Object Oriented Design

1. Object-oriented Design
   • Designing systems using self-contained objects and object classes
2. Objectives
   • To explain how a software design may be represented as a set of interacting objects that manage their own state and operations
   • To describe the activities in the object-oriented design process
   • To introduce various models that describe an object-oriented design
   • To show how the UML may be used to represent these models
3. Topics covered
   • Objects and object classes
   • An object-oriented design process
   • Design evolution
4. Characteristics of OOD
   • Objects are abstractions of real-world or system entities and manage themselves
   • Objects are independent and encapsulate state and representation information.
   • System functionality is expressed in terms of object services
   • Shared data areas are eliminated. Objects communicate by message passing
   • Objects may be distributed and may execute sequentially or in parallel
5. Interacting objects
6. Advantages of OOD
   • Easier maintenance. Objects may be understood as stand-alone entities
   • Objects are appropriate reusable components
   • For some systems, there may be an obvious mapping from real world entities to system objects
7. Object-oriented development
   • Object-oriented analysis, design and programming are related but distinct
   • OOA is concerned with developing an object model of the application domain
   • OOD is concerned with developing an object-oriented system model to implement requirements
   • OOP is concerned with realising an OOD using an OO programming language such as Java or C++
8. Objects and object classes
   • Objects are entities in a software system which represent instances of real-world and system entities
   • Object classes are templates for objects. They may be used to create objects
   • Object classes may inherit attributes and services from other object classes
9. Objects An object is an entity which has a state and a defined set of operations which operate on that state. The state is represented as a set of object attributes. The operations associated with the object provide services to other objects (clients) which request these services when some computation is required. Objects are created according to some object class definition. An object class definition serves as a template for objects. It includes declarations of all the attributes and services which should be associated with an object of that class.
10. The Unified Modeling Language
    • Several different notations for describing object-oriented designs were proposed in the 1980s and 1990s
    • The Unified Modeling Language is an integration of these notations
    • It describes notations for a number of different models that may be produced during OO analysis and design
    • It is now a de facto standard for OO modelling
11. Employee object class (UML)
12. Object communication
    • Conceptually, objects communicate by message passing.
    • Messages
    • The name of the service requested by the calling object.
    • Copies of the information required to execute the service and the name of a holder for the result of the service.
    • In practice, messages are often implemented by procedure calls
    • Name = procedure name.
    • Information = parameter list.
13. 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) ;
14. Generalisation and inheritance
    • Objects are members of classes which define attribute types and operations
    • Classes may be arranged in a class hierarchy where one class (a super-class) is a generalisation of one or more other classes (sub-classes)
    • A sub-class inherits the attributes and operations from its super class and may add new methods or attributes of its own
    • Generalisation in the UML is implemented as inheritance in OO programming languages
15. A generalisation hierarchy
16. Advantages of inheritance
    • It is an abstraction mechanism which may be used to classify entities
    • It is a reuse mechanism at both the design and the programming level
    • The inheritance graph is a source of organisational knowledge about domains and systems
17. Problems with inheritance
    • Object classes are not self-contained. they cannot be understood without reference to their super-classes
    • Designers have a tendency to reuse the inheritance graph created during analysis. Can lead to significant inefficiency
    • The inheritance graphs of analysis, design and implementation have different functions and should be separately maintained
18. Inheritance and OOD
    • There are differing views as to whether inheritance is fundamental to OOD.
    • View 1. Identifying the inheritance hierarchy or network is a fundamental part of object-oriented design. Obviously this can only be implemented using an OOPL.
    • View 2. Inheritance is a useful implementation concept which allows reuse of attribute and operation definitions. Identifying an inheritance hierarchy at the design stage places unnecessary restrictions on the implementation
    • Inheritance introduces complexity and this is undesirable, especially in critical systems
19. UML associations
    • Objects and object classes participate in relationships with other objects and object classes
    • In the UML, a generalised relationship is indicated by an association
    • Associations may be annotated with information that describes the association
    • Associations are general but may indicate that an attribute of an object is an associated object or that a method relies on an associated object
20. An association model
21. Concurrent objects
    • The nature of objects as self-contained entities make them suitable for concurrent implementation
    • The message-passing model of object communication can be implemented directly if objects are running on separate processors in a distributed system
22. Servers and active objects
    • Servers.
    • The object is implemented as a parallel process (server) with entry points corresponding to object operations. If no calls are made to it, the object suspends itself and waits for further requests for service
    • Active objects
    • Objects are implemented as parallel processes and the internal object state may be changed by the object itself and not simply by external calls
23. Active transponder object
    • Active objects may have their attributes modified by operations but may also update them autonomously using internal operations
    • Transponder object broadcasts an aircraft’s position. The position may be updated using a satellite positioning system. The object periodically update the position by triangulation from satellites
24. An active transponder object
25. Java threads
    • Threads in Java are a simple construct for implementing concurrent objects
    • Threads must include a method called run() and this is started up by the Java run-time system
    • Active objects typically include an infinite loop so that they are always carrying out the computation
26. An object-oriented design process
    • Define the context and modes of use of the system
    • Design the system architecture
    • Identify the principal system objects
    • Develop design models
    • Specify object interfaces
27. 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. 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.
28. 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. 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.
29. Layered architecture
30. System context and models of use
    • Develop an understanding of the relationships between the software being designed and its external environment
    • System context
    • A static model that describes other systems in the environment. Use a subsystem model to show other systems. Following slide shows the systems around the weather station system.
    • Model of system use
    • A dynamic model that describes how the system interacts with its environment. Use use-cases to show interactions
31. Subsystems in the weather mapping system
32. Use-cases for the weather station
33. Use-case description
34. Architectural design
    • Once interactions between the system and its environment have been understood, you use this information for designing the system architecture
    • Layered architecture is appropriate for the weather station
    • Interface layer for handling communications
    • Data collection layer for managing instruments
    • Instruments layer for collecting data
    • There should be no more than 7 entities in an architectural model
35. Weather station architecture
36. Object identification
    • Identifying objects (or object classes) is the most difficult part of object oriented design
    • There is no 'magic formula' for object identification. It relies on the skill, experience and domain knowledge of system designers
    • Object identification is an iterative process. You are unlikely to get it right first time
37. Approaches to identification
    • Use a grammatical approach based on a natural language description of the system (used in Hood method)
    • Base the identification on tangible things in the application domain
    • Use a behavioural approach and identify objects based on what participates in what behaviour
    • Use a scenario-based analysis. The objects, attributes and methods in each scenario are identified
38. Weather station object classes
    • Ground thermometer, Anemometer, Barometer
    • Application domain objects that are ‘hardware’ objects related to the instruments in the system
    • Weather station
    • The basic interface of the weather station to its environment. It therefore reflects the interactions identified in the use-case model
    • Weather data
    • Encapsulates the summarised data from the instruments
39. Weather station object classes
40. Further objects and object refinement
    • Use domain knowledge to identify more objects and operations
    • 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
    • Active or passive objects
    • In this case, objects are passive and collect data on request rather than autonomously. This introduces flexibility at the expense of controller processing time
41. Design models
    • Design models show the objects and object classes and relationships between these entities
    • Static models describe the static structure of the system in terms of object classes and relationships
    • Dynamic models describe the dynamic interactions between objects.
42. Examples of design models
    • Sub-system models that show logical groupings of objects into coherent subsystems
    • Sequence models that show the sequence of object interactions
    • State machine models that show how individual objects change their state in response to events
    • Other models include use-case models, aggregation models, generalisation models,etc.
43. Subsystem models
    • Shows how the design is organised into logically related groups of objects
    • In the UML, these are shown using packages - an encapsulation construct. This is a logical model. The actual organisation of objects in the system may be different.
44. Weather station subsystems
45. Sequence models
    • Sequence models show the sequence of object interactions that take place
    • Objects are arranged horizontally across the top
    • Time is represented vertically so models are read top to bottom
    • Interactions are represented by labelled arrows, Different styles of arrow represent different types of interaction
    • A thin rectangle in an object lifeline represents the time when the object is the controlling object in the system
46. Data collection sequence
47. Statecharts
    • Show how objects respond to different service requests and the state transitions triggered by these requests
    • If object state is Shutdown then it responds to a Startup() message
    • In the waiting state the object is waiting for further messages
    • If reportWeather () then system moves to summarising state
    • If calibrate () the system moves to a calibrating state
    • A collecting state is entered when a clock signal is received
48. Weather station state diagram
49. Object interface specification
    • Object interfaces have to be specified so that the objects and other components can be designed in parallel
    • Designers should avoid designing the interface representation but should hide this in the object itself
    • Objects may have several interfaces which are viewpoints on the methods provided
    • The UML uses class diagrams for interface specification but Java may also be used
50. Weather station interface
51. Design evolution
    • Hiding information inside objects means that changes made to an object do not affect other objects in an unpredictable way
    • Assume pollution monitoring facilities are to be added to weather stations. These sample the air and compute the amount of different pollutants in the atmosphere
    • Pollution readings are transmitted with weather data
52. Changes required
    • Add an object class called ‘Air quality’ as part of WeatherStation
    • Add an operation reportAirQuality to WeatherStation. Modify the control software to collect pollution readings
    • Add objects representing pollution monitoring instruments
53. Pollution monitoring
54. • OOD is an approach to design so that design components have their own private state and operations
    • Objects should have constructor and inspection operations. They provide services to other objects
    • Objects may be implemented sequentially or concurrently
    • The Unified Modeling Language provides different notations for defining different object models
    Key points
55. Key points
    • A range of different models may be produced during an object-oriented design process. These include static and dynamic system models
    • Object interfaces should be defined precisely using e.g. a programming language like Java
    • Object-oriented design simplifies system evolution
    • Object-Oriented Programming Concepts
    • Contents
      • What is OOP?
      • Classes and Objects
      • Principles of OOP
      • Inheritance
      • Abstraction
      • Encapsulation
      • Polymorphism
    • What is OOP?
    • What is OOP?
      • Object-oriented programming (OOP) is an engineering approach for building software systems
      • Based on the concepts of classes and objects that are used for modeling the real world entities
      • Object-oriented programs
      • Consist of a group of cooperating objects
      • Objects exchange messages, for the purpose of achieving a common objective
      • Implemented in object-oriented languages
    • OOP in a Nutshell
      • A program models a world of interacting objects
      • Objects create other objects and “send messages” to each other (in Java, call each other’s methods)
      • Each object belongs to a class
      • A class defines properties of its objects
      • The data type of an object is its class
      • Programmers write classes (and reuse existing classes)
    • What are OOP’s Claims To Fame?
      • Better suited for team development
      • Facilitates utilizing and creating reusable software components
      • Easier GUI programming
      • Easier software maintenance
      • All modern languages are object-oriented: Java, C#, PHP, Perl, C++, ...
    • Classes and Objects
    • What Are Objects?
      • Software objects model real-world objects or abstract concepts
      • E.g. dog, bicycle, queue
      • Real-world objects have states and behaviors
      • Dogs' states: name, color, breed, hungry
      • Dogs' behaviors: barking, fetching, sleeping
    • What Are Objects?
      • How do software objects implement real-world objects?
      • Use variables/data to implement states
      • Use methods/functions to implement behaviors
      • An object is a software bundle of variables and related methods
    •  checks  people  shopping list …  numbers  characters  queues  arrays Things in the real world Things in the computer world Objects Represent
    • Classes
      • Classes provide the structure for objects
      • Define their prototype
      • Classes define:
      • Set of attributes
      • Also called state
      • Represented by variables and properties
      • Behavior
      • Represented by methods
      • A class defines the methods and types of data associated with an object
    • Objects
      • Creating an object from a class is called instantiation
      • An object is a concrete instance of a particular class
      • Objects have state
      • Set of values associated to their attributes
      • Example:
      • Class: Account
      • Objects: Ivan's account, Peter's account
    • Classes – Example Account +Owner: Person +Ammount: double +suspend() +deposit(sum:double) +withdraw(sum:double) Class Attributes Operations
    • Classes and Objects – Example Account +Owner: Person +Ammount: double +suspend() +deposit(sum:double) +withdraw(sum:double) Class ivanAccount +Owner="Ivan Kolev" +Ammount=5000.0 peterAccount +Owner="Peter Kirov" +Ammount=1825.33 kirilAccount +Owner="Kiril Kirov" +Ammount=25.0 Object Object Object
    • Messages
      • What is a message in OOP?
      • A request for an object to perform one of its operations (methods)
      • All communication between objects is done via messages
    • Interfaces
      • Messages define the interface to the object
      • Everything an object can do is represented by its message interface
      • The interfaces provide abstractions
      • You shouldn't have to know anything about what is in the implementation in order to use it (black box)
      • An interface is a set of operations (methods) that given object can perform
    • The Principles of OOP
    • The Principles of OOP
      • Inheritance
      • Abstraction
      • Encapsulation
      • Polymorphism
    • Inheritance
      • A class can extend another class, inheriting all its data members and methods
      • The child class can redefine some of the parent class's members and methods and/or add its own
      • A class can implement an interface, implementing all the specified methods
      • Inheritance implements the “is a” relationship between objects
    • Inheritance
      • Terminology
      subclass or derived class superclass or base class extends subinterface superinterface extends class interface implements
    • Inheritance Person +Name: String +Address: String Employee +Company: String +Salary: double Student +School: String Superclass Subclass Subclass
    • Inheritance in Java
      • In Java, a subclass can extend only one superclass
      • In Java, a subinterface can extend one superinterface
      • In Java, a class can implement several interfaces
      • This is Java’s form of multiple inheritance
    • Interfaces and Abstract Classes in Java
      • An abstract class can have code for some of its methods
      • Other methods are declared abstract and left with no code
      • An interface only lists methods but does not have any code
      • A concrete class may extend an abstract class and/or implement one or several interfaces, supplying the code for all the methods
    • Inheritance Benefits
      • Inheritance plays a dual role:
      • A subclass reuses the code from the superclass
      • A subclass inherits the data type of the superclass (or interface) as its own secondary type
    • Class Hierarchies
      • Inheritance leads to a hierarchy of classes and/or interfaces in an application:
      Game GameFor2 BoardGame Chess Backgammon Solitaire
    • Inheritance
      • An object of a class at the bottom of a hierarchy inherits all the methods of all the classes above
      • It also inherits the data types of all the classes and interfaces above
      • Inheritance is also used to extend hierarchies of library classes
      • Allows reusing the library code and inheriting library data types
    • Abstraction
      • Abstraction means ignoring irrelevant features, properties, or functions and emphasizing the relevant ones...
      • ... relevant to the given project (with an eye to future reuse in similar projects)
      • Abstraction = managing complexity
      “ Relevant” to what?
    • Abstraction
      • Abstraction is something we do every day
      • Looking at an object, we see those things about it that have meaning to us
      • We abstract the properties of the object, and keep only what we need
      • Allows us to represent a complex reality in terms of a simplified model
      • Abstraction highlights the properties of an entity that we are most interested in and hides the others
    • Abstraction in Java
      • In Java abstraction is achieved by use of
      • Abstract classes
      • Interfaces
    • Abstract Data Types
      • Abstract Data Types (ADT) are data types defined by a set of operations
      • Examples:
    • Abstraction in AWT/Swing
      • java.lang.Object
      • |
      • +--java.awt.Component
      • |
      • +--java.awt.Container
      • |
      • +--javax.swing.JComponent
      • |
      • +--javax.swing. AbstractButton
    • Encapsulation
      • Encapsulation means that all data members ( fields ) of a class are declared private
      • Some methods may be private, too
      • The class interacts with other classes (called the clients of this class) only through the class’s constructors and public methods
      • Constructors and public methods of a class serve as the interface to class’s clients
    • Encapsulation
      • Ensures that structural changes remain local :
      • Usually, the internal structure of a class changes more often than the class’s constructors and methods
      • Encapsulation ensures that when fields change, no changes are needed in other classes (a principle known as “locality”)
      • Hiding implementation details reduces complexity  easier maintenance
    • Encapsulation – Example
      • Data Fields are private
      • Constructors and accessor methods are defined
    • Polymorphism
      • Ability to take more than one form
      • A class can be used through its parent class's interface
      • A subclass may override the implementation of an operation it inherits from a superclass (late binding)
      • Polymorphism allows abstract operations to be defined and used
      • Abstract operations are defined in the base class's interface and implemented in the subclasses
    • Polymorphism
      • Why use an object as a more generic type?
      • To perform abstract operations
      • To mix different related types in the same collection
      • To pass it to a method that expects a parameter of a more generic type
      • To declare a more generic field (especially in an abstract class) which will be initialized and “specialized” later
    • Polymorphism – Example Square::calcSurface() { return size * size; } Circle::calcSurface() { return PI * radius * raduis; } Abstract class Abstract action Concrete class Overriden action Overriden action
    • Polymorphism
      • Polymorphism ensures that the appropriate method is called for an object of a specific type when the object is disguised as a more generic type:
      Figure f1 = new Square(...); Figure f2 = new Circle(...); // This will call Square::calcSurface() int surface = f1.calcSurface(); // This will call Square::calcSurface() int surface = f2.calcSurface();
    • Polymorphism in Java
      • Good news: polymorphism is already supported in Java
      • All you have to do is use it properly
      • Polymorphism is implemented using a technique called late method binding :
      • Exact method to call is determined at run time before performing the call
    • Questions ? OOP Concepts
    • Problems
      • Describe the term object in OOP.
      • Describe the term class in OOP.
      • Describe the term interface in OOP.
      • Describe the term inheritance in OOP.
      • Describe the term abstraction in OOP.
      • Describe the term encapsulation in OOP.
      • Describe the term polymorphism in OOP.