CS 501: Software Engineering

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

2. Administration

3. 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. 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. 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. 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. Exam Question: Candidate Classes System general term WeatherMap Data same as MeteorologicalData WeatherStationis this a general term? MeteorologicalDatahow does this relate to WeatherStation? DataSummaryhow does this relate to MeteorologicalData? AreaComputer hardware Database general term DigitizedMap

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

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

11. 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. Notation: Statechart diagrams State diagrams can be used for both requirements specification and for program design. 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.

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

14. State Transition Diagram Select field Start Enter Enter (lock off) Beam on Patients Fields Setup Ready Stop (lock on) Select patient

15. Interaction: Bouncing Ball Diagrams Example: execution of http://www.cs.cornell.edu/ domain name service TCP connection HTTP get Client Server(s)

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

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

18. 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. Actions on Objects returnCopy(c) call return send create destroy okToBorrow() local status notifyReturn(b) asynchronous signal <<create>> stereotypes <<destroy>>

20. Sequence Diagram: Borrow Copy of a Book libMem: LibraryMember theBook:Book BookBorrower theCopy:Copy borrow(theCopy) okToBorrow borrow borrow

21. Sequence Diagram: Change in Cornell Program :MEngStudent Cornellian 1 : getName() 1.1 : name 2: <<create>> PhDStudent(name) :PhDStudent 3: <<destroy>> sequence numbers added to messages

22. Sequence Diagram: Painting Mechanism :Thread :Toolkit :ComponentPeer target:HelloWorld run run callbackLoop handleExpose paint

23. 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. Release work order Reschedule Assign tasks Activity Diagram (Flowchart) Activity diagrams can be used for both requirements specification and for program design. branch [materials not ready] [materials ready] guard expression

25. Decompress Stream audio Stream video Activity Diagram: Parallel Activities start state fork join stop state

26. 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. 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. Software Reuse Examples (Tools) Standard functions • mathematical methods • formatting User interface • toolkits (e.g. Quickdraw) • class libraries, (e.g., Swing)

29. 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. 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. 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. 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. 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. 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. 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. *