Software Engineering Object Oriented Analysis

Objectives 1.To outline an Object Oriented Analysis process and modelling language 2.To study the process and notation associated with use-cases 3.To provide an example of use-cases in action

[1] Object Oriented Analysis lGet from real world domain lDecomposed into process (algorithm) and models (output of algorithm)

Process lThe steps taken in order to complete an analysis (the algorithm) lThe OOA process landscape:  Booch: evolutionary process encompassing both a ‘macro’ and ‘micro’ development process  Rumbaugh: OMT (Object Modeling Technique) producing object, dynamic and functional models  Jacobson: OOSE (Object Oriented Software Engineering) emphasises Use-Cases  Coad and Yourdon: Viewed as one of the easiest  Wirfs-Brock: No clear distinction between analysis and design lBUT All similar with small annoying differences. lBooch, Rumbaugh and Jacobson now combined (Objectory process)

Generic Process lMost OOA processes have the following steps in common: 1.Elicit customer requirements 2.Identify scenarios or use-cases 3.Extract candidate classes 4.Identify attributes and methods 5.Define a class hierarchy 6.Build an object-relationship model 7.Build an object-behaviour model 8.Review the OO analysis against the use-cases 9.Repeat as required

Modeling Language lA modeling language is a means of specifying, visualizing and documenting the artifacts of a software systems. lThese models are the primary means of communication between users and developers lModeling languages specify a notation lIt is important that they be standardized

Unified Modeling Language (UML) lA notational System (including semantics for its notations) that is principally graphical and aimed at modeling systems using object oriented concepts. lUML is not a process, methodology or proprietary lCombines the notations of Booch, Rumbaugh and Jacobson lStandardized by the OMG (Object Management Group) lDefines a notation (syntax) and a meta-model (defining the notation using the notation) lConsists of:  Views (shows different faces of the system and links with the process)  Diagrams (graphs that describe the contents of a view)  Model elements (concepts used in a diagram)

Views lUser model view. This view represents the system (product) from the user’s (called “actors” in UML) perspective. lStructural model view. Data and functionality is viewed from inside the system. That is, static structure (classes, objects, and relationships) is modeled. lBehavioral model view. This part of the analysis model represents the dynamic or behavioral aspects of the system. lImplementation model view. The structural and behavioral aspects of the system are represented as they are to be built. lEnvironment model view. The structural and behavioral aspects of the environment in which the system is to be implemented are represented.

Analysis = Process + Models ProcessModel Output 1. Elicit customer requirements and identify use-cases Use-Case Diagrams 2. Extract candidate classes, Identify attributes and methods, Define a class hierarchy Class Responsibility Collaborator (CRC) Cards 3. Build an object-relationship model Class Diagram 4. Build an object-behaviour model Interaction Diagram

[2] Use Cases la view of a system that emphasizes the behavior as it appears to outside users. A use case model partitions system functionality into transactions (‘use cases’) that are meaningful to users (‘actors’). lA use-case scenario is a typical interaction between a user and a computer system (a thread of usage of a system) lProperties:  Captures some user-visible function  Achieves a discrete goal for the user  No attempt to represent order or number of times actions are executed lCaptured (in the simplest usage) by talking to a typical user and discussing what they want to achieve with the system. lUse-cases can be used to derive structural and behavioural models and construct test cases.

Use Case Diagrams lGraphically shows use-cases, actors and their relationships. Use Case Communication Relationship Actor Analyze Risk Trader Price Deal Capture Deal Valuation Limits Exceeded > Uses Relationship Extends Relationship

Use Cases lFundamentally a system transaction lArranged in a hierarchy.  At the top level a system box can enclose the use-cases  At lower levels each use-case is decomposed into several more detailed use-cases lUse cases often start with a verb in order to emphasize that they are processes (Buy Items, Price Deal) Valuation Share Trade System

Actors lactors represent roles people or devices play as the system functions. They communicate with the system and are external to it. lusers can play a number of different roles for a given scenario.  Example: an operator on a production line might have many roles (programmer, tester, monitor, troubleshooter). Each of these would represent an actor in the use-case lA single actor may perform many use-cases; a use-case may have several actors performing it lSystem (non-human) actors should only be shown when they are the ones that need the use case  Example: If the system generates a file that is later picked up by the accounting system then the accounting system is a relevant actor. Trader

Relationships lCommunication:  Flow of data and control between an actor and use-case lUses:  Use uses when you are repeating yourself in two or more separate use cases and you want to avoid repetition lExtends:  Use extends when you are describing a variation on normal behaviour  Useful for identify core and extended functionality >

Use Case Narratives lFor each use-case provide a narrative document lFor example: Use Case:Buy Item Actors:Customer, Cashier Description: - The use case begins when the customer arrives at a checkout with items to purchase. - The Cashier records each item. If there is more than one of an item, the Cashier can enter the quantity as well. - The system determines the item price and adds the information to the running sales transaction. The description and price of the current item are presented. - On completion of item entry, the cashier indicates that item entry is complete. - The system calculates and presents the sale total.

Developing a Use Case lAsk yourself these questions:  What are the main tasks or functions that are performed by the actor?  What system information will the the actor acquire, produce or change?  Will the actor have to inform the system about changes in the external environment?  What information does the actor desire from the system?  Does the actor wish to be informed about unexpected changes?

Modeling Tips lMake sure that each use case describes a significant chunk of system usage that is understandable by both domain experts and programmers lWhen defining use cases in text, use nouns and verbs accurately and consistently to help with later derivation of objects. lA use case diagram should  contain only use cases at the same level of abstraction  include only actors who are required lTry to describe use cases independent of implementation lA use case can have many scenarios (threads of execution)

[3] A Use-Case Example lExample: Home Security System (SafeHome) lProject Brief (provided at the start of the project):  Build a micro-processor based home security system that will protect against and/or recognize a variety of undesirable situations such as illegal entry, fire and flooding.  The product will use appropriate sensors to detect each situation, can be programmed by the homeowner and will automatically telephone a monitoring agency when necessary.

lActors:  homeowner (the user)  sensors (devices attached to the system)  monitoring and response subsystem (central station that monitors SafeHome) lInteractions (for the Homeowner):  Enters a password to allow all other interactions  Inquires about the status of a security zone  Inquires about the status of a sensor  Presses the panic button in an emergency  Activates/deactivates the security system Actors and Interactions

Use-Case Diagrams l(a) High-Level l(b) Intermediate Level Interacts expanded

Use-Case Narrative lUse-Case: Activates System lUsers: Homeowner lDescription:  The homeowner observes the control panel to determine if the system is ready for input. If the system is not ready, the homeowner must physically close window/doors so that the ready indicator is present [a not ready indicator implies that a sensor is open].  The homeowner uses the keypad to key in a four-digit password. The password is compared with the valid password stored in the system. If the password is incorrect, the control panel will beep once and reset itself for additional input. If the password is correct, the control panel awaits further action.  The homeowner selects and keys in stay or away to activate the system. Stay activates only perimeter sensors. Away activates all sensors.  When activation occurs, a red alarm light can be observed by the homeowner.