Composite   The value of composite for the aggregation property of an association end signifies a whole-part relationship between two classifiers where the whole has responsibility for the parts. An association, with one of its ends specifying a value of composite, is commonly called a composition relationship. It is stronger than an unadorned association end, adding the concept of whole (filled diamond end) and part relationship. It is also stronger than an aggregate, because the classifier at the whole end has responsibility for the lifetime of any parts. Only one end of a binary association can be adorned with a composition symbol. Formally it is one of three values that can be set for the aggregation property of an association end the others being none and aggregate.

Explanation

A composition relationship is a strong type of whole-part association, where the element at the solid diamond has responsibility for the element or elements at the other end of the relationship.

Important Points

		The instance on the whole end is responsible for the lifetime of the instances on the part end.		The whole (with the diamond) is responsible for the lifetime of its parts (without the diamond). This includes their creation; their destruction and their propagation; if the whole is copied so are all its parts.
		The part can only "belong" to one whole at a given point in time.		The whole is responsible for its parts and as such a part cannot be a part in another composite relationship, otherwise the responsibility of the whole would be compromised.
		This is the strongest form of association, and implies the whole has responsibility for the part.		If a whole is destroyed it does not necessarily imply that the part must be destroyed. The whole must take responsibility for the part, but it has a number of options it can destroy the part or pass on the responsibility to another instance.
		When a whole is copied the parts are also copied.		Whenever the whole is copied or propagated the parts are always copied as well. In accordance with the principle that the whole is responsible for the lifetime of the parts which make it up.

Related Entries

Aggregate  Graphically Similar An aggregation is a type of association that is weaker, or that has less responsibility than the weaker composition The aggregation adornment is used when the whole part (hollow diamond end) is composed of one or more parts (tail end).

Association  Graphically Similar,  More Specific An association is the base type of relationship to which the aggregation adornment is added. The association indicates no whole and part relationship, which is added by the aggregation.

Attribute  Similar Usage An attribute is related to a composite in that a classifier with an attribute, whose type is a classifier, has the same meaning as a classifier connected to another classifier with an adornment of composite. They are alternate notations one showing the structural nature of the relationship the other showing the contents of the classifier.