UML syntax: aggregation and composition

[ronaldtse]

• An aggregation is an association representing a whole-parts hierarchy.
• An aggregation is used to describe how something whole is logically composed of its parts.
• A composition is a strict form of aggregation, where the existence of its parts depends on the whole. The whole is the owner of its parts.
• An aggregation or composition is a property of the metamodel class property (i.e. a property of the property).
• The type is an enumeration called AggregationKind with the literals none, shared (which refers to aggregation) and composite.
• So, formally, neither aggregation nor composition is a form of relationship of its own.

Proposal:

class Whole {
  aggregation_of Part[0..1] "consists of"
}

class Part {
}

Proposal:

class Whole {
  aggregation_of Part
  composition_of ExistentiallyDependentPart
}

class Part
class ExistentiallyDependentPart

Proposal:

class Car {
  aggregation_of Engine[0..1][1] "has"
}
class Boat {
  aggregation_of Engine[0..1][1] "has"
}
class Engine {
  of_aggregation Car
  of_aggregation Boat
}

Proposal:

class Car {
  composition_of Engine[0..1][1] "has"
}
class Boat {
  composition_of Engine[0..1][1] "has"
}
class Engine {
  of_aggregation Car
  of_aggregation Boat
}

Composition

• If a variable multiplicity for the parts is specified (e.g. 1..*), then they don’t have to be created together with the aggregate; they can be created later.
• From the time of their creation, however, they normally belong immeediatly to the whole.
• Similarly, they can be destroyed before the aggregate at any time.
• However, generally the latest they are destroyed is when the aggregate is destroyed.

[w00lf]

@ronaldtse do we need the direction arrow of association label(has in the case below)?

class Car {
  composition_of Engine[0..1][1] "has"
}
class Boat {
  composition_of Engine[0..1][1] "has"
}

[ronaldtse]

@ronaldtse do we need the direction arrow of association label(has in the case below)?

From UML spec:

This triangle indicates the "association name:

The direction of this association name/label follows the direction of the arrow.

So we don't need to define it the direction of this solid triangle.