A Stamp Dispenser State Machine

Occasionally, you can describe the behavior of objects in state machine terms. For example, you could express the behavior of the stamp dispenser example from Guideline 3 in terms of a state machine that has:

• Four states: HAS_0, HAS_5, HAS_10, HAS_15
• Three messages: add5, add10, returnCoins
• Four actions: dispenseStamp, ret5, ret10, ret15

A stamp dispenser's current state indicates how much money has been inserted. If no money has been inserted, the stamp dispenser is in HAS_0 state. If a nickel has been inserted, the stamp dispenser is in HAS_5 state, and so on. No HAS_20 state appears in the list, because as soon as 20 cents is inserted, a stamp is automatically issued and any change is returned.

The three messages represent the actions a stamp dispenser user can take: inserting a nickel (add5), inserting a dime (add10), or pressing the coin return lever (returnCoins). The four actions the stamp dispenser can take are return a nickel (ret5), return a dime (ret10), return 15 cents (ret15), or dispense a 20 cent stamp (dispenseStamp).

Figure 4-1 shows the state-transition diagram that defines the actions taken and next state that results from a stamp dispenser receiving each message in each state.

Figure 4-1. The stamp dispenser state-transition diagram