Object Containment

1. Objects that contain other objects are called complex or composite objects. There can be multiple levels of containment, forming a containment hierarchy among objects.
2. Example: A bicycle design database:

    
   Figure 8.6:  Containment hierarchy for bicycle-design database.
   

   Fig. 8.6 shows the containment relationship in a schematic way by listing class names. Thus the links between classes must be interpreted as is-part-of, rather than the is-a interpretation of links in an inheritance hierarchy.

3. Containment allows data to be viewed at different granularities by different users. E.g., wheel by wheel designer but bicycle by a sales-person.

   The containment hierarchy is used to find all objects contained in a bicycle object.

4. In certain applications, an object may be contained in several objects. In such cases, the containment relationship is represented by a DAG rather than by a hierarchy.