18.1. Visibility Between Objects
The designs created for the system events (enterItem, and so on) illustrate messages between objects. For a sender object to send a message to a receiver object, the sender must be visible to the receiver—the sender must have some kind of reference or pointer to the receiver object.
For example, the getSpecification message sent from a Register to a ProductCatalog implies that the ProductCatalog instance is visible to the Register instance, as shown in Figure 18.1.
Figure 18.1. Visibility from the Register to ProductCatalog is required.[1]
[1] In this and subsequent code examples, language simplifications may be ...
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