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ObjectObject--OrientedOrientedModeling and DesignModeling and Design

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understanding problems

communicating with application experts

modeling enterprises

preparing documentation

Object-Oriented models are useful for :

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Emphasis is on doing things (algorithms)Large programs are divided into smaller programs

(functions)

Most of the functions share global data

Data move openly around the system from fn.. to fnFunctions transforms data from one form to

anotherEmploys top-down approach in program design

What is Procedure-OrientedProgramming ?

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Relationship of data and functionsin Procedural Programming

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What is Object-Oriented Programming ?

Emphasis is on data rather than procedurePrograms are divided into Objects

(Abstracted into classes)

Data is encapsulated in the objects

Objects communicate with each other

through functions

Follows bottom-up approach in program

design.

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What is Object-Oriented Programming ?

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Characteristics of Object-Oriented Model

Abstraction

Encapsulation

PolymorphismInheritance

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Abstraction

Focus is an what an Object is and does

before deciding how it should be implemented.

Use of abstraction preserves the freedom

to make decisions as long as possible byavoiding premature commitments to details.

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Encapsulation

Separates the external aspects of an object,which are accessible to other objects, from

the internal

implementation details of the object.

Prevents a program from becoming so

inter-dependent that a small change has

massive ripple effects.

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Polymorphism

The same operation may behave

differently on different classes.

An operation is an action or transformation

that an object performs or is subject to.

Example : 'move' operation may behavedifferently on the Window andChess Piece Classes.

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Inheritance

Is the sharing of attributes and operations

among classes based on a hierarchicalrelationship.

The properties of a Superclass need not

be repeated in the subclass.Example : Scrolling Window and

Fixed Window are subclasses ofSuperclass Window.

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OBJECT MODELINGCONCEPTS

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An Object Model

captures the static structure of the system

shows the objects, the relationship

between them,

and

the attributes and operations thatcharacterize each class of objects.

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Objects

is a concept, abstraction, or thing with crisp

boundaries and meaning for an application.Purpose

promotes understanding of the real

world provides a practical basis for computer

information.Objects have identity and are

distinguishable.Example : Two apples with the same color,

shape and texture are still individual entities.

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Classes

It describes a group of objects, with common

attributes, operations and semantics.Example : Fruit is a class.

Objects in a class have the same attributes

and behaviour patterns.A object's class is an implicit propertyof the object.

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OBJECT DIAGRAMS

OMT symbol for a class in a box.

CLASS Person

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ATTRIBUTES

is a property of the objects in a class

is a data value held by the objects in a class

different Object instances may have the sameor different values for a given attribute.

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ATTRIBUTES

Each attribute name is unique within a class.

An attribute should be a pure data value, not anobject.

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OPERATIONS

An operation is a function or transformation that

may be applied to or by objects in a class. Same operations may apply to many differentclasses (Polymorphic Operation).

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Methods

is the implementation of an operation for a class.

Example : Operation 'Print' in class 'File'

can be implemented for printing ASCII

files, printing binary files are for printingdigitized picture files.

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Links and Associations

A link is a physical or conceptual connectionobject instances i.e. represents the relationship

between them.

Example : Joe Smith works for Simplex Company

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Links and Associations

An association describes a group of links with

common structure and common semantics.

Example : A persons works for a company.

Association and links often appear as verbs in theproblem statement.

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Representation ofLinks / Associations

Associations may be binary, ternary or higher order.

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Representation ofLinks / Associations

used to represent ternary and n-ary association.

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Multiplicity of an Associations

Specifies how many instances of one classmay to a single instance of an associated class.

Usually described as "one" or "many".

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Multiplicity of an Associations

In OMT notation,

* Solid Ball => "many" meaning zero

or more

* Hollow ball => "optional" meaning

zero or one* A line without balls => one-to-one association.

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Multiplicity of an Associations

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Link and Association Concepts

LinkAttribute is a property of the links in an

association.In OMT notation, it is represented by abox attached to the association by a loop.

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Link and Association Concepts

Role Names

* Role is one end of an association.

* Role name is a name that uniquely

identifies one end of an association.

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Qualified Association

Qualifier is a special attribute that reduces the

effective multiplicity of an association.

This reduces the effective multiplicity of thisassociation from one-to-many to one-to-one.

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Generalization

* Is the relationship between a class and one

or more refined versions of it. The class being refined is called the Super Class.

Each refined version is called the Sub Class.

Is also called the "is-a" relationship.

In OMT notation, generalization is represented by atriangle connection a superclass to its subclass.

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Generalization

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Inheritance

Each subclass is said to inherit the

features of its superclass.

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Inheritance

Each subclass not only inherits all the features

of its ancestors but adds its own specific

attributes and operations as well.

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Multiple Inheritance

A class may have more than one superclass

and inherits features from all panels

Such a class is called a "Join Class".

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Multiple Inheritance

Example:

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Generalization as Extension and Restriction

Extension : When a subclass adds new

features.

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Generalization as Extension and Restriction

Restriction : When a Subclass constrainsthe ancestor attributes.

Example:

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Generalization, Inheritance andSpecialization

Generalization is the relationship among

classes.

Inheritance is the mechanism of sharingattributes and operations using the

generalization relationship.

Generalization and Inheritance:

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Aggregation:

Is the "part-whole" or "a-part-of" relationship

Objects representing the components are

associated with an object representing the entire

assembly.

Aggregation is a special form of association.

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Aggregation:

In OMT notation, aggregation is represented by a

small diamond at the assembly end of the

relationship.

Example:

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Aggregation Versus Generalization

Aggregation relates instances

two distinct parts are involved, one is a part of

the other.

Generalization relates classes

Subclass inherits the properties of the

Superclass.

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Aggregation Versus Generalization

Aggregation and Generalization

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ABSTRACT CLASSES

An abstract class is a class that has no direct

instances but whose descendent classes have direct

instances.

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CONCRETE CLASSES

A Concrete class is a class that is instantiable.Example:

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META DATA

In RDBMS, a data table may store information

such as capital of India is New Delhi.

Capital of Japan is Tokyo. And so on .

A meta table will store the fact that acountry has a Capital City.

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Homomorphisms

Is the mapping between two associations.

A frequent usage is the mapping between

a descriptor and a part tree

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Homomorphisms Contd..

Homomorphisms mostly occur for complex

applications that deal with meta data

It is an special type of relationship between

relationships.

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Dynamic Modeling

examines changes to the objects

and their relationships overtime.

represents control information : The sequence of

events, states and operations that occur within

a system of objects.

describes the sequences of operations that occur in

response to external stimuli.

represents the control structure of a system.

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Events and States

State is the attribute values and links held by an object

An event results in a series of changes to the

object states.

The pattern of events, states and state

transitions for a given class can be abstracted

and represented as a state diagram.

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Events Contd..

Example:

airplane flight departs

mouse button pushed

input string entered

phone receiver lifted

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Scenarios

Scenario is a sequence of events that

occurs during one particular execution

of a system.

Scenario can be the historical record of executing

a system.

Scenario can be a thought experiment of

executing a proposed system.

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Scenarios Contd..

Example : Scenario for making a telephone call.

caller lifts receiverdial tone begins

caller dials digits

called phone begins ringing

called party picks up the receiverphones are connected

caller hangs upphones are disconnected.

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Events and Traces:

Event trace is the representation of the sequence

of events and the objects exchanging events as an

augmented scenario.

Sender and Receiver objects of each event is

identified

Object is represented as a vertical line and eachevent as a horizontal arrow from the sender to the

receiver object.

Sequence of events is important.

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Events and Traces:Contd..

Event Trace Fora Phone Call

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State:

Is an abstraction of the attribute values and

links of an object.

A state specifies the response of the object to

input events.

The response of an object to an event may include

an action or a change of state by the objects.

Example : After the receiver is lifted and before the first

digit is dialed, the phone line is in state Dial

tone.

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State Diagrams

It relates events and states

It is a graph whose nodes are statesand whose directed arcs are transitions

labelled by event names.

Describes the behaviour of a single

class of objects

Example : State diagram for phone line

Figure 5.5 Page 90 (R

ambaugh)

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Relationship between Object and Dynamic

Models

The dynamic model structure is related

to and constrained by Object model structure.

A sub-state refines the attribute and link

values that the object can have.

The dynamic model specifies allowable

sequences of changes to objects from the

object model.

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Relationship between Object and Dynamic

Models

The dynamic model structure is related

to and constrained by Object model structure.

A sub-state refines the attribute and link

values that the object can have.

The dynamic model specifies allowable

sequences of changes to objects from the

object model.

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Functional Modeling

Describes computations within a system.

It shows how output values in a computation are

derived from input values, without regard for the

order in which the values are computed.

Consists of multiple DFD's, which show the flow of

values from external inputs, through operations

and internal data stores, to external outputs.

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Data Flow Diagrams (DFD's)

Shows the functional relationships of values

computed by a system, including input values and

output values and internal data stores.

A data flow diagram is a graph showing the flow

of data values from their sources in objects

through processes that transform them to

their destinations in other objects.

It does not show central information.

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DFD's Contd..

A DFD contains

processes, that transform data

data flows, that move data

actor objects, that produce and consume data

data store objects that store data passively.

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Processes

A process transforms data values.

The final model only indicates the possible

functional paths, it does not show which path will

actually occur.

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Processes

The results of such a process depends on the

behaviour of the system, as specified by the

dynamic model.

Process may or may not have side effects on the

non-functional components such as data stores and

external objects.

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Processes Contd..

Representation in DFD is as an ellipse, containing

a description of the transformation.

Each process has a fixed number of input and

output data arrows.

Input and Outputs are labeled.

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Processes Contd..

Example:

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Data Flows

A data flow connects the output of an object or

process to the input of another object or process.

It represents an intermediate data value within a

computation.

The value is not changed by the data flow.

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Data Flows Contd..

Representation in DFD's is by an arrow between

the procedure and consumer of the data value.

The arrow is labeled with a description of the

data.

The same value can be sent to several objects,

this is indicated by a fork.

Output arrows if not split into its components,

may be unlabeled.

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Data Flows Contd..

Example:

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Actors

An actor is an active object that drives the data

flow graph by producing or consuming values.

Actors are attached to the inputs and outputs of a

data flow graph.

Actors act as sources and sinks of data, so also

called as terminators.

Actors lie on the boundary of the data flow graph

but terminate the flow of data.

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Actors Contd..

Representation in DFD's is by a rectangle

Arrows between the actor and the diagram are

inputs and outputs of the diagram.

Actions of actors are a part of the Dynamic Model.

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Actors Contd..

Example:

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Data Stores

A data store is a passive object within a data

flow diagram, that stores data for later access.

A data store does not generate any operations on

its own but merely responds to requests to store

and access data.

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Data Stores Contd..

A data store allows values to be accessed in a

different order, than they are generated.

Aggregate data stores, such as lists and labels,

provide access of data by insertion order or index

keys.

Representation of a data store is by a pair of

parallel lines containing the name of the store.

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Data Stores Contd..

Input arrows indicate information or operations

that modify the stored data (like adding,

modifying, deleting elements)

Output arrows indicate information retrieved from

the data store the data store

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Data Stores Contd..

Example:

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Relationship between Object, Dynamic and

Functional Models

Functional model shows what "has to be done" by a

system

Object model shows the "doer" - the objects of the

system.

Dynamic model shows the sequence in which theoperations are performed.

Processes in the final model shows objects that

are related by functions.

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Relative to the Functional Model

The Object Model shows the structure of the

actors, data stores, and flows in the functionalmodel.

The Dynamic Model shows the sequence in

which processes are performed.

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Relative to the Object Model

The final model shows the operations in the

classes and the arguments of each operation.

The Dynamic Model shows the states of each

object and the operations that are performed

as it receives events and changes state.

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Relative to Dynamic Model

The final Model shows the definitions of the leaf

actions and activities that are undefined with thedynamic model.

The Object Model shows what changes state

and undergoes operations.

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OBJECT ORIENTED ANALYSIS (OOA)

Stages in OOA

Problem / Requirement is stated

Problem / Requirement is understood

Essential features are abstracted

into an analysis model (Object,Dynamic

and functional Models)

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OOA Contd..

Iterate the analysis, refine the analysis model

and Restate the requirements if required.

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Goal of OOA

Is to fully specify the problem and application

domain without introducing a bias to any particular

implementation.

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Goal of OOA Contd..

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Problem orRequirements Statements

Problem scope

What is needed

Application context

Assumptions

Performance needs

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Problem orRequirements Statements

Contd..

The analyst must work with the requestto refine the requirements, so they

represent the requestor's true intent.

Page 151 include Fig. 8.3 + explanation of paragraph.

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Object Modeling

Object Model shows the static data structure of

the real-world system, as object classes and

relationship between classes.

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Object Modeling Contd..

Information for the Object Model comes from

the problem statement

the expert knowledge of the application

domain, and

the general knowledge of the real world.

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Steps for construction of an Object Model

Identify all possible Objects and classes

Eliminate Redundant classes

Eliminate irrelevant classes

Eliminate operations that are applied toobjects

Eliminate Implmentation Constructs.

1. Identify Objects and Classes

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ATM ExampleATM classes identified from knowledge of

problem domain :

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iminating unnecessary Classes.BAD CLASSES

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Good Classes

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2. Preparing a Data DictionaryWrite a paragraph precisely describing

each object class

Describe the scope of the class withinthe current problem

Mention any assumptions or restrictions

on its membership or use

Briefly describe associations, attributes

and operations

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3. Identifying AssociationsIdentify all possible associations between the

classes.

Eliminate associations between eliminated classes.

Eliminate Irrelevant orImplementation associations

EliminateActions that describe a transient event.

Convert Ternary associations into Binaryassociations (But without loss of information)

Eliminate Derived associations.

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ATM Example

Some possible Associations from ATM

problem Statement

Banking network includes Cashiers and ATMsConsortium shares ATMs

Cashier enters transaction for account

Bank provides Bank Computer

Bank owns Cashier station

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ATM Example Contd..

ATMs communicate with Central Computer

about transaction

Central Computer clear transaction with bank

ATM accepts Cash Card

ATM interact with user

ATM dispenses Cash

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ATM Example Contd..

ATM prints receipts

System handles concurrent access

Consortium consists of banks

Bank holds account

Consortium owns Central Computer

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4. Identifying Object Attributes

should describe the properties of

individual objects

Identify link attributes too.

Eliminate unnecessary and incorrectattributes.

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ATM ExampleSome classes in ATM object model with attributes

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ATM Example

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6. Grouping Classes into Modules

Group classes into sheets and modules

T

ightly coupled classes should be groupedtogether

Diagrams may be divided into sheets of uniform

size for convenience in drawing, printing and

viewing...

ATM example for grouping classes into

Modules.

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6. Grouping Classes into Modules Contd..

Possible Modules :

Tellers - Cashier, entry station, Cashier Station, ATM

Account - account, Cash Card, Customer, Cashier,

Transaction

Banks - Consortium, Bank.

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Dynamic Modeling

It shows the time-dependent behaviour of the

system and objects

This model is important for interactive systems

only.

Look for externally visible stimuli and responses.

Summarize permissible event sequences for

each object with a state diagram.

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Steps for construction of an Dynamic Model1. Preparation of a Scenario

Scenarios show the major interactions, externaldisplay formats and information exchanges

Scenario is a sequence of events

Event occurs whenever information is exchanged

between an object in the system and an outside

agent, such as a user, a sensor or another task.

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Contd..ATM Example : Possible Scenario

The ATM asks the user to insert a Card

The user inserts a CashCard.

The ATM accepts the Card and reads serial number

The ATM requests the password

The user enters the 'password

The ATM verifies the serial number and password

with the consortium and so on .........

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2. Identifying Events betwwn Objects

Events include all signals, inputs, decisions,

interrupts, transitions and action to or from useror external devices.

Show each scenario as a event-trace or event-flow

diagram.

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Contd..

ATM Example:

"Enter Password" is an event sent from

external agent User to application object ATM.

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3. Prepare a State Diagram

for each Object class with retrieval

dynamic behaviour

showing the events the objects receives and sends

Early scenario or event trace corresponds to apath through the state diagram.

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ATM Example :

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4. Matching Events between Objects

Check for completeness and consistency at

the system level

Every event should have a sender and a receiver

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4.Matching Events between Objects Contd..

ATM Example:

An "account" can potentially be accessed

concurrently by more than one machine.

Access to account needs to be controlled

to ensure that only one update at a timeis applied.

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Steps for Construction of Functional Model

1. Identify Input and Output Values

List all the input and output values.

It is the interaction between the system and the

outside world.

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Contd..

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2. Building Data Flow Diagrams

Show how each output value is computed from

input values.

Construct the DFD's in layers.

DFD's specify only dependencies amongoperations.

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3. Describng Functions

Refine the DFD's

Describe each function

What it does

Not flow to implement it

Description can be declarative or procedural.

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ATM Example

Update-account-function ()

If amount on withdrawal > the current account balance

reject transaction.

If amount on withdrawal < the current account balance

debit the account and dispence the

amount request and so on ......

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4. Identify Constraints between Objects

Constraints are functional dependencies

between objects that are not related by an

input-output dependency.

Constraints can be on two objects at

the same time.

or between instances of the same

object and different times

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4. Identify Constraints between Objects

or between instances of different objects and

different hours.

Constraints are the preconditions on the

functions.

ATM Example:

A possible constraint

'No account balance may ever be negative'

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5. Specifying Optimization Criteria

Specify values to be maximized, minimized or

optimized

In case of conflict, indicate the trade-off.

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5. Specifying Optimization CriteriaATM Example:

Possible Optimization Criteria

Minimize the number of physical messages

sent between different sites.

Minimize the time an account is locked for

concurrency reason.

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Final Stage :

Refine the analysis model

Restate the Requirements if required.

Verify the analysis model.

Final verified analysis model serves as the basisfor system architecture, design and

implementation.

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Summary

Purpose of OOA is to state and understand the

problem and application domain, so that a correct

design can be constructed

A good analysis captures the essential features of

the problem without introducing implementation

artifacts.

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SYSTEM DESIGN

During analysis, the focus is on what is to be

done, independent of how it is done. During design,

decisions are made about how the problem will be

solved, first at high level, then at increasingly

detailed levels.

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SYSTEM DESIGN

System Design is the high level strategy for

solving the problem and building a solution.

The overall organization of a system into

components is called the System Architecture.

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SYSTEM DESIGN

The system designee must make the following

decisions :

Organise the system into sub-systems

Identify concurrency inherent in the problem

Allocate sub-system to processor and tasks

Choose an approach for management of

data stores

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SYSTEM DESIGN

The system designee must make the following

decisions : Contd..

Handle access to global resources

Choose the implementation of control in software

Handle boundary conditions

Set trade off priorities

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Contd..

There are a number of architectural styles suitable

for various applications. They differ in placing

emphasis on the three models - Object, Dynamic and

Functional.

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Breaking System into sub-systems

The first step in system design is to divide the system

into a small number of components called subsystem.

A subsystem is not an object nor a function but a

package of classes, associations, operations, events

and constraints that are inter-related and that have

a reasonably well-defined and small interface withother systems. It defines a coherent way of looking at

one aspect of the problem.

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Breaking System into sub-systems

Subsystem can be identified by the services it

provide. A service is a group of related functions

that share some common purpose.

Each subsystem should have well-defined interface

to the rest of the system.

S

ubsystem should be defined so that mostinteractions are within subsystems, rather than

across the subsystem boundaries, in order to

reduce the dependencies among the subsystem.

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contd..A system should be divided into small (reasonable)

number of subsystems. The lowest level subsystems

are called modules.

The relationship between two subsystems can be

client-supplier or peer-to-peer.

The subsystem should have simple topology

(pipeline as preferred to star)

The decomposition of system into subsystem can be

organized as a sequence ofhorizontal layers or

vertical partitions.

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contd..

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Layers

The system is divided into subsystems as a

horizontal layer where each layer knows about

the layers below it but has no knowledge of

layers above it. A supplier client relationship

exists between lower layers and upper layers.

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Layers Contd..

closed open

Layer architecture comes in two forms;

a layer uses features

only of immediately lower

layer

reduces dependencies

layers and allows easier

changes

can use features of any

lower layer to any depth

reduces need to redefineoperations at each level

and results in more

efficient and compact

code

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Layers Contd..

closed open

Layer architecture comes in two forms;

does not observe

information hiding so it

is less robust.

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Partitions

Partitions vertically divide a system into several

independent or weakly-coupled subsystems,

each providing one kind of service

e.g. Subsystems in OS.

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Hybrid

Layers and partitions can be used in combination.

Layers can be partitioned

Partitions can be layered

For example, simulation of applicationand interactive graphics

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Hybrid Contd..

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Identifying Concurrency

System design needs to identify which objects

must be active concurrently and which objects

have activity that in mutually exclusive.

The later objects can be folded together

into a single thread of control or task.

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Identifying Concurrency Contd..There are two ways of identifying concurrency.

1. Identify inherent concurrency

The dynamic model is the guide to identifying

concurrency. Two objects are inherently concurrent

if they can receive events at the same time without

interacting. If the events are unsynchronized, the

objects can not be folded into a single threadof control.

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Identifying Concurrency Contd..

2. Defining Concurrent tasks

By examining the state diagrams of individualobjects and the exchange of events among them,

many objects can be folded together onto a single

thread of control. On each thread of control, only

a single object at a time is active. Thread of control

are implemented as tasks in computer

systems.