container pattern

Collection.

Combine a number of elements to a larger structure.

A set of multiple data elements.

Combine multiple independent elements on the same level to refer to them as a joint group.

• Explicitly list all member elements which belong to the container.
• Specify a method to check whether an element belongs to the container.

• A directory of files in a file system.
• An archive containing a set of files.
• A set of records in a database.
• A repeatable entity or relationship in a schema. In fact the concept of repeatability is an instance of the container pattern.
• An entity type in a schema is the set of all of its instances.

A single record with its properties does not constitute a container because properties depend on the record instead of being independent.

• A container may hold a single member element only, making the collection difficult to distinguish from the element as such.
• A container may be empty, making it difficult to list member elements.

• Explicitly listing member elements requires a sequence.
• A membership function is a form of derivation.
• Empty containers often involve an implicit element (void).
• Collections are used to refer to elements (or to a type of elements) with a human readable label.
• Each collection defines the property of “belonging to the collection”. An alternative pattern to group by same properties is normalization.
• Collections may be abbreviated (etcetera pattern).
• Containers are also used to wrap or abstract from sets of data. This goal can better be achieved by atomicity.

• A container is a special kind of embedding with member elements embedded into the collection as host element.
• Unless abbreviated, containers have a specific number of member elements which implies the size pattern.

A sequence and a graph typically consist of collections of elements.