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ITEC 3010 “Systems Analysis and Design, I”LECTURE 7:LECTURE 7:

The Object-Oriented Approach to The Object-Oriented Approach to RequirementsRequirements

  

[Prof. Peter Khaiter]

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Lecture OutlineLecture Outline

Unified Modeling Language (UML)Unified Modeling Language (UML) Object-Oriented Requirements (OOR)Object-Oriented Requirements (OOR) System ActivitiesSystem Activities Use Case DiagramUse Case Diagram Activity DiagramActivity Diagram System Sequence DiagramSystem Sequence Diagram State Machine DiagramState Machine Diagram Integrating Object-Oriented ModelsIntegrating Object-Oriented Models

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• OOA considers an IS as a set of objects that work together to carry out the function. OO analysis involves two steps:

(1) Definition the set of objects that will make up the system and describing the interactions or communications among the various objects (these interactions take the form of messages between objects)

(2) Describing the internal processes that go on within each object to respond to messages from other objects, and to initiate messages to other objects

• UML is a notation used in OO modeling • A preliminary version of UML was presented in 1995 • In 1997 UML was presented to the Object Management Group

(OMG) as a standard modeling technique • OMG (est. 1989) is a consortium of software vendors, developers and

organization with the goal to provide a common architectural framework for OO applications – www.omg.org

Unified Modeling Language (UML)Unified Modeling Language (UML)

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Object-Oriented Requirements Object-Oriented Requirements (OOR)(OOR)

Object-oriented system requirements are specified and documented through process of building models

Modeling process starts with identification of use cases and problem domain classes (things in users’ work environment)

Business events trigger elementary business processes (EBP) that new system must address as use cases

Use cases define functional requirements

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OO models (or diagrams) to define system requirements are called use case models:• Use case diagram shows the various user roles and how they will use the system. It is an extension of the event table. It is a convenient way to document the function that the system must support.• Sequence diagram shows inputs and outputs and sequence of messages between an external actor and the system objects during a use case or scenario. • The flow of information between the objects is in form of messages

OO models (or diagrams) to define classes of objects and their states are called domain models :• Domain class diagram (see lecture 5) – identifies and classifies the objects that will make up the new system along with their properties (or attributes)

• State machine diagram describes the states and behavior of each object: life of an object in state and transitions

OOR - ModelsOOR - Models

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The System Activities—The System Activities—A Use Case/Scenario ViewA Use Case/Scenario View Use case model used to identify and define all

elementary business processes that system must support

Use cases are identified at two levels:Overview level (event table and use case diagram of all the use cases)

Detailed level (for each use case – use case description, activity diagram, sequence diagram)

Use case – an activity a system carried out, usually in response to a user request

Actor (person, other system, or device that receive services from the system and who actually interact with the system)

Role played by user

Outside automation boundary

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Techniques for Identifying Use Techniques for Identifying Use Cases (Review from Chapter 5)Cases (Review from Chapter 5)

Identify user goals

Each goal at the elementary business process (EBP) level is a use case

EBP – task performed by one user in one place and in response to business event that adds measurable business value, and leaves system and data in consistent state

Event decomposition technique (event table)

CRUD analysis technique (create, read/report, update, delete) to ensure coverage

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Use Case DiagramUse Case Diagram Graphical UML diagram that summarizes information about

actors and use cases

Simple diagram shows overview of functional requirements

Symbols: use case is symbolized by an oval with the name of the use case inside; stick figure represents an actor (i.e., a role)

Two approaches to use case diagrams

By subsystem

By actor

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Simple Use Case with an ActorSimple Use Case with an Actor

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Use Case Diagram with Use Case Diagram with Automation Boundary and Automation Boundary and Alternate Actor NotationAlternate Actor Notation

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All Use Cases Involving Customer All Use Cases Involving Customer as Actoras Actor

Direct access to the system

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A use case diagram of the CSS for RMO organizes by subsystems

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Use Cases of RMO Use Cases of RMO Order EntryOrder Entry Subsystem Subsystem

A packaged notation is used: a tabbed rectangle that groups similar component together

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<<Includes>> Relationship<<Includes>> Relationship

Documents situation in which one use case requires the services of a common subroutine

Another use case is developed for this common subroutine

A common use case can be reused by multiple use cases

E.g., two use cases “Create new order” and “Update order” may need to validate the customer account. A common use case may be defined to carry out this function

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Example of Order-Entry Example of Order-Entry Subsystem with <<Includes>> Subsystem with <<Includes>> Use CasesUse Cases

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• Objective of the use case diagram is to provide an overview of the system (including actors and the functions they perform)

– A use case diagram is like a context diagram in the sense of defining the scope of the system

– However, individual use cases appear more like a DFD fragment in that they identify an individual function that the system must support

• Difference with structured techniques

– The use case diagram begins by defining the automation boundary while in DFD this boundary is often not defined until the entire process has been detailed

– In a DFD an external agent is always the original source or destination of the information and may not necessarily be the one interacting with the system;

– In a use case diagram, an actor is the one who actually interacts with the system whether or not that actor is the original source of information

Use Case Diagram vs. Structured Use Case Diagram vs. Structured TechniquesTechniques

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Example: in the RMO case, a customer may call an RMO clerk and make an order.

The DFD would identify the external entity as the customer; the clerk’s activity would be embedded in a process “Enter customer order”.

In the use case diagram, the clerk would be identified as the actor who uses the system to enter or create a customer order

The use case diagram does not indicate data flows, i.e. information flowing into and out of the system is not shown (this is done with the next level of modeling – the sequence diagrams)

Important to identify every possible role that will use the system.

Use Case Diagram vs. Structured Use Case Diagram vs. Structured Techniques (cont’d)Techniques (cont’d)

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Developing a Use Case DiagramDeveloping a Use Case Diagram

Two main approaches to developing a use case diagram:

First approach: start with an event table and analyze each event to determine:

• How the system is used to support the event

• The actors who initiate the event

• Other use cases that will need to be invoked

• Generally, each event will become a use case

Second approach: identifying all the actors who use the system and the EBPs with the user goal technique Actors must actually contact the system Assume perfect technology condition (no technical activities!) Actors – are roles played by users (e.g., order clerk, manager) Identify goal for each actor (e.g., “accept a return”)

Finalize with a CRUD analysis to ensure completeness (every class has sufficient use cases to support its all methods) – a cross-check!

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• A use case only shows that an actor interacts with the system to carry out a business activity• There may be a whole series of individual steps to accomplish the use case (these steps are described with a narrative called a flow of activities) - steps within a use case• A use case may have several alternative internal activities. In the RMO example, for the use case “Create new order” there are at least two sequences when:

– A customer creates telephone order through clerk (a clerk interacts with the system), or– A customer creates web order (a customer interacts with the system)

• A particular sequence of activities within a use case is called scenario. It represents a unique path through the use caseE.g., two possible scenarios for the RMO use case “Create new order”: in the first instance, the second actor “Order clerk” is involved who is actually interfacing with the system

ScenariosScenarios

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Two scenarios for RMO Two scenarios for RMO ““Create new order”Create new order”

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Activity DiagramsActivity Diagrams

Used to document workflow of business process activities for each use case or scenario

Standard UML 2.0 diagram as seen in Chapter 4

Can support any level of use case description; a supplement to use case descriptions

Helpful in developing system sequence diagrams

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Activity Diagram— Telephone Order Scenario

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Activity Diagram— Web Order Scenario

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Identifying Inputs and Outputs—Identifying Inputs and Outputs—The System Sequence DiagramThe System Sequence Diagram

Interaction diagram – a communication diagram or a sequence diagram

System sequence diagram (SSD) is type of UML 2.0 interaction diagram

Used to model input and output messaging requirements for a use case or scenario

Shows sequence of interactions as messages to/from actors and internal objects during flow of activities

Emphasis: how the actor interacts with the system by entering input data and receiving output data

Object notation instead of class notation is used to show that message is sent to an individual object, not the class

System is shown as one object: a “black box”

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Object and class names in SSDObject and class names in SSD

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SSD NotationSSD Notation

Lifeline or object lifeline is a vertical dashed line under object or actor to show the sequence of messages over time: from top to bottom

Message is labelled on arrows to show messages sent to or received by actor or system, including input data: request operation in the destination object

Actor is role interacting with the system with messages

Object is the component that interacts with actors and other objects

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System Sequence Diagram (SSD) System Sequence Diagram (SSD) NotationNotation

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SSD NotationSSD Notation

Activation line

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SSD LifelinesSSD Lifelines

Vertical line under object or actorShows passage of time

If vertical line dashedCreation and destruction of thing is not important for scenario

Long narrow rectanglesActivation lifelines emphasize that object is active only during part of scenario

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SSD MessagesSSD Messages

Internal events identified by the flow of objects in a scenario

Requests from one actor or object to another to do some action

Invoke a particular method

Input message – solid line

Return message – dashed line

Complete syntax:

* [true/false condition] return-value := message-name (parameter list) 

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SSD for the RMO Look up item SSD for the RMO Look up item availabilityavailability

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Repeating Message

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Developing a System Sequence Developing a System Sequence DiagramDiagram

Begin with detailed description of use case from fully developed form or activity diagram

Identify input messages Describe message from external actor to

system using message notation Identify and add any special conditions on

input message, including iteration and true/false conditions

Identify and add output return messages

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Activity Diagram of the Telephone Activity Diagram of the Telephone Order ScenarioOrder Scenario

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Resulting SSD for the Telephone Resulting SSD for the Telephone Order ScenarioOrder Scenario

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SSD of the Web Order Scenario for the Create New Order Use case

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SSD includes objects from the class diagram and actors from the use case diagram

The relationship of use case The relationship of use case diagram, class diagram and diagram, class diagram and sequence diagramssequence diagrams

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Developing a System Sequence Developing a System Sequence Diagram: StepsDiagram: Steps

1. Identify all the objects and actors that are involved in the scenario (they are only actors have been identified in the use case diagram and objects from the classes that are identified in the class diagram)

2. Based on the flow of activities, identify each message that will be required to carry out the scenario along with both source object (or actor) for the message and the destination object (or actor)

3. Determine whether each message is always sent or sent only under certain conditions

4. Sequence the messages correctly and attach them to the appropriate lifelines of actors and objects

5. Add the formal syntax on the messages for conditions, message names and passed parameters

6. Add response messages and communications

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A complete SSD for the RMO use case Create new order

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Just for Fun!Just for Fun!

http://www.visualjokes.com

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Identifying Object BehaviourIdentifying Object Behaviour—— The State Machine Diagram (SMD)The State Machine Diagram (SMD)

SSDs give external view of object behaviour – messages passed around but do not show, what an object does when it gets a message

There is a need to specify internal logic for each object, i.e., a description of the actions that the objects perform themselves

State machine diagram is UML a 2.0 diagram that models object internal behaviour, states and transitions

Complex problem domain classes can be modeled

Each object is an instance of a class, and comes into existence in some manner

During its existence it is “in” certain states and makes transitions from state to state

These states and the changes an object makes from state to state are shown in SMD (two main symbols: state and transitions)

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Identifying Object BehaviourIdentifying Object Behaviour—— The State Machine DiagramThe State Machine Diagram

State of an object

A condition that occurs during its life when it satisfies some criterion, performs some action, or waits for an event

Each state has unique name (e.g., “on”, “working”, “loading equipment”, etc.) and is a semipermanent condition or status (because external events can interrupt them)

Action is an activity performed by an object in a particular state A state is represented by a rectangle with rounded corners (with

the name of the state inside) Any actions that must be performed during the period of the state

are placed below the state name in the rectangle Transition - movement of an object from one state to

another stateConsidered to be short in duration and cannot be interrupted Once started, a transition runs to completion by taking the object to the new state (called destination state)

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Simple SMD for a PrinterSimple SMD for a Printer

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State Machine TerminologyState Machine Terminology

Pseudostate – the starting point of a state machine, indicated by a black dot

Origin state – the original state of an object from which the transition occurs

Destination state – the state to which an object moves after the completion of a transition

Message event – the trigger for a transition, which causes the object to leave the origin state

Guard condition – a true/false test to see whether a transition can fire

Action expression – a description of the activities performed as part of a transition

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Composite States and Composite States and ConcurrencyConcurrency——States within a States within a StateState

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Concurrent Paths for Printer in Concurrent Paths for Printer in the On Statethe On State

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State Machine Diagram and SSDState Machine Diagram and SSD

•Each column is labelled with the name of a class (the columns identify all of the classes in the system); each row is for one sequence diagram; the Xs in the cells show which classes are involved in which sequence diagram).

• An SMD for a class is based on the entire column for that class

• Every cell with X provides information about the messages to and from the class

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Rules for Developing State Rules for Developing State Machine DiagramMachine Diagram

Review domain class diagram, select important ones, and list all state and exit conditions

Begin building state machine diagram fragments for each class

Sequence fragments in correct order and review for independent and concurrent paths

Expand each transition with message event, guard-condition, and action-expression

Review and test each state machine diagram

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States and Exit Transitions for States and Exit Transitions for OrderItemOrderItem

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Partial State Machine for Partial State Machine for OrderItemOrderItem

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Final State Machine for OrderItemFinal State Machine for OrderItem

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Order Domain Class for RMOOrder Domain Class for RMO——States and Exit TransitionsStates and Exit Transitions

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First-Cut State Machine Diagram First-Cut State Machine Diagram for Orderfor Order

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Second-Cut State Machine Second-Cut State Machine Diagram for OrderDiagram for Order

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Integrating Object-Oriented Integrating Object-Oriented ModelsModels

Complete use case diagram is needed to understand total scope of new system

Domain model class diagrams should also be as complete as possible for entire system

With iterative approach, construct use case descriptions, activity diagrams, and system sequence diagrams for use cases in iteration

Development of a new diagram often helps refine and correct previous diagrams

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Relationships Between OO Relationships Between OO Requirements ModelsRequirements Models

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ReadingsReadings

Today’s lecture: Chapter 7 – “The Object-Oriented Approach to Requirements”

For next lecture: Chapter 8 – “Evaluating Alternatives for Requirements, Environments, and Implementation”

Thank

you

!!!