1 CS 501 Spring 2005 CS 501: Software Engineering Lecture 16 Object Oriented Design 2

2 CS 501 Spring 2005 Administration

3 CS 501 Spring 2005 Candidate Classes: Solution Classes Application classes and solution classes: Application classes represent application concepts. Noun identification is an effective technique to generate candidate application classes. Solution classes represent system concepts, e.g., user interface objects, databases, etc.

4 CS 501 Spring 2005 From Candidate Classes to Completed Design Methods used to move to final design: Reuse: Wherever possible use existing components, or class libraries. They may need modification. Restructuring: Change the design to improve, understandability, maintainability, etc. Techniques include merging similar classes, splitting complex classes, etc. Optimization: Ensure that the system meets anticipated performance requirements, e.g., by changed algorithms or restructuring. Completion: Fill all gaps, specify interfaces, etc.

5 CS 501 Spring 2005 An Exam Question: Object Oriented Design A system generates weather maps using data collected from unattended weather stations. Each weather station collects meteorological data and produces summaries of the data. On request, it sends the summary information to an area computer. The area computer uses a database of digitized maps to generate a set of local weather maps.

6 CS 501 Spring 2005 Exam Question: Noun Identification A system generates weather maps using data collected from unattended weather stations. Each weather station collects meteorological data and produces summaries of the data. On request, it sends the summary information to an area computer. The area computer uses a database of digitized maps to generate a set of local weather maps.

7 CS 501 Spring 2005 Exam Question: Candidate Classes Systemgeneral term WeatherMap Datasame as MeteorologicalData WeatherStationis this a general term? MeteorologicalDatahow does this relate to WeatherStation? DataSummaryhow does this relate to MeteorologicalData? AreaComputerhardware Databasegeneral term DigitizedMap

8 CS 501 Spring 2005 Exam Question: Observations about the Candidate Classes WeatherMapis a DigitizedMap is derived from 1...* DataSummary WeatherStationhas a set of MeteorologicalData MeteorologicalData DataSummaryis derived from MeteorologicalData DigitizedMap Can Meteorological Data be an attribute of WeatherStation? Can DataSummary be combined with WeatherMap?

9 CS 501 Spring 2005 Exam Question: Attributes and Operations WeatherStation location metereologicalData collectData() getSummary() WeatherMap location date-time geographicData weather gatherData() printMap() DigitizedMap location geographicData printMap() Or should MetereologicalData be a separate object?

10 CS 501 Spring 2005 Exam Question: Class Diagram DigitizedMap 1...* 1 WeatherStation location metereologicalData collectData() getSummary() WeatherMap location date-time geographicData weather gatherData() printMap() summary

11 CS 501 Spring 2005 Modeling Dynamic Aspects of Systems Interaction diagrams: set of objects and their relationships including messages that may be dispatched among them Sequence diagrams: time ordering of messages Collaboration diagrams: structural organization of objects that send and receive messages Activity diagram: flow chart showing flow of control from activity to activity Statechart diagram: models a state machine

12 CS 501 Spring 2005 Notation: Statechart diagrams Waiting A state machine is a behavior that specifies the sequence of states an object or an interaction goes through during its lifetime in response to events. State diagrams can be used for both requirements specification and for program design.

13 CS 501 Spring 2005 State Diagram: Notation State diagram for class Book in a library system not borrowable returned() borrowable borrowed()[not last copy] borrowed()[last copy] guard expression

14 CS 501 Spring 2005 State Transition Diagram Patients Fields SetupReady Beam on Enter Start Stop Select field Select patient (lock on) (lock off)

15 CS 501 Spring 2005 Interaction: Bouncing Ball Diagrams Example: execution of Client Server(s) domain name service TCP connection HTTP get

16 CS 501 Spring 2005 Notation for Classes and Objects ClassesObjects AnyClass attribute1 attribute2 operation1() operation2() AnyClass or anObject:AnyClass :AnyClass anObject The names of objects are underlined. or

17 CS 501 Spring 2005 Notation: Active Class EventManager eventlist suspend() flush() An active class is a class whose objects own one or more processes or threads and therefore can initiate control activity.

18 CS 501 Spring 2005 Notation: Interaction display An interaction is a behavior that comprises a set of messages exchanged among a set of objects within a particular context to accomplish a specific purpose.

19 CS 501 Spring 2005 Actions on Objects call return send create destroy returnCopy(c) okToBorrow()local status notifyReturn(b)asynchronous signal > stereotypes

20 CS 501 Spring 2005 Sequence Diagram: Borrow Copy of a Book BookBorrower libMem: LibraryMember theCopy:Copy theBook:Book borrow(theCopy) okToBorrow borrow

21 CS 501 Spring 2005 Sequence Diagram: Change in Cornell Program Cornellian :MEngStudent 1 : getName() sequence numbers added to messages :PhDStudent 1.1 : name 2: > PhDStudent(name) 3: >

22 CS 501 Spring 2005 Sequence Diagram: Painting Mechanism :Thread:Toolkit:ComponentPeertarget:HelloWorld run callbackLoop handleExpose paint

23 CS 501 Spring 2005 Other Diagrams in UML Activity diagram is a statechart diagram that shows the flow from activity to activity within a system. Component diagram shows the organization and dependencies among a set of components. Deployment diagram shows the configuration of processing nodes and the components that live on them.

24 CS 501 Spring 2005 Activity Diagram (Flowchart) Release work order Assign tasksReschedule [materials not ready] [materials ready] guard expression branch Activity diagrams can be used for both requirements specification and for program design.

25 CS 501 Spring 2005 Activity Diagram: Parallel Activities DecompressStream audioStream video fork join start state stop state

26 CS 501 Spring 2005 Software Reuse Better software at lower cost Potential benefits of reuse: Reduce development time and cost Improved reliability of mature components Shared maintenance cost Potential disadvantages of reuse: Difficulty in finding appropriate components Components may be a poor fit for application The discussion of design for reuse draws from the book by Bruegge and Dutoit in the readings.

27 CS 501 Spring 2005 Software Reuse: Examples Software developers rely heavily on software components provided by others System software device drivers file systems exception handling network protocols Subsystems database management systems firewalls web servers

28 CS 501 Spring 2005 Software Reuse Examples (Tools) Standard functions mathematical methods formatting User interface toolkits (e.g. Quickdraw) class libraries, (e.g., Swing)

29 CS 501 Spring 2005 Software Reuse (Application Packages) An application package supports a standard application (e.g., payroll) Functionality can be enhanced by: => configuration parameters (e.g., table driven) => extensibility at defined interfaces => custom written source code extensions

30 CS 501 Spring 2005 Reuse and Object Oriented Languages Example: Java Java is a relatively straightforward language with a very rich set of class hierarchies. Java programs derive much of their functionality from standard classes Learning and understanding the classes is difficult Experienced Java programmers can write complex systems quickly Inexperienced Java programmers write inelegant and buggy programs

31 CS 501 Spring 2005 Design for Reuse The software design should anticipate possible changes in the system over its life-cycle. New vendor or new technology Components are replaced because a supplier goes out of business, ceases to provide adequate support, increases its price, etc., or because better software from another sources provides better functionality, support, pricing, etc. This can apply to either open-source or vendor-supplied components.

32 CS 501 Spring 2005 Design for Reuse New implementation The original implementation may be problematic, e.g., poor performance, inadequate back-up and recovery, difficult to trouble-shoot, or unable to support growth and new features added to the system. Example. The portal nsdl.org was originally implemented using uPortal. This did not support important extensions that were requested and proved awkward to maintain. It was reimplemented using PHP/MySQL.

33 CS 501 Spring 2005 Design for Reuse Additions to the requirements When a system goes into production, it is usual to reveal both weaknesses and opportunities for extra functionality and enhancement to the user interface design. For example, in a data-intensive system it is almost certain that there will be requests for extra reports and ways of viewing the data. Requests for enhancements are often the sign of a successful system. Clients recognize latent possibilities.

34 CS 501 Spring 2005 Design for Reuse Changes in the application domain Most application domains change continually, e.g., because of business opportunities, external changes (such as new laws), mergers and take-overs, new groups of users, etc., etc., It is rarely feasible to implement a completely new system when the application domain changes. Therefore existing systems must be modified. This may involve extensive restructuring.

35 CS 501 Spring 2005 Reuse Techniques Implementation inheritance: Reuse of code by subclassing an existing class and modifying its behavior. Specification inheritance: Subclasses are subtypes of their superclass. Objects of the subclass can be substituted for any object of the superclass type. Delegation: Implementation of an operation by sending a message to another class. *