Abstract Data Type(s)

  We abstract the notion of a FiniteStateMachine and the related notions of a Stimulus, a Transition, an Action, and a State.

FiniteStateMachine

A FiniteStatMachine maintains a set of States, one of which is designated as the current State. A FiniteStatMachine provides the following services:

addState

a given State is added to the machine;

initiate

an already given State is designated as the current one;

addTransition

a Transition that links two already given States (or an already given State with itself) is defined for a given Stimulus and a given Action, one State is the beginning of the Transition and the other is the end;

fire

a Stimulus is given to the FiniteStateMachine, and if the current State is the beginning of a Transition with that Stimulus, the current State is changed to the end State of the Transition and the Action of the Transition is carried out.

Stimulus

A Stimulus is a key that selects a Transition based upon the current State.

State

A State is an entity that provides the current focus for a FiniteStateMachine; pairs of States (or a single State repeated twice) serve as the beginning and ending of a Transition.

Action

An Action provides a single service:

doIt

a procedure with no input that carries out some algorithm (for the purpose of this assignment, it can merely print a message).

As a test that your Finite State Machine can do something useful, write a simple example program that uses it to simulate a coin-operated turnstyle. The turnstyle is initially ``locked.'' In a locked state it cannot be ``rotated.'' Upon ``deposit'' of a coin, it becomes ``unlocked.'' While in an unlocked state, it will not accept deposit of a coin. After being rotated (i.e., when a person passes through), it returns to the locked state.