Identifying Operations

An operation is a service that objects of a class provide to clients of the class. Consider the operations of some real-world objects. A radio’s operations include setting its station and volume (typically invoked by a person adjusting the radio’s controls). A car’s operations include accelerating (invoked by the driver pressing the accelerator pedal), decelerating (invoked by the driver pressing the brake pedal or releasing the gas pedal), turning and shifting gears. Software objects can offer operations as well—for example, a software graphics object might offer operations for drawing a circle, drawing a line, drawing a square and the like. A spreadsheet software object might offer operations like printing the spreadsheet, totaling the elements in a row or column and graphing information in the spreadsheet as a bar chart or pie chart.

We can derive many of the operations of each class by examining the key verbs and verb phrases in the requirements document. We then relate each of these to particular classes in our system (Fig. 22.16). The verb phrases in Fig. 22.16 help us determine the operations of each class.

Fig. 22.16. Verbs and verb phrases for each class in the ATM system.