1. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 1 Chapter 12 Object-Oriented Design

2. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 2 Objectives l To explain how a software design may be represented as a set of interacting objects that encapsulate their own state and operations. l To describe the activities in the object- oriented design process.

3. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 3 Objectives l To introduce various models used to describe an object-oriented design l To show how the UML may be used to represent these models

4. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 4 Topics covered l Objects and object classes l An object-oriented design process l Design evolution

5. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 5 Characteristics of OOD l Allows designers to think in terms of interacting objects that maintain their own state and provide operations on that state instead of a set of functions operating on shared data. l Objects hide information about the represen- tation of state and hence limit access to it. l Objects may be distributed and may work sequentially or in parallel.

6. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 6 A design strategy based on “information hiding”… Another way to think about “information hiding”: Potentially changeable design decisions are isolated (i.e., “hidden”) to minimize the impact of change. - David Parnas

7. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 7 Interacting objects

8. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 8 Advantages of OOD l Easier maintenance. Objects may be understood as stand-alone entities (and ripple effect is reduced). l Objects are appropriate reusable components. l For some systems, there is an obvious mapping from real world entities to system objects.

9. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 9 Object-oriented development l OO Analysis: concerned with developing an object model of the application domain. l OO Design: concerned with developing an object-oriented system model to implement requirements l OO Programming: concerned with realising an OOD using an OO pro- gramming language such as Java or C++.

10. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 10 The history of OO Development as reflected by Sommerville’s text l 1 st Edition (1982): No mention of OO design! l 2 nd Edition (1985): …it has been sug- gested that a better design methodology is OO design… To place such comments in perspective, many large systems have been built using top-down design. Few large systems have been built using an object-oriented approach.

11. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 11 The history of OO Development as reflected by Sommerville’s text l 3 rd Edition (1989): It is only within the past few years that an alternative mode of decomposition, OO design, has been recognized as of value.

12. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 12 The history of OO Development as reflected by Sommerville’s text l 5 th Edition (1995): Until relatively recently, the most commonly used software design strategy involved decomposing the design into functional components with system state infor- mation held in a shared data area…It is only since the late 1980’s that…OO design has been widely adopted.

13. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 13 The history of OO Development as reflected by Sommerville’s text l 6 th Edition(2001): An OO approach to the whole (of) software development is now commonly used… Coverage of functional design is now included in the new chapter on legacy systems.

14. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 14 The history of OO Development as reflected by Sommerville’s text Although many software developers consider function-oriented design to be an outdated approach, OO development may not offer significant advantages (in some) situations. An interesting challenge…is to ensure that function- oriented and OO systems can work together.

15. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 15 Objects and object classes l Objects are entities with state and a defined set of operations on that state.  State is represented as a set of object attributes.  Operations provide services to other objects when requested.

16. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 16 Objects and object classes l Object classes are templates for objects.  An object class definition includes declarations of all attributes and operations associated with an object of that class.  They may inherit attributes and services from other object classes.

17. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 17 The Unified Modeling Language l Several different notations for OOD were proposed in the 1980s and 1990s. (Booch, Rumbaugh, Jacobson, Coad & Yourdon, Wirfs, …) l UML is an integration of these notations. l It describes a number of different models that may be produced during OO analysis and design (user view, structural view, behavioural view, implementation view, …) l The de facto standard for OO modelling.

18. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 18 Employee object class (UML) Object attributes Services to other objects

19. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 19 Object communication l Conceptually, objects communicate by message passing. l Messages include:  The name of the service requested,  A copy of the information required to carry out the service, and  the name of a holder for the result of the service. l In practice, messages are often imple- mented by procedure calls

20. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 20 Message examples // Call a method associated with a buffer // object that returns the next value // in the buffer v = circularBuffer.Get () ; // Call the method associated with a // thermostat object that sets the // temperature to be maintained thermostat.setTemp (20) ; Info required Name of service Holder for result

21. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 21 Generalization and inheritance l Objects are members of classes which define attribute types and operations. l Classes may be arranged in a hierarchy where one class (a super-class) is a generalization of one or more other classes (sub-classes)

22. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 22 Generalization and inheritance l A sub-class inherits the attributes and operations from its super class and may add new methods or attributes of its own.

23. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 23 A UML generalisation hierarchy Definition on p. 18

24. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 24 Advantages of inheritance l It is an abstraction mechanism which may be used to classify entities. l It is a reuse mechanism at both the design and the programming level. l The inheritance graph is a source of organisational knowledge about domains and systems. (OO Analysis)

25. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 25 Problems with inheritance l Object classes are not self-contained (i.e., they cannot be understood without reference to their super-classes). l Designers have a tendency to reuse the inheritance graph created during analysis. (Inheritance graphs of analysis, design and implementation have different functions.) Due to inherited attributes/ops Discipline/education problem?

26. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 26 Inheritance and OOD l Inheritance is a useful implementation concept which allows reuse of attribute and operation definitions. l Some feel that identifying an inheritance hierarchy or network is also a funda- mental part of object-oriented design. (Obviously, this can only be implemented directly using an OOPL.)

27. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 27 Inheritance and OOD l Others feel this places unnecessary restrictions on the implementation.* l Inheritance introduces complexity and this is undesirable, especially in critical systems. * This appears to be Sommerville’s view

28. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 28 UML associations l Objects and object classes participate in various types of relationships with other objects and object classes. l In the UML, a generalized relationship is indicated by an association.

29. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 29 UML associations l Associations may be annotated with information that describes their nature. l Associations can be used to indicate that an attribute of an object is an associated object or that a method relies on an associated object. (semantics)

30. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 30 An association model annotations

31. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 31 Concurrent objects l The nature of objects as self-contained entities make them well suited for con- current implementation. l The message-passing model of object communication can be implemented directly if objects are running on separate processors in a distributed system. (as opposed to using procedure calls)

32. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 32 Concurrent object implementation: servers and active objects l Servers (Passive objects): implemented as parallel processes with entry points correspon- ding to object operations. If no calls are made to it, the object suspends itself and waits for further requests for service. l Active objects: implemented as parallel processes and the internal object state may be changed by the object itself and not simply by external calls.

33. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 33 Example: an active transponder object l A transponder object broadcasts an aircraft’s position. (on demand) l The object periodically updates the position by triangulation from satellites. (autonomously)

34. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 34 An active transponder object ON DEMAND IN BACKGROUND Java-based interface description

35. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 35 Active object implementation: Java threads, Ada tasks, etc. l Threads in Java are a simple construct for implementing concurrent objects. l Threads must include a method called run() and this is started up by the Java run-time system. l Active objects typically include an infinite loop so that they are always carrying out the computation.

36. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 36 An object-oriented design process (an iterative, boot-strapping process) 1.Define the context and modes of use of the system. 2.Design the system architecture. 3.Identify the principal system objects. 4.Develop design models (static and dynamic). 5.Specify object interfaces.

37. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 37 Weather system description A weather data collection system is required to generate weather maps on a regular basis using data collected from remote, unattended weather stations and other data sources such as weather observers, balloons and satellites. Weather stations transmit their data to the area computer in response to a request from that machine.

38. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 38 Weather system description The area computer validates the collected data and integrates it with the data from different sources. The integrated data is archived and, using data from this archive and a digitised map database a set of local weather maps is created. Maps may be printed for distribution on a special-purpose map printer or may be displayed in a number of different formats.

39. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 39 Weather station description A weather station is a package of software controlled instruments which collects data, performs some data processing and transmits this data for further processing. The instruments include air and ground thermometers, an anemometer, a wind vane, a barometer and a rain gauge. Data is collected every five minutes.

40. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 40 Weather station description When a command is issued to transmit the weather data, the weather station processes and summarises the collected data. The summarised data is transmitted to the mapping computer when a request is received.

41. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 41 1.Define system context and modes of use l Goal: develop an understanding of the relationships between the software being designed and its external environment. l System context: a static model that describes other systems in the environ- ment. l The context of the weather station is illustrated below using UML packages.

42. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 42 Context of weather station

43. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 43 1.Define system context and modes of use (cont.) l Modes of system use: a dynamic model that describes how the system will interact with its environment. l Modes of weather station use are illustrated below using a UML use-case model.

44. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 44 Use-cases for the weather station External entity (weather data collection sys) Possible interactions

45. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 45 Use-case description basis for “information hiding”

46. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 46 2.Design system architecture l A layered architecture is appropriate for the weather station:  Interface layer for handling communications  Data collection layer for managing instruments  Instruments layer for collecting data l Rule of Thumb: There should be no more than 7 entities in an architectural model. (See Miller, "The Magical Number Seven, Plus or Minus Two,“ WOR. )

47. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 47 Weather station architecture UML “nested packages” UML annotations

48. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 48 3.Identify principal system objects l Identifying objects (or object classes) is the most difficult part OO design. l There is no “magic formula” – it relies on the skill, experience, and domain knowledge of system designers l An iterative process – you are unlikely to get it right the first time.

49. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 49 Approaches to object identification l Use a grammatical approach based on a natural language description of the system (Abbott’s heuristic). l Associate objects with tangible things in the application domain (e.g. devices). l Use a behavioural approach: identify objects based on what participates in what behaviour.

50. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 50 Approaches to object identification (cont.) l Use scenario-based analysis. The objects, attributes and methods in each scenario are identified. l Use an information-hiding based approach.* Identify potentially change- able design decisions and isolate these in separate objects. * “Bonus” approach! (No extra charge.)

51. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 51 Weather station object classes l Weather station – interface of the weather station to its environment. It reflects interactions identified in the use-case model. l Weather data – encapsulates summarised data from the instruments. l Ground thermometer, Anemometer, Barometer – application domain “hardware” objects* related to the instruments in the system. * hardware-controlling SOFTWARE

52. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 52 Weather station object classes

53. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 53 Other objects and object refinement l Use domain knowledge to identify more objects, operations, and attributes.  Weather stations should have a unique identifier.  Weather stations are remotely situated so instrument failures have to be reported automatically. Therefore, attributes and operations for self-checking are required.

54. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 54 Other objects and object refinement (cont.) l Active or passive objects?  Instrument objects are passive and collect data on request rather than autonomously. This introduces flexibility (how?)* at the expense of controller processing time.  Are any active objects required? * Hint: consider potentially changeable design decisions

55. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 55 4.Develop design models l Design models show the relationships among objects and object classes. l Static models describe the static structure of the system in terms of object and object class relationships. l Dynamic models describe the dynamic interactions among objects.

56. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 56 Examples of design models l Sub-system models show logical groupings of objects into coherent sub- systems. (static) l Sequence models show the sequence of object interactions associated with system uses. (dynamic)

57. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 57 Examples of design models l State machine models show how indi- vidual objects change their state in re- sponse to events. (dynamic) l Other models include use-case models, aggregation models, generalisation (inheritance) models, etc.

58. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 58 Subsystem models l In the UML, these are shown using packages, an encapsulation construct. l This is a logical model – the actual organization of objects in the system as implemented may be different.

59. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 59 Weather station subsystems Annotations go here Active object

60. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 60 Sequence models l Objects are arranged horizontally across the top. l Time is represented vertically; models are read top to bottom. l Interactions are represented by labelled arrows – different styles of arrows represent different types of interaction. l A thin rectangle in an object lifeline represents the time when the object is the controlling object in the system.

61. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 61 Data collection sequence Return of control No reply expected

62. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 62 Weather station state machine model

63. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 63 5.Object interface specification l Designers should avoid revealing data representation information in their interface design. (operations access and update all data) l Objects may have several logical interfaces which are viewpoints on the methods provided. (supported directly in Java) l The UML uses class diagrams for interface specification but pseudocode may also be used.

64. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 64 Weather station interface (Java-based)

65. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 65 Design evolution l Hiding information in objects means that changes made to an object need not affect other objects in an unpredictable way. l Assume pollution monitoring facilities are to be added to weather stations. l Pollution readings are transmitted with weather data.

66. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 66 Changes required l Add an object class called “Air quality” as part of WeatherStation. l Add an operation reportAirQuality to WeatherStation. Modify the control software to collect pollution readings. l Add objects representing pollution monitoring instruments. …and that’s all.

67. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 67 Pollution monitoring

68. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 68 l OOD results in design components with their own private state and operations. l Objects should have constructor and inspection operations. They provide services to other objects. l Objects may be implemented sequentially or concurrently. l The Unified Modeling Language provides different notations for defining different object models. Key points as do algebraic specs

69. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 12Slide 69 Key points l A range of different models may be produced during an object-oriented design process. These include static and dynamic system models (See the OMG website for sources.) l Object interfaces should be defined precisely. l Object-oriented design simplifies system evolution.