1 CS 501 Spring 2008 CS 501: Software Engineering Lectures 15 Object Oriented Design 1

2 CS 501 Spring 2008 Administration

3 CS 501 Spring 2008 Design in the modified Waterfall Model Requirements System design Testing Operation & maintenance Program design Implementation (coding) Acceptance & release Feasibility study Requirements Design Implementation

4 CS 501 Spring 2008 Program Design The task of program design is to represent the software system functions in a form that can be transformed into one or more executable programs. Given a system architecture, the program design specifies: programs, components, packages, classes and class hierarchies interfaces, protocols security mechanisms, operational procedures UML models (diagrams and specifications) can be used for almost all aspects of program design

5 CS 501 Spring 2008 Models: Levels of Abstraction The complexity of a model depends on its level of abstraction: High-levels of abstraction show the overall system. Low-levels of abstraction are needed for implementation. Two approaches: Model entire system at same level of abstraction, but present diagrams with different levels of detail. Model parts of system at different levels of abstraction.

6 CS 501 Spring 2008 UML Models A UML model consists of: (a)A diagram. This gives a general overview of the model, showing the principal elements and how they relate to each other. A diagram is the graphical representation of a set of elements, usually rendered as a connected graph of vertices (things) and arcs (relationships). (b)A specification. This provides details about each element of the model. Specification for models used in program design should have sufficient detail that they can be used to write code from.

7 CS 501 Spring 2008 List of all Diagrams in UML Use case diagram shows a set of use cases and actors (a special kind of class) and their relationships. Principally used for requirements. Component diagram shows the organization and dependencies among a set of components. Principally used for system architecture. Deployment diagram shows the configuration of processing nodes and the components that live on them. Principally used for system architecture.

8 CS 501 Spring 2008 List of all Diagrams in UML These models are used principally for program design. Class diagram shows a set of classes, interfaces, and collaborations with their relationships. Object diagram shows a set of objects and their relationships.

9 CS 501 Spring 2008 These models are for interactive aspects of systems. They can be used for requirements or program design. 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 Statechart diagram shows a state machine consisting of states, transitions, events, and activities. Activity diagram is a statechart diagram that shows the flow from activity to activity within a system. List of all Diagrams in UML

10 CS 501 Spring 2008 Using Rational Rose for Program Design

11 CS 501 Spring 2008 Class Diagrams Window origin size open() close() move() display() name attributes (local, instance, and class variables) operations (methods) responsibilities (optional text) A class is a description of a set of objects that share the same attributes, operations, relationships and semantics.

12 CS 501 Spring 2008 The "Hello, World!" Example import java.awt.Graphics; class HelloWorld extends java.applet.Applet { public void paint (Graphics g) { g.drawString ("Hello, World!", 10, 10); } Example from: BJR

13 CS 501 Spring 2008 The HelloWorld Example HelloWorld paint() class name operations

14 CS 501 Spring 2008 Abstraction for HelloWorld HelloWorld paint() g.drawString ("HelloWorld", 0, 10)" class name operations annotation

15 CS 501 Spring 2008 Annotation some text note A note is a symbol for rendering constraints and comments attached to an element or a collection of elements.

16 CS 501 Spring 2008 A Typical Class Diagram

17 CS 501 Spring 2008 Another Typical Design

18 CS 501 Spring 2008 Specification

19 CS 501 Spring 2008 Specification Fields

20 CS 501 Spring 2008 General Specification Fields

21 CS 501 Spring 2008 Notation: Grouping A package is a general-purpose mechanism for organizing elements into groups. Business rules

22 CS 501 Spring 2008 Packaging Classes applet awt lang HelloWorld java Graphics package

23 CS 501 Spring 2008 Notation: Relationships A dependency is a semantic relationship between two things in which a change to one may effect the semantics of the other * employer employee An association is a structural relationship that describes a set of links, a link being a connection among objects.

24 CS 501 Spring 2008 Relationships Parking ParkingSpace location is_available()

25 CS 501 Spring 2008 Notation: Relationships (continued) A generalization is a specialization/generalization relationship is which objects of the specialized element (child) are substitutable for objects of the generalized element (parent). childparent A realization is a semantic relationship between classifiers, wherein one classifier specifies a contract that another classifier guarantees to carry out.

26 CS 501 Spring 2008 Generalization Applet HelloWorld paint() Graphics generalization dependency Note that the Applet and Graphics classes are shown elided, i.e., just the name is shown, not the attributes or operations.

27 CS 501 Spring 2008 Notation: Interface An interface is a collection of operations that specify a service of a class or component, i.e., the externally visible behavior of that element. ISpelling

28 CS 501 Spring 2008 Class Inheritance Diagram Object Component Container Panel Applet HelloWorld ImageObserver interface

29 CS 501 Spring 2008 Modeling Classes Given a real-life system, how do you decide what classes to use? What terms do the users and implementers use to describe the system? They are candidates for classes. Is each candidate class crisply defined? For each class, what is its set of responsibilities? Are the responsibilities evenly balanced among the classes? What attributes and operations does each class need to carry out its responsibilities?

30 CS 501 Spring 2008 Coupling and Cohesion Coupling is a measure of the dependencies between two subsystems. If two systems are strongly coupled, it is hard to modify one without modifying the other. Cohesion is a measure of dependencies within a subsystem. If a subsystem contains many closely related functions its cohesion is high. Aim for high cohesion within classes and weak coupling between them.

31 CS 501 Spring 2008 Candidate Classes: Application Classes and 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.

32 CS 501 Spring 2008 Noun Identification: A Library Example The library contains books and journals. It may have several copies of a given book. Some of the books are reserved for short-term loans only. All others may be borrowed by any library member for three weeks. Members of the library can normally borrow up to six items at a time, but members of staff may borrow up to 12 items at one time. Only members of staff may borrow journals. The system must keep track of when books and journals are borrowed and returned and enforce the rules.

33 CS 501 Spring 2008 Noun Identification: A Library Example The library contains books and journals. It may have several copies of a given book. Some of the books are reserved for short-term loans only. All others may be borrowed by any library member for three weeks. Members of the library can normally borrow up to six items at a time, but members of staff may borrow up to 12 items at one time. Only members of staff may borrow journals. The system must keep track of when books and journals are borrowed and returned and enforce the rules.

34 CS 501 Spring 2008 Candidate Classes Librarythe name of the system Book Journal Copy ShortTermLoanevent LibraryMember Weekmeasure MemberOfLibraryrepeat Itembook or journal Timeabstract term MemberOfStaff Systemgeneral term Rulegeneral term

35 CS 501 Spring 2008 Relations between Classes Bookis anItem Journalis anItem Copyis a copy of a Book LibraryMember Item MemberOfStaffis aLibraryMember Is Item needed?

36 CS 501 Spring 2008 Operations LibraryMemberborrowsCopy LibraryMemberreturnsCopy MemberOfStaffborrowsJournal MemberOfStaffreturnsJournal Item not needed yet.

37 CS 501 Spring 2008 Class Diagram MemberOfStaffBookCopyJournal is a copy of 1..* 1 LibraryMember 1 0..* on loan

38 CS 501 Spring 2008 Rough Sketch: Wholesale System A wholesale merchant supplies retail stores from stocks of goods in a warehouse. What classes would you use to model this business?

39 CS 501 Spring 2008 Rough Sketch: Wholesale System RetailStore Warehouse Order Invoice Product Shipment Merchant

40 CS 501 Spring 2008 Rough Sketch: Wholesale System Warehouse Order Invoice Product Merchant RetailStore name address contactInfo financialInfo Shipment Responsibilities -track status of shipped products Reversal damaged() return() wrongItem() responsibility (text field)

41 CS 501 Spring 2008 Expanding a Class: Modeling Financial Information RetailStore Transaction 1 * association Invoice Payment Which class is responsible for the financial records for a store?

42 CS 501 Spring 2008 Modeling Invoice Shipment Invoice invoiceNumber +goodsShipped() -sendInvoice() goodsShipped PartsList adornments + public - private RetailStore ??? invoiceRecord

43 CS 501 Spring 2008 Lessons Learned Design is empirical. There is no single correct design. During the design process: Eliding: Elements are hidden to simplify the diagram Incomplete: Elements may be missing. Inconsistency: The model may not be consistent The diagram is not the whole design. Diagrams must be backed up with specifications.

44 CS 501 Spring 2008 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.

45 CS 501 Spring 2008 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.

46 CS 501 Spring 2008 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.

47 CS 501 Spring 2008 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

48 CS 501 Spring 2008 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?

49 CS 501 Spring 2008 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?

50 CS 501 Spring 2008 Exam Question: Class Diagram DigitizedMap 1...* 1 WeatherStation location metereologicalData collectData() getSummary() WeatherMap location date-time geographicData weather gatherData() printMap() summary *