Object-Oriented Analysis: Concepts and Use Cases

1. Software Engineering Object-Oriented Analysis (Use Cases) James Gain (jgain@cs.uct.ac.za) http://people.cs.uct.ac.za/~jgain

2. Objectives • Outline the history and concepts of Object-Oriented Analysis • Introduce the Unified Modelling Language (UML) • Study the process and notation associated with Use-Cases • Provide a case study and exercise on Use-Cases in action

3. [1] Analysis Process • The steps taken in order to complete an analysis (the algorithm) • The 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 • BUT All are similar with small annoying differences • Booch, Rumbaugh and Jacobson now combined (into Objectory - a heavyweight life-cycle model)

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

5. Modelling Language • A modelling language is a means of specifying, visualizing and documenting the artifacts of a software systems • These models are the primary means of communication between users and developers • Modelling languages specify a notation • It is important that they be standardized

6. [2] Unified Modelling Language • A notational System (including syntax, semantics and pragmatics for its notations) that is principally graphical and aimed at modelling systems using object-oriented concepts. • UML is not a process, methodology or proprietary • Combines the notations of Booch, Rumbaugh and Jacobson • Standardized by the OMG (Object Management Group) • Defines a notation and a meta-model (defining the notation using the notation) • Consists of: • Views (shows different faces of the system and links with the process, e.g. user, structural, behavioural, etc.) • Diagrams (graphs that describe the contents of a view) • Model elements (concepts used in a diagram)

7. Analysis = Process + Models

8. [3] Use Cases • A view of a system that emphasizes the behaviour as it appears to outside users. • Partitions system functionality into transactions (‘use cases’) that are meaningful to users (‘actors’). • Consists of scenarios - typical interaction between a user and a computer system (a thread of usage of a system) • Properties: • Captures some user-visible function • Achieves a discrete goal for the user • No attempt to represent order or number of times actions are executed • Captured by talking to a typical user and discussing what they want to achieve with the system. • Use-cases can be used to derive structural and behavioural models and construct test cases.

9. Valuation Trader [3] Use Case Diagrams • Graphically shows use-cases, actors and their relationships. Uses Relationship Analyze Risk <<uses>> Actor Price Deal <<uses>> Capture Deal Communication Relationship Use Case <<extends>> Extends Relationship Limits Exceeded

10. Valuation Use Cases • Fundamentally a system transaction • Arranged 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 • Use cases often start with a verb in order to emphasize that they are processes (Buy Items, Price Deal) Share Trade System

11. Trader Actors • actors represent roles people or devices play as the system functions. They communicate with the system and are external to it. • users 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 • A single actor may perform many use-cases; a use-case may have several actors performing it • System (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.

12. <<uses>> <<uses>> <<Extends>> Relationships • Communication: • Flow of data and control between an actor and use-case • Uses: • Use uses when you are repeating yourself in two or more separate use cases and you want to avoid repetition • Extends: • Use extends when you are describing a carefully controlled variation on normal behaviour • Useful for identify core and extended functionality • Needs to have extension points (specific aspects that are extended) • Generalizes • A casual description of a variation on normal behaviour

13. Use Case Narratives • For each use-case provide a narrative document • For 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.

14. Developing a Use Case • Ask 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?

15. Modelling Tips • Make sure that each use case describes a significant chunk of system usage that is understandable by both domain experts and programmers • When defining use cases in text, use nouns and verbs accurately and consistently to help with later derivation of objects. • A use case diagram should • contain only use cases at the same level of abstraction • include only actors who are required • Try to describe use cases independent of implementation • A common mistake is to use chains of uses and extends to describe internal details • A use case can have many scenarios (threads of execution)

16. [4] Case Study: Use Cases • Reminder: • Convert a photographic image into a simulated mosaic with square tiles • Tiles must follow curves but still be closely spaced • User Interface: • Select image • Draw contours • Set parameters (e.g. tile size) • Initiate simulator

17. High-Level Use Case Narrative • Use Case: Mosaic Builder • Actors: Imager, Draftsman, Designer • Description: • Imager selects a reference picture in electronic image format. • Draftsman draws edges onto a transparent contour image aligned with the reference picture OR • System automatically detects edges in the reference picture, places these edges into the contour image. The Draftsman then cleans up the results - strengthening or erasing edges. • Designer flood-fills coloured regions onto a region image to distinguish tile sizes. Designer associates coloured regions with tile sizes. • Designer specifies number of tiles, colour palette and tile spacing. • System generates and displays the mosaic.

18. High-Level Use Case Diagram

19. Refined Use Case Narrative • Refine each stage as necessary producing separate Use-Cases: Use Case: 2a - Draw Edges Actors: Draftsman Description: A transparent overlay image is placed over the original reference picture. The draftsman draws black edges of a consistent thickness onto the overlay using the mouse. The draftsman is also able to erase edges that are incorrect. The draftsman can save the overlay as a black and white image with the same resolution as the reference picture.

20. [5] Exercise • Example: Home Security System (SafeHome) • Project 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.

21. Components and Interactions • Components: • homeowner (the user) • sensors (devices attached to the system) • monitoring and response subsystem (central station that monitors SafeHome) • Interactions: • Enters a password to allow all other interactions. This password needs to be validated by the system • Inquires about the status of a security zone which relies on the status of a combination of sensors • Inquires about the status of a sensor • Presses the panic button in an emergency • Activates/deactivates the security system

22. Low-level Use Case Narrative Use Case: Activates System Actors: Homeowner Description: 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. Task: Create the corresponding Use-Case Diagram