What represents dynamic models of how objects change their states in response to events?

The Dynamic Model: Sequence and State Chart Diagrams

Dynamic Model

• Remember that the dynamic model describes the internal behavior of a system
• This can be illustrated in UML with interaction diagrams, statechart diagrams, and activity diagrams
• Interaction diagrams describe behavior in terms of messages exchanged between objects
  • The sequence diagram is a specific kind of interaction diagram
• Statechart diagrams describe the states of an individual object and the possible transitions between states
• Activity diagrams model behavior in terms of activities. Usually this illustrates a more general control flow

When are they used?

• During requirements analysis
  • Help refine use case descriptions
  • Help find participating objects
  • Help identify responsibilities of classes
  • Help identify necessary communication between objects
• During system design
  • Can help refine subsystem interfaces
  • Can help identify control flow and messages between objects, once concrete classes are being designed

Notation

• A sequence diagram represents a graphical description of objects participating in a single use case or scenario (illustrates communication between objects and low of control)
• Arranged in general columns
• First item in each column is an actor, object, or class (using usual notation for these things):
  • Actor a stick figure
  • Class a rectangle with class name inside
  • Object a rectangle with underlined name (and type)
• A message is represented by an arrow from one column to another. Usually labelled with an operation name, looking like a function call.
• An activation is represented with narrow rectangle in a column. Represents duration of an invoked operation
• Dashed lines between activations on the rest of the column -- just shows that everything in the column is part of the same object/class
• A message arrow originates from one of the activations -- indicates not only which object sent a message, but from inside which activated operation (kind of like a function calling another function)
• Sequence diagrams can be based on either a use case or a scenario
  • Can represent a use case with a more general flow of events, usually the "main" flow. Can't really capture all variations with this
  • Easier to represent a scenario -- a single example of the general use case. Specific flow of events and choices

General guidelines for sequence diagrams

• Layout:
  • Make 1st column correspond to the actor that initiates the use case (or scenario)
  • Make the 2nd column a boundary object (actor uses this to initiate use case)
  • Make the 3rd column a control object, which manages the rest of the use case
• Object creation:
  • Control objects should be created by boundary objects that initiate use cases
  • Control objects can create other boundary objects
• Access:
  • Entity objects are accessed by boundary and control objects
  • Entity ojects shouldn't call boundary or control objects. This makes entity ojects easier to share across use cases

Examples

Some other sequence diagrams can be found in the textbook:

• (pg 60) Figure 2-34: setting the time on a watch
• (pg 186-187) Figures 5-8, 5-9, 5-10: Sequence diagrams for ReportEmergency use case, for the book's FRIEND case study
• (pg 213-214) Figures 5-26, 5-27, 5-28: Sequence diagrams for AnnounceTournament use case, ARENA system case study

UML State Chart diagrams

Purpose

• Can help identify important object attributes
• Can help refine the behavior description of an object
• Should focus on the important attributes that affect the object's behavior
• Should be used to represent non-trivial behavior of a given object type (or can apply to a subsystem)

Notation

• Similar to a finite state machine diagram (computer theory)
• A state
  • is a condition satisfied by the attributes of the object (i.e. determined by the current values in the attributes)
  • is represented by a rounded rectangle
• A transition
  • is a change of state triggered by an external event, a condition, or perhaps time elapsed
  • is represented by a directed edge (arrow) between states
• Transitions should be labelled to indicate the triggering event

Examples

Explanation of states in this example:

• new -- represents a newly created Thread object. Not yet running a task
• runnable -- a thread is now executing a task
  • Note: this is referred to as runnable instead of running because the processor still has to schedule threads and processes to run and take turns
• asleep (timed waiting) -- a thread has been put to sleep with a sleep() function call
• waiting -- a thread is blocked, waiting on a resource to become available
• terminated -- a thread has finished its task and has been shut down

Some other statechart examples from the textbook

• (pg 34) Figure 2-4: Statechart diagram for SetTime use case in the watch example
• (pg 63) Figure 2-37: Statechart for Incident class of the FRIEND case study
• (pg 63) Figure 2-38: A statechart for the watch example
• (pg 64-65) Figures 2-39, 2-40: More detailed statecharts for the watch example
• (pg 195) Figure 195: A more detailed statechart for Incident -- FRIEND case study

Which of the following represent dynamic models of how objects change their states in response to events?

What does dynamic model represent?

Which model is known as dynamic model?

Which model diagram represents dynamic behavior of the system?