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Systems Analysis and Design in a Changing World, Fifth Edition

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Learning Objectives

• Understand the models and processes of defining object-oriented requirements
• Develop use case diagrams and activity diagrams
• Develop system sequence diagrams
• Develop state machine diagrams to model object behavior
• Explain how UML diagrams work together to define functional requirements for the object-oriented approach

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Overview

• The objective of requirements definition is understanding – understanding the users’ needs, the business processes, and the systems to support business processes
• Understand and define requirements for a new system using object-oriented analysis models and techniques
• Line between object-oriented analysis and object-oriented design is somewhat fuzzy
• Iterative approach to development
• Models built in analysis are refined during design

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Object-Oriented Requirements

• Object-oriented modeling notation is Unified Modeling Language (UML 2.0)
• UML was accepted by Object Management Group (OMG) as standard modeling technique
• Purpose of Object Management Group
• Promote theory and practice of object-oriented technology for development of distributed systems
• Provide common architectural framework for OO

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Object-Oriented Requirements (continued)‏

• 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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Object-Oriented Requirements Models

• Use case model – a collection of models to capture system requirements
• Use case diagram – identify actors and their roles and how the actor roles utilize the system
• Systems sequence diagrams (SSDs) – define inputs and outputs and sequence of interactions between user and system for a use case

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Object-Oriented Requirements Models (continued)

• Message – the communication between objects within a use case
• Domain model – describes the classes of objects and their states
• State machine diagrams – describe states of each object

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Requirements Models—Traditional vs OO

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Figure 7-1

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The System Activities—
A Use Case/Scenario View

• Use case analysis used to identify and define all business processes that system must support
• Use case – an activity a system carried out, usually in response to a user request
• Actor
• Role played by user
• Outside automation boundary

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Techniques for Identifying Use 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 Diagram

• Graphical UML diagram that summarizes information about actors and use cases
• Simple diagram shows overview of functional requirements
• Can have multiple use case diagrams
• By subsystem
• By actor

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Simple Use Case with an Actor

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Figure 7-2

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Use Case Diagram with Automation Boundary and Alternate Actor Notation

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Figure 7-3

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All Use Cases Involving Customer as Actor

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Figure 7-4

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Use Cases of RMO Order Entry Subsystem

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Figure 7-5 (partial figure)‏

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<<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

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

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Figure 7-6

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Developing a Use Case Diagram

• Underlying conditions for describing use cases
• Based on automated system, e.g. users “touch” the system
• Assume perfect technology condition
• Iterate through these two steps
• Identify actors as roles
• List goals, e.g. use cases, for each actor. A goal is a unit of work.
• Finalize with a CRUD analysis to ensure completeness

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Activity 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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Figure 7-8

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Activity Diagram—
Web Order Scenario

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Figure 7-9

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Identifying Inputs and Outputs—
The 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 during flow of activities
• System is shown as one object: a “black box”

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SSD Notation

• Lifeline or object lifeline is a vertical line under object or actor to show passage of time for object
• Message is labeled on arrows to show messages sent to or received by actor or system
• 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) Notation

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Figure 7-10

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SSD Lifelines

• Vertical line under object or actor
• Shows passage of time
• If vertical line dashed
• Creation and destruction of thing is not important for scenario
• Long narrow rectangles
• Activation lifelines emphasize that object is active only during part of scenario

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SSD 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

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Repeating Message

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Figure 7-11

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Developing a System Sequence Diagram

• 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 Order Scenario

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Figure 7-12

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Resulting SSD for the Telephone Order Scenario

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Figure 7-13

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

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Figure 7-14

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Identifying Object Behavior—
The State Machine Diagram

• State machine diagram is UML 2.0 diagram that models object states and transitions
• Complex problem domain classes can be modeled
• 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 and is a semipermanent condition or status
• Transition
• The movement of an object from one state to another state

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Simple State Machine Diagram for a Printer

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Figure 7-15

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State 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 Concurrency—States within a State

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Figure 7-16

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Concurrent Paths for Printer in the On State

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Figure 7-17

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Rules for Developing State Machine 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 OrderItem

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Figure 7-18

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Partial State Machine for OrderItem

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Figure 7-19

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Final State Machine for OrderItem

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Figure 7-20

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Order Domain Class for RMO—
States and Exit Transitions

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Figure 7-21

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First-Cut State Machine Diagram for Order

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Figure 7-22

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Second-Cut State Machine Diagram for Order

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Figure 7-23

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Integrating Object-Oriented Models

• 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, only 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
Requirements Models

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Figure 7-24

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Summary

• Object-oriented approach has complete set of diagrams that define system requirements
• Requirements specified using following models
• Domain model class diagram (Chapter 5)‏
• Use case diagrams (Chapters 7)
• Use case detailed models, either descriptive formats or activity diagrams (Chapter 5 & 7)‏
• System sequence diagrams (Chapter 7)‏
• State machine diagrams (Chapter 7)‏

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