Page 315 -

282 Part 3 • the analysis Process

2. Synchronous messages, which are calls to functions or subroutines. The calling object
stops and waits for control to be returned to it, along with an optional message.
3. Temporal events, which occur at a predetermined time. These usually do not involve an
actor or any external event.
Material objects have persistence; that is, they exist for a long period of time. Airplane flights,
concerts, and sporting events have shorter persistence (they may have states that transition in a shorter
time). Some objects, called transient objects, do not survive the end of a session. These include main
memory, Web URL (or location) data, web pages, CICS displays, and so on. The only way to save
transient objects is to store information about them, such as storing Web data in a cookie.
Each time an object changes state, some of the attributes change their values. Furthermore,
each time an object’s attributes change, there must be a method to change the attributes. Each of the
methods would need a display or Web form to add or change the attributes. These become the inter-
face objects. The display or Web form would often have more controls (or fields) on them than just
the attributes that change. They would usually have primary keys, identifying information (such as
a name or address), and other attributes that are needed for a good user interface. The exception is a
temporal event, which may use database tables or a queue containing the information.

A State Transition Example
Consider a student enrolling at a university and the various states that he or she would go through.
Three of the states are listed below in detail:

State: Potential Student
Event: Application Submitted
Method: new( )
Attributes changed: Number
Name
Address
User interface: Student Application Web Form
State: Accepted Student
Event: Requirements Met
Method: acceptStudent( )
Attributes changed: Admission Date
Student Status
Return Acceptance Letter
User interface: Accept Student Display
State: Dorm Assigned Student
Event: Dorm Selected
Method: assignDorm( )
Attributes changed: Dorm Name
Dorm Room
Meal Plan
User interface: Assign Student Dorm Display

The other states are Program Student, Current Student, Continuing Student, and
Graduated Student. Each state would have an event, methods, attributes changed, and a user
interface associated with it. This series of states can be used to determine the attributes and meth-
ods that make up part of the class.
The states and events that trigger the changes may be represented on a statechart diagram
(or a state transition diagram). The statechart diagram for Student is illustrated in Figure 10.22.
States are represented by rectangles, and events or activities are the arrows that link the states
and cause one state to change to another state. Transition events are named in the past tense
because they have already occurred to create the transition.