Lecture 5 - A Generalised Object Orientated Model

Lecture 5 - A Generalised Object Orientated Model

1. Dynamic Polymorphism

   • When a method is called but overwritten at runtime
   • 'Abstract' means the at the concept (method) is common to all subclasses - but the details are not
     • Precise version of method used may be determined at run-time

2. Meta Objects

   • All classes possess some properties and methods
     • e.g. Methods for creating instance of class
   • Allows each class to be represented as an object (meta objects)
     • The meta-object's meta-class is a class of the original class

3. Meta Classes

   • Contains (or inherits) properties and methods related to a class
     • 'Normal' objects store information about specific values i.e., as addresses or student id
     • Meta objects store concepts a particular class has
       • i.e., store the FACT that a person has an address, rather than the value a particular address
   • Each meta-class is represented as an objects, and has its own meta class

4. Root Classes

   • Often there is one meta class
     • Called meta-class
     • it it its own meta class
     • i.e., there is an instance of class meta-class
       • which represents information on class meta-class
       • Because meta-class possesses the same concepts as any other class
         • Meaning one of its OWN objects can store data about it
   • Objects generally share some properties
     • e.g. They all have a class they belong to
     • These properties must be associated with a class
     • ROOT Class which all other classes inherit

5. Meta Data

   • Meta-Class has general properties and methods relating to classes
   • Root has information relating to all objects
     • e.g. indicate class instance (returns a meta object)
   • Company has information of companies
     • Company name
     • Uniform colour
     • Also acts as a meta-class
       • Because it inherits class meta-class

6. Modelling the Hierarchy

   • Information on the class hierarchy is now stored within the class hierarchy itself
   • Creating a new class is a matter of creating an instance of its meta-class
   • Although not always desirable
     • It is possible to modify the class hierarchy dynamically
     • A minimal 'bootstrap' is required to start the development process

7. Object Orientated Models may be generalised by modelling classes as Objects

8. Produces a very simple, clean, and very flexible system

9. Many modern systems not sufficiently flexible to incorporate all concepts at present

10. Generalised systems are beginning to appear

    • Especially to deal with multimedia documents