6.4 Identify Object Associations

Early in the analysis, some objects seem to relate to others even though it is not always clear exactly how. The first step is to identify the existence of such associations. We discuss the characterization of associations later in this chapter.

There are a few strategies for the identification of object relationships. Each relies on the fact that objects send messages to other objects and every message implies an association. Table 6-7 shows the most common strategies used to identify associations.

Table 6-7. Object Association Strategies

Strategy	Description
Identify messages	Each message implies an association between the participating objects.
Identify message sources	The sensors that detect information or events and the creators of information or events are all message sources. They pass information on to other objects for handling and storage.
Identify message storage depots	Message storage depots store information for archival purposes or provide a central repository of information for other objects. The depots have associations with the message sources as well as the users of that information.
Identify message handlers	Some objects centralize message dispatching and handling. They form connections to either or both message sources and message storage depots.
Identify whole/part structures	Whole/part relations will become aggregation or composition relationships among objects. Wholes often send messages to their parts.
Identify more/less abstract structures	Some objects are related to each other by differences in levels of abstraction. It is not uncommon for a single higher-level abstraction to be implemented by a tightly knit set of objects at a more concrete level of abstraction. The larger object in this case will become a composite object and the smaller objects, its component parts. The relationship among them will become a composition relation.
Apply scenarios	Walk through scenarios using the identified objects. The scenarios explicitly show how the messages are sent between objects.

In our elevator example, there are a number of associations, shown in Table 6-8.

Table 6-8. Elevator Object Associations

Message Source	Message Target	Message
Elevator request button	Elevator gnome	Requests an elevator
Elevator gnome	Elevator	Request status
Elevator gnome	Elevator	Add destination for elevator
Elevator	Elevator gnome	Accept destination
Elevator	Floor speaker	Arrival event beep
Elevator floor sensor	Elevator	Location
Cable tension sensor	Locking clamps	Engage
Central station	Locking clamps	Release
Cable tension sensor	Central station	Alarm condition
Elevator	Central station	Status
Floor request button	Elevator	Add destination
Run-stop switch	Elevator	Stop/run
Run-stop switch	Central station	Stop/run
Alarm button	Central station	Alarm condition
Elevator	Door	Open/close

Class and object diagrams capture these associations. A line drawn between two objects represents a link (instance of an association) between those objects, supporting the transmission of a message from one to the other.

The object diagram in Figure 6-11 shows the general structure of the identified objects and their neighbors. The system consists of 38 high-level objects: 8 shafts, 8 elevators, 20 floors, and one central station and one elevator gnome. The numbers at the upper lefthand corner of the object represents the instance count, that is, the number of instances of the object in the enclosing context. The shaft, elevator, and floor are all composites that strongly aggregate their components. The instance counts of their components as per instance of their context. That is, each Elevator has 20 instances of Floor Request Button.

Figure 6-11. First-Cut Elevator Object Diagram