Computers as Components, 2nd Edition

340 CHAPTER 6 Processes and Operating Systems

6.7.2 Specification

Figure 6.21 shows the class diagram for the answering machine. In addition to

the classes that perform the major functions, we also use classes to describe the

incoming and OGMs. As seen below, these classes are related.

The definitions of the physical interface classes are shown in Figure 6.22. The

buttons and lights simply provide attributes for their input and output values. The

phone line, microphone, and speaker are given behaviors that let us sample their

current values.

The message classes are defined in Figure 6.23. Since incoming and OGM types

share many characteristics, we derive ...