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.
- counter examples
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.
- related patterns
- 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.
- implied patterns
- specialized patterns