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:

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 di erent granularities by di erent users. E.g., wheel by wheel designer but bicycle by a sales-person. The containment hierarchy is used to nd 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.