Content over container: object-oriented programming with multiplicities

In object-oriented programs, the relationship of an object to many objects is usually implemented using a collection. This is in contrast to a relationship to one object, which is usually realized as a direct value. However, using collections for relationships to many objects does not only mean that accessing the related objects always requires accessing the collection first, it also presents a lurking maintenance problem that manifests itself when a relationship needs to be changed from to-one to to-many or vice versa. To address these issues, we suggest the extension of object-oriented programming with multiplicities, that is, with expressions that evaluate to an arbitrary number of objects not wrapped in a container.