Communication Diagrams

Figure 22.23 shows a communication diagram that models the ATM executing a BalanceInquiry. Objects are modeled in the UML as rectangles containing names in the form objectName : ClassName. In this example, which involves only one object of each type, we disregard the object name and list only a colon followed by the class name. [Note: Specifying the name of each object in a communication diagram is recommended when modeling multiple objects of the same type.] Communicating objects are connected with solid lines, and messages are passed between objects along these lines in the direction shown by arrows. The name of the message, which appears next to the arrow, is the name of an operation (i.e., a member function) belonging to the receiving object—think of the name as a service that the receiving object provides to sending objects (its “clients”).

Fig. 22.23. Communication diagram of the ATM executing a balance inquiry.

The solid filled arrow in Fig. 22.23 represents a message—or synchronous call—in the UML and a function call in C++. This arrow indicates that the flow of control is from the sending object (the ATM) to the receiving object (a BalanceInquiry). Since this is a synchronous call, the sending object may not send another message, or do anything at all, until the receiving object processes the message and returns control to the sending object—the sender just waits. For example, in Fig. 22.23, the ATM calls member function execute of a BalanceInquiry and may not send another message until execute has finished and returns control to the ATM. [Note: If this were an asynchronous call, represented by a stick arrowhead, the sending object would not have to wait for the receiving object to return control—it would continue sending additional messages immediately following the asynchronous call. Asynchronous calls often can be implemented in C++ using platform-specific libraries provided with your compiler. Such techniques are beyond the scope of this book.]