(Optional) Software Engineering Case Study: Identifying Objects States and Activities

(Optional) Software Engineering Case Study Identifying Objects States and Activities

In Section 4.15, we identified many of the class attributes needed to implement the ATM system and added them to the class diagram in Fig. 4.24. In this section, we show how these attributes represent an object's state. We identify some key states that our objects may occupy and discuss how objects change state in response to various events occurring in the system. We also discuss the workflow, or activities, that objects perform in the ATM system. We present the activities of BalanceInquiry and Withdrawal transaction objects in this section.

State Machine Diagrams

Each object in a system goes through a series of 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 several 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 5.29 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. 4.24. This attribute is initialized to false, or the "User not authenticated" state, according to the state diagram.

Figure 5.29. 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 an 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.

Software Engineering Observation 5.5

Activity Diagrams

Like a state diagram, an activity diagram models aspects of system behavior. Unlike a state diagram, an activity diagram models an object's workflow (sequence of events) during program execution. An activity diagram models the actions the object will perform and in what order. The activity diagram in Fig. 5.30 models the actions involved in executing a balance-inquiry transaction. We assume that a BalanceInquiry object has already been initialized and assigned a valid account number (that of the current user), so the object knows which balance to retrieve. The diagram includes the actions that occur after the user selects a balance inquiry from the main menu and before the ATM returns the user to the main menua BalanceInquiry object does not perform or initiate these actions, so we do not model them here. The diagram begins with retrieving the balance of the account from the database. Next, the BalanceInquiry displays the balance on the screen. This action completes the execution of the transaction. Recall that we have chosen to represent an account balance as both the availableBalance and totalBalance attributes of class Account, so the actions modeled in Fig. 5.30 refer to the retrieval and display of both balance attributes.

Figure 5.30. Activity diagram for a BalanceInquiry object.

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The UML represents an action in an activity diagram as an action state modeled by a rectangle with its left and right sides replaced by arcs curving outward. Each action state contains an action expressionfor example, "get balance of account from database"that specifies an action to be performed. An arrow with a stick arrowhead connects two action states, indicating the order in which the actions represented by the action states occur. The solid circle (at the top of Fig. 5.30) represents the activity's initial statethe beginning of the workflow before the object performs the modeled actions. In this case, the transaction first executes the "get balance of account from database" action expression. The transaction then displays both balances on the screen. The solid circle enclosed in an open circle (at the bottom of Fig. 5.30) represents the final statethe end of the workflow after the object performs the modeled actions. We used UML activity diagrams to illustrate the flow of control for the control statements presented in Chapters 45.

Figure 5.31 shows an activity diagram for a withdrawal transaction. We assume that a Withdrawal object has been assigned a valid account number. We do not model the user selecting a withdrawal from the main menu or the ATM returning the user to the main menu because these are not actions performed by a Withdrawal object. The transaction first displays a menu of standard withdrawal amounts (shown in Fig. 2.19) and an option to cancel the transaction. The transaction then receives a menu selection from the user. The activity flow now arrives at a decision (a fork indicated by the small diamond symbol). [ Note: A decision was known as a branch in earlier versions of the UML.] This point determines the next action based on the associated guard condition (in square brackets next to the transition), which states that the transition occurs if this condition is met. If the user cancels the transaction by choosing the "cancel" option from the menu, the activity flow immediately skips to the final state. Note the merge (indicated by the small diamond symbol) where the cancellation flow of activity joins the main flow of activity before reaching the activity's final state. If the user selects a withdrawal amount from the menu, Withdrawal sets amount (an attribute originally modeled in Fig. 4.24) to the value chosen by the user.

Figure 5.31. Activity diagram for a withdrawal transaction.

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After setting the withdrawal amount, the transaction retrieves the available balance of the user's account (i.e., the availableBalance attribute of the user's Account object) from the database. The activity flow then arrives at another decision. If the requested withdrawal amount exceeds the user's available balance, the system displays an appropriate error message informing the user of the problem, then returns to the beginning of the activity diagram and prompts the user to input a new amount. If the requested withdrawal amount is less than or equal to the user's available balance, the transaction proceeds. The transaction next tests whether the cash dispenser has enough cash remaining to satisfy the withdrawal request. If it does not, the transaction displays an appropriate error message, then returns to the beginning of the activity diagram and prompts the user to choose a new amount. If sufficient cash is available, the transaction interacts with the database to debit the withdrawal amount from the user's account (i.e., subtract the amount from both the availableBalance and totalBalance attributes of the user's Account object). The transaction then dispenses the desired amount of cash and instructs the user to take the cash that is dispensed. Finally, the main flow of activity merges with the cancellation flow of activity before reaching the final state.

We have taken the first steps in modeling the behavior of the ATM system and have shown how an object's attributes participate in performing the object's activities. In Section 6.14, we investigate the behaviors for all classes to give a more accurate interpretation of the system behavior by "filling in" the third compartments of the classes in our class diagram.

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