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An introduction to object-oriented design

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Chapter 11 An introduction to object-oriented design – PowerPoint PPT presentation

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Title: An introduction to object-oriented design


1
Chapter 11
  • An introduction to object-oriented design

2
Objectives
  • To introduce object-oriented design
  • To define objects, classes, attributes,
    operations and information hiding
  • To introduce public and private operations,
    accessors and mutators
  • To list the steps required to create an
    object-oriented design solution to a problem

3
11.1
  • Introduction to object-oriented design

4
Introduction to object-oriented design
  • Objects
  • Can be considered as a container for a set of
    data and the operations that need to be performed
    on it
  • Objects have the following properties
  • It has an identity that is unique for the
    lifetime of the object
  • It has data in a form of a set of characteristics
    or attributes
  • A set of operations or methods that can be
    performed on the date
  • It is a instance (example) of a class

5
Introduction to object-oriented design
  • Classes and objects
  • Object is created from a template or pattern,
    called a class which defines the basic
    relationship, attributes and operations available
    to the objects of that class
  • Each class in a system should bear a unique name
  • Process of creating objects from classes is
    called instantiation and an object is described
    as an instance of its class

6
Introduction to object-oriented design
  • Attributes
  • An objects data is stored in attributes
  • Attributes are the properties or characteristics
    that describe that particular object
  • Objects of the same class will have an identical
    set of attributes

7
Introduction to object-oriented design
  • Methods
  • Objects usually have a set of operations called
    methods
  • Includes all operations required to be performed
    on an object, and are the same as modules in
    procedural programming

8
Introduction to object-oriented design
  • Inheritance
  • Allows a new object to inherit the same
    attributes and methods as an existing object

9
Introduction to object-oriented design
  • Class diagram
  • Object-oriented programming requires classes,
    attributes and methods to be represented in a
    class diagram
  • Consists of a rectangular box divided into three
    section with the name of the class at the top,
    the attributes of the class in the middle and the
    methods at the bottom

10
Introduction to object-oriented design
  • Encapsulation and information hiding
  • Objects are said to be encapsulate (enclose
    together in a single indivisible unit)
  • In object-oriented design, each object can be
    regarded as a black box whose internal workings
    are hidden from all other objects

11
11.2
  • Public and private access methods

12
Public and private access methods
  • Necessary to consider whether the attributes and
    operations of an object are to have private or
    public access
  • This concept is called visibility
  • Private access means that the attribute and
    methods are invisible to the rest of the system
  • Public access means that the operation is visible
    other objects

13
Public and private access methods
  • Instantiating objects
  • Every time an object is instantiated from a
    class, a special operation, or set of
    instructions known as a constructor method is
    called or invoked
  • Constructors may
  • Have no parameters new objects is assigned all
    the default vales for its attribute
  • Have parameters that initialise the attributes
    with specific values

14
Public and private access methods
  • Accessors and mutators
  • The values in the attributes of an object should
    be available to all operations in that object,
    but hidden from external objects
  • Accessor operations pass attribute values to
    external objects
  • Mutator operations enable objects to change the
    values stored in attributes

15
Public and private access methods
  • Message
  • Communication is achieved when one object passes
    a message to another
  • Message from one object to another usually
    initiates the processing of an operation in the
    receiving object

16
11.3
  • Steps in creating an object-oriented solution

17
Steps in creating an object-oriented solution
  • Three steps in creating an object-oriented
    solution for a problem with just one class
  • Identify the objects and their attributes,
    responsibilities and operations
  • Design the algorithms for the operations and
    methods using structured design
  • Develop a test or driver algorithm to test the
    solution

18
Summary
  • Object-oriented design focuses on the objects
    that make up a program rather than on the
    processes.
  • An object can be defined as a container for both
    a set of characteristics and a set of operations
    that can be performed on the data.
  • Objects encapsulate their data and operations,
    and can be regarded as black boxes for the
    purposes of large system design.

19
Summary
  • Operations that are accessible by external
    objects are described as having public access.
  • Operations that are internal to the object are
    described as having private access.

20
Summary
  • The steps in designing an object-oriented
    solution for a programming problem are
  • Identify the classes and their attributes,
    responsibilities and operations.
  • Design the algorithms for the operation, using
    structured design.
  • Develop a test or driver algorithm to test the
    solution.
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