Modeling Operations

To identify operations, we examine the verb phrases listed for each class in Fig. 22.16. The “executes financial transactions” phrase associated with class ATM implies that class ATM instructs transactions to execute. Therefore, classes BalanceInquiry, Withdrawal and Deposit each need an operation to provide this service to the ATM. We place this operation (which we’ve named execute) in the third compartment of the three transaction classes in the updated class diagram of Fig. 22.17. During an ATM session, the ATM object will invoke the execute operation of each transaction object to tell it to execute.

Fig. 22.17. Classes in the ATM system with attributes and operations.

The UML represents operations (which are implemented as member functions in C++) by listing the operation name, followed by a comma-separated list of parameters in parentheses, a colon and the return type:

Each parameter in the comma-separated parameter list consists of a parameter name, followed by a colon and the parameter type:

For the moment, we do not list the operations’ parameters—we’ll identify and model the parameters of some of the operations shortly. For some, we do not yet know the return types, so we also omit them from the diagram. These omissions are perfectly normal at this point. As our design and implementation proceed, we’ll add the remaining return types.