State Machine Diagrams

Each object in a system goes through a series of discrete states. An object’s current state is indicated by the values of the object’s attributes at a given time. State machine diagrams (commonly called state diagrams) model key states of an object and show under what circumstances the object changes state. Unlike the class diagrams presented in earlier case study sections, which focused primarily on the structure of the system, state diagrams model some of the behavior of the system.

Figure 22.13 is a simple state diagram that models some of the states of an object of class ATM. The UML represents each state in a state diagram as a rounded rectangle with the name of the state placed inside it. A solid circle with an attached stick arrowhead designates the initial state. Recall that we modeled this state information as the Boolean attribute userAuthenticated in the class diagram of Fig. 22.12. This attribute is initialized to false, or the “User not authenticated” state, according to the state diagram.

Fig. 22.13. State diagram for the ATM object.

The arrows with stick arrowheads indicate transitions between states. An object can transition from one state to another in response to various events that occur in the system. The name or description of the event that causes a transition is written near the line that corresponds to the transition. For example, the ATM object changes from the “User not authenticated” state to the “User authenticated” state after the database authenticates the user. Recall from the requirements document that the database authenticates a user by comparing the account number and PIN entered by the user with those of the corresponding account in the database. If the database indicates that the user has entered a valid account number and the correct PIN, the ATM object transitions to the “User authenticated” state and changes its userAuthenticated attribute to a value of true. When the user exits the system by choosing the “exit” option from the main menu, the ATM object returns to the “User not authenticated” state in preparation for the next ATM user.