Jim Shaw has asked me to document some thoughts on these topics. Here they are...
They are a
work in progress and feedback most welcome.
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[size=16]Meronymy[/size]Meronymy (n): the semantic relation that holds between a part and the whole [synonym: part to whole relation]. In UML terms, this is related to the notion of aggregation.
The AggregationKind adornmentWe are familiar with the UML AggregationKind enumeration type and its corresponding diamond shaped adornment. Since there are only two effective AggregationKind literals (shared and composite), and we are about to define four types of meronymy, we won’t use the terms shared and composite aggregation, but will use the diamond adornment – either filled or open as required.
PoliciesSome aspects of the following attributes and behaviours may be controlled by the adoption of various policy directions and their accompanying policy variant implementation. For example, it can be a policy decision as to whether an item may self-remove from a collection or grouping. The policy would be entitled: Item self-removal. The two implementations would be: Item can self-remove and the other Item cannot self-remove. As each instance of the whole is created, the appropriate policy implementations are applied and the behaviour adjusted accordingly.
Where a behaviour may be policy controlled the denotation [Policy] is added.
[size=16]Containers / Collections / Compositions / Nests[/size]NestNests hold items structurally (similar to composites). They have to be able to hold the kinds of items you need to put in them. [Policy]
Nested items are either all classifiers or all instances (not a mixture).
Nested items cannot be references.
There can be only one item of a given name in a nest.
The types of items held in nests are usually (but certainly not exclusively) not of the same metatype as the container. [Policy] Where the same metatype may be nested, then we have the possibility of recursion.
The types of the nested items do not define the nest.
Each classifier item can be in more than one nest at a time. But a specific instance item is always only in one nesting instance.
1The item needs to know which nest it's in.
The nest may restrict access to (some or all) the items. [Policy]
Destroying the nest necessarily destroys the nested items.
2Destroying the nested items never destroys the nest.
Nesting without Stereotype. (But see footnote
1)
ContainerContainers contain items (physically but not structurally). They have to be able to contain the kinds of items you need to put in them. [Policy]
The types of items held in containers are usually (but certainly not exclusively) not of the same type as the container. [Policy]
Contained items may be references.
The types of the contained items do not define the container.
Each item can be on only one container at a time. (The item can only be inside its innermost container)
The item needs to know which container it's in.
The container may restrict access to (some or all) the items. [Policy]
Destroying the container doesn't necessarily destroy the contained items (but often may). [Policy]
Destroying the contained items never destroys the container.
If the container can destroy the items: Filled diamond Association with Stereotype: «contain».
If the container cannot destroy the items: Open diamond Association with Stereotype: «contain».
CollectionAll containers define collections (even if only implicitly - the objects contained)
Not all collections are containers. [Policy]
Collections collect (group) things. They have to be able to collect the kinds of items you need to add to them. [Policy]
The types of items held in collections are usually (but certainly not exclusively) not of the same type as the collection.
Collected items may be references.
The types of the collected items do not define the collection.
An item can be in more than one collection at the same time. [Policy]
Often the item knows which collections it's in, but sometimes it doesn't. [Policy]
The collection may never restrict access to the items.
Destroying the collection never destroys the items.
Destroying the items doesn't necessarily destroy the collection, but may do.
Open diamond Association with Stereotype: «include».
CompositeComposite objects define the types of the composing items. The items are functionally and/or structurally related to the composite. [Policy]
The types of the items define the composite (but some types are there for adornment and are not intrinsically part of the composite). [Policy]
An item instance can only be in one composition at a time.
The composite may restrict access to (some or all) the items. [Policy]
Composing items cannot be references.
Destroying the composite always destroys the item.
Destroying the items destroys the composite.
Filled diamond Association with Stereotype «comprise».
NOTE: in all cases, the items can be removed from the container / collection / composite / nest. It may be a policy to allow an item to self-remove.
1 NOTE: For browser purposes, where a single nesting is defined, the nestling shall be physically placed (nested) under the nest. Where more than one nesting is defined, one will be designated the «physical» nesting and the nestling placed under that nest. The other nesting(s) will be designated «symbolic».
2 If an item is nested in more than one nest, then only that nestling is destroyed. If that nestling is the «physical» nesting, then the «physical» nesting is transferred to one of the remaining nestings.
