin went further in 1997, suggesting the relationship could be ignored altogether, and quoted the UML 0.8 document: "if you don't understand [aggregation] don't use it." What a hollow diamond does say, in Martin's words, is that the class it touches is "in some way the whole" and the other class is somehow part of it.

Which end the diamond goes on

The diamond touches the whole. UML 2.5.1 adds it at the end of the line away from the part, and asks for it to be noticeably smaller than the large diamond used for an association between three or more classes. Kansas State's textbook describes it as a diamond "fletching" at the aggregating class's end, with an optional arrowhead at the part's end. A number at the part's end is a Multiplicity: Martin marks the Shape end of his example with *, for many shapes.

Look-alikes

Diamonds standing on their own, not at the end of a line, are other symbols: a Decision and merge node in an activity diagram, a decision on a flowchart, or a relationship in a Chen ER diagram.