OBJECT-ORIENTED ANALYSIS

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OBJECT-ORIENTED ANALYSIS

• Chapter 11

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OBJECT-ORIENTED ANALYSIS (OOA)

• Semi-formal specification technique
• Several methods
• Booch, OMT, OOSE, Objectory--Essentially
  equivalent
• Nowadays--represent OOA using UML (Unified
  Modeling Language)

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THE THREE STEPS OF OOA

• 1. Use-case Modeling
• Determine how the various results are computed by
  the product (without regard to sequencing)
• Largely action-oriented
• 2. Class Modeling (Object Modeling)
• Determine the classes and their attributes
• Purely data-oriented
• 3. Dynamic Modeling
• Determine the actions performed by and to each
  class
• Purely action-oriented

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Elevator ProblemOOA

• 1. Use-case Modeling
• Use cases Describe the behavior of the system as
  seen from actors point of view.
• Actors external entities that interact with the
  system (e.g. A system adm, a bank customer, a
  bank teller, a central database, etc.)
• A Scenario a concise description of how an actor
  may interact with the system
• a scenario is an instance of a use case

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1. Use-case Modeling

• Identify the actors
• Identify a list of "typical" scenarios to get
  insight into the systems behavior
• Identify use-cases
• a use-case specifies all possible scenarios for
  a given functionality.
• a use case is initiated by an actor.
• A use case represents a flow of events thru the
  system.

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Elevator Problem OOA
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Normal Scenario
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Abnormal Scenario
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2. Object Modeling

• The object model describes the real-world object
  classes of the systems and their relationship to
  each other with an entity-relationship diagram
• It shows the static structure of the system.
• 1. Identify classes/objects
• 2. Identify associations between objects
• 3. Identify attributes of objects
• 4. Represent them using an entity-relationship
  diagram
• 5. Prepare a data dictionary

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Identifying Object Classes

• An Object is an instance of a class that consists
  of
• Data (attributes, state variables, instance
  variables, fields, data members)
• Actions (methods , member functions)
• An object has a state (represented by data
  members) that changes during execution.

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Identifying Object Classes

• Objects include physical entities (house,
  employees, button, etc.) and concepts (payment
  schedule, seating assignment, trajectory, etc.)
• Avoid computer implementation constructs (an
  array, a linked list, a binary tree, etc.)
• Dont be concerned with operations at this points.

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Identifying Object Classes

• Two approaches
• Deduce from use cases and their scenarios
• often many scenarios
• too many candidate classes
• Noun Extraction
• The two approaches are complementary

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Noun Extraction

• Stage 1 Concise Problem Definition
• Define product in a single sentence
• Buttons in elevators and floors are to be used
  to control motion of n elevators in building with
  m floors.
• Stage 2 Incorporate constraints
• buttons illuminate when pressed to request
  elevator to stop at specific floor illumination
  is cancelled when request has been satisfied. If
  elevator has no requests, it remains at its
  current floor with its doors closed.

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Noun Extraction

• Stage 3 Identify nouns.
• Nouns buttons, elevator, floor, movement,
  building, illumination, door
• floor, building, door are outside the problem
  boundarygtexcluded
• Movement, illumination are abstract nouns
  gtexcluded (may become attributes)
• Candidate classes elevator, button
• Subclasses Elevator button and floor button

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Noun Extraction
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Identify Associations

• An association is any dependency between two or
  more classes.
• Example
• Person works-for Company
• User has Home Directory
• Directory contains Files
• contains and part-of are special types of
  associations called aggregation
• Inheritance is also a special type of
  association.

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Identifying Associations

• Problem statement
• location phrases next to, contained in, etc
• communication talk to
• ownership phrases has, part of
• satisfaction of some condition works for,
  married to, manages
• Some depend on knowledge of application domain.

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Identifying Associations

• Example Buttons in elevators and floors are to
  be used to control motion of n elevators in
  building with m floors.
• Buttons control elevators
• There are two types of buttons floor buttons and
  elevator buttons
• However, buttons do not really control elevator
  they simply communicate with it.

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Identify Associations

• Identify association multiplicity
• 1 to 1
• 1 to many
• Many to many
• Identify aggregations
• Identify inheritance relations

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First Iteration of Class Model

• Problem buttons do not communicate directly with
  elevatorgt class Elevator Controller

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Second Iteration of Class Model
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Identify Attributes

• Attributes are properties of individual objects
• Problem Statement
• Correspond to nouns followed by possessive
  phrase collar of the car, position of the
  cursor, salary of the employee, etc.
• Attributes are less likely to occur in statement
  of the problem. Knowledge of application domain.
  
• Try to get only the most important attributes at
  this stage. Others can be added later

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Prepare Data Dictionary

• Write a paragraph describing each object class.
  Described the scope of the class within the
  current problem
• Describe associations and attributes
• Data dictionaries also include description of
  operations.

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CRC Cards

• Used since 1989 for OOA
• For each class, fill in card showing
• name of Class
• Responsibility functionality
• Collaboration list of classes it invokes

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Elevator Controller--CRC
1. Turn on elevator button Totally unacceptable
in OOA
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Elevator Controller--CRC

• 1. Turn on Elevator Button
• Should be
• 1. Send message to Elevator Button to turn on
  button
• What about ?
• 5. Open elevator doors
• Note Elevator doors have a state that changes
  during execution (class characteristic)
• gt add class Elevator Doors

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Elevator Controller--CRC
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3. Dynamic Modeling

• The dynamic model shows the time-dependent
  behavior of the system
• Aim
• Produce a UML state diagram for each object
  class.
• A state diagram is less formal than an FSM
• States, events, and conditions

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States and Events

• The state of an object are defined by the values
  of its attributes.
• Objects change state when stimulated by other
  objects or external actors.
• A event is an individual stimulus from one object
  to another.
• The response to an event by the object receiving
  it can include a change of state or the sending
  of an another event (to the original object or
  another object)

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Events

• An event is something that happens at a point in
  time user depresses left-button, phone receiver
  lifted, call is routed, input string entered.
• An event has no duration assumed to be
  instantaneous
• An event coveys information from one object to
  another
• signal that something has occurred
• Send data values
• left mouse-button pushed (location)

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Example

• Draw a state diagram for a phone line

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Conditions

• A condition is a Boolean function of an object
  values. Example
• Temperature is below freezing
• Floor button is lit
• No cars on N/S left lanes
• A condition is valid over an interval of time
• Generally, a condition indicates the state of
  another object in the system.

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Conditions

• Conditions are used as guards on transitions
• A transition fires when its event occurs, only if
  the guard condition is true
• When you go out in the morning (event), if the
  temperature is below freezing (condition), then
  put on your gloves (next state)

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Traffic Light Controller

• Straight N/S or W/E
• After N/S times-out,
• if there are cars on N/S left lane, N/E left-turn
  signal is turned on
• else straight W/E light is turned on
• Same for W/E left-lane

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Time-out cars in N/S left lane
N/S may go straight
N/S may turn left
Time-out No cars in N/S left lane
Time-out
Time-out
Time-out No cars in W/E left lane
W/E may go straight
W/E may turn left
Time-out cars in W/E left lane
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Operations--Activities Actions

• Operations are performed in response to states or
  events
• Activity an operation that takes time to
  complete.
• Continuous operations display a picture
• Sequential operations that terminate after an
  interval of time performing a computation, play
  a recorded message

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Activities

• An activity continues until complete or
  interrupted by an event that causes transition to
  a another state
• e.g. ringing a telephone bell
• Activities are associated with states

State1 do activity1 do activity2
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Operations--Activities Actions

• Action is associated with an event
• Instantaneous duration is insignificant compared
  to the resolution of the state diagram
• e.g. when callee hangs-up disconnect phone line
• Changing attribute values
• e.g. increment count when some event occurs.

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Summary of Notation
Event condition/action
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CASE Tools for OOA Phase

• Paradigm Plus
• Software through Pictures
• Rose