COMP 325 Lecture 12: Object Oriented Analysis: System Dynamics
major resources: Introduction to Object Oriented Analysis, Brown 2002, Object-Oriented and Classical Software Engineering 6ed, Schach 2005, Object-Oriented Software Engineering, Schach 2008.

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Modeling System Dynamics

• Any object that is created and later deleted has a life cycle (born-lives-dies).
• So far we have focused on static aspects of an object, now turn to dynamics (what happens over time)
• object diagram includes only static aspects of the system
• dynamics are modeled in UML using
• statechart diagrams
• collaboration diagrams
• sequence diagrams
• In Schach OOA, this addresses dynamic modeling and use-case realization
• In Brown OOA, this addresses KRB Step 7 find behaviors

UML Statechart diagrams

• Any object that has attributes has state (set of attribute values).
• Any object having behaviors/associations which change its attributes can undergo state transitions.
• State transition occurs when object state changes
• State transitions are triggered by events.
• Many object state transitions are of interest for analysis modeling - in particular, those that model changes of state that user's-world object goes through. Others occur but are not yet of interest.
• Statechart diagrams (similar to state transition diagrams or finite state machines) are included in UML to model life cycles and state transitions.
• class should probably have a statechart diagram if it has an operational life cycle (pattern repeated many times throughout life, such as states of a library book: shelved -> checked out -> checked in -> shelved).
• What kinds of things are relevant to the "experience" of changing state?
  • the source state, or state before the transition
  • the target state, or state after the transition
  • the event trigger, or event that directly causes the transition
  • the guard condition, an optional condition that must be true at the time event is received for the transition to occur
  • the transition action, an optional and atomic operation carried out "during" the transition
• Some Examples
• Operation of an elevator
• consider single elevator system, such as in Towers Hall
• what states does it have?
• what events cause state transitions, and from what to what?
• what events (if any) do not cause state transition?
• develop a UML statechart diagram
• note that this is an operational cycle, not a life cycle
• Operation of a credit-card gasoline pump
• Operation of an ATM machine (consider 3 classes: customer, machine, account DB)
• Basic symbols (from http://www.smartdraw.com/tutorials/software-uml/uml5.htm):

state		event-driven transition
initial state		final state

UML Collaboration diagrams

• Class diagrams and statecharts do not accurately model execution sequences
• a CRC card models associations and collaborations with other classes from the viewpoint of one class but not dynamics
• a class diagram models associations among all classes in the diagram but not dynamics
• a statechart models class and object dynamics (not associations), but only partially models sequence
• the UML tools to model sequence are collaboration diagrams and sequence diagram
• Collaboration diagram is comparable in power to sequence diagram (see below) for illustrating dynamics, except sequence must be made explicit by numbering arrows.
• Basic symbols (from http://www.smartdraw.com/tutorials/software-uml/uml5.htm):

Object and/or class (do not include attributes) can use stereotype figure or stick person
message passing (method invocation) include sequence number optional condition in [ ]
Association between classes

UML Sequence diagrams

• What a sequence diagram shows
• diagram represents an execution sequence from the viewpoint of one class
• can be developed during use-case role playing exercise (at same time as CRC cards are developed)
• viewpoint is typically that of a "client" class which requests things from other classes
• a 2-dimensional chart
• leftmost "column" is client class
• other "columns" represent classes that it interacts with
• each "row" represents an interaction (event, message, response)
• rows are time ordered from top (first interaction) to bottom (last interaction) -- relative length of time is not modeled
• To see examples of sequence diagrams on the web, search for "uml sequence diagram"
• See http://www.agilemodeling.com/style/sequenceDiagram.htm for some examples

Object and/or class (do not include attributes)		Actor (other stereotype figures can be used)
Object lifeline (starts when created)		Active section of timeline
Object destruction (X at lifeline bottom)		message passing (method invocation)