Lecture 4 - A Basic Object Orientated Model

Lecture 4 - A Basic Object Orientated Model

1. Classical Software Engineering

   • Stepwise Refinement
   • Problems with Classical Approach

2. Basic Features of Object-Orientation

   • Encapsulation
     • Objects
     • Classes
   • Inheritance
     • Multiple Inheritance

3. Classical Software Engineering

   • Programming Language Evolution let to larger and more complex programs incomprehensible to humans
   • Methodology required to build larger more complex programs
   • 'Structured Programming' (60s and 70s)
     • Essentially Divide and Conquer
     • Write high level algorithm for problem
     • Divide algorithm into a series of significant sub-algorithms each forming a program module
     • Repeat process on sub-algorithms until they become small enough to be expressed as program code (Stepwise Refinement)

4. Real World Analogy

   • Entities
     • e.g. John, Room G1A, Canterbury
     • Possess Properties
     • Everyday tasks involved these entities
     • These entities are called Objects in OOP

5. Object Orientation

   • Components of a system are abstract 'objects' with relevant attributes
     • Example: Uni Admin System
       • Object: Student
         • Name
         • ID No.
         • Course
     • An Object Encapsulates properties related to an entity in the system
       • Does not include irrelevant attributes
   • Encapsulation
     • Objects have their own properties
       • e.g. Lecture title, theme, Slide running order
     • Objects have relations to each other
       • A given lecture involves many objects

6. Classes

   • Group together objects with common properties to form a class
   • Identifying Classes major part of design process
     • Dependent on system requirements
   • Class Definition
     • Classes have a number of attributes
     • Classes also have operations
       • Functions

7. Definitions

   • Message
     • Information sent to an object requesting an operation to be performed
   • Attribute
     • Property of an object often expressed as a variable
   • Operation
     • A Function which an object
   • Method
     • An implementation of an operation
       • Method is completely encapsulated within the class

8. Inheritance

   • Allows classes to extend the definition of other classes (superclass)
   • But the subclasses can still include their own attributes