Telephone is an interface type, so that it will be easy to replace a non-GUI implementation of a telephone with a GUI implementation.

MailSystemTester launches the VMSS. Note the telephone variable, which is of interface type.

• Here, telephone refers to a TelephoneNoGUI object. This object presents a console-based user interface.
• The telephone variable can easily be made to reference a TelephoneFX object instead, which presents a graphical user interface (GUI).

The TelephoneNoGUI class extends the Console class.

• Here, the handleInput method is overridden to send telephone input to the Connection object.
• If handleInput were not overridden, it would default to the implementation in the Console class, which simply echoes the input to the display.

Use cases for the voice mail program can be divided into those carried out by a caller and those carried out by the voice mailbox owner.

The use cases for a voice mail caller are to Reach An Extension and to Leave A Message.

Note how the second use case involves the first one.

The use cases for a voice mailbox owner are to Log In and to perform various menu actions.

Note that these use cases can come with variations.

This section describes how use case walkthroughs can yield new information for class descriptions using CRC cards.

This section describes UML diagrams that result from the voice mail system design process.

Sequence diagrams are formal results of the use case walkthrough process.