Objects

1. Objects • Objects are at the heart of the Object Oriented Paradigm • What is an object?

2. Objects • We are surrounded by objects. • In this class room there are desks, blackboards, lights , chairs and so on. • Each object has specific attributes • The desk is made of wood. The seat has a blue cover and so on.

3. Objects have associations and relationships with each other . • Objects have associations and relationships with each other . • Seats have desktops attached. • All are attached to the floor • Seats are adjacent to other seats. • They are oriented in the same way. • Lights are controlled by switches

4. Object Roles and Functions • Each Object in the room has a specific role or function or behaviour. • Seats are to be sat on. • Board is to be written on. • Lights can be turned on and off • Boards can be moved up and down and seats can be put up and down.

5. Objects can be classified • Within the room objects can be grouped into different classes • For example we could have the furniture objects e.g. chairs, tables, worktop • We could also have the electrical objects. • Lights, projector, laptop and so on.

6. The nature of Objects • Clearly Objects constitute a complex multi faceted concept. • Their definition is dependent on many elements- their attributes , their behaviour, their classification and as we will see much , much more • To investigate the nature of objects , it is helpful to consider an important knowledge representation scheme which is the basis for theoretical frameworks of the object oriented paradigm • This framework is known as semantic networks

7. Semantic Nets • Semantic networks are a popular scheme which elegantly reflect these ideas. • A network consists of nodes repesenting objects, concepts and events and links between the nodes representing their interrelations.

8. Example • Using the example Birds have wings • a typical semantic net would be

9. Origin • The development of semantic networks had its origins in psychology. Ross Quillian in 1968 designed two semantic network based systems that were intended primariliy as psychological models of associateive memory. • Semantic Networks quickly found application in AI. B. Raphaels SIR system, also 1968, was one of the first programs to use this type of representation scheme. • SIR was a question Answering system and could answer questions requiring a variety of simple reasoning tasks and relationships

10. Meaning of Semantic Nets • The semantics of net structures however depends solely on the program that manipulates them and there are no fixed conventions about their meaning. • A wide variety of network based systems have been implemented that use totally different procedures for making inferences.

11. likes Dog bone Another Example

12. Evolving Features • While there are no fixed conventions, a number of important features of Semantic nets have emerged , that are widely used. • These have largely emerged because of the application of Semantic Nets to Object Oriented Theory.

13. Object Definitions • The central aspect of the object paradigm is how it defines objects. • The basic mechanism of representation is the articulation of class hierarchies. • Instances of Objects exist. In turn Objects belong to classes and these in turn can belong to other classes

14. Example • Consider Fido ( who is a dog) • Fido is instance of the object “dog”. • Dogs belong to the class pets, which for example could also include other classes such as cats. • Pets in turn belong to a class animals and so on

15. Animals Pets Cats Dogs Fido

16. Labels used in Semantic Nets • Objects and Instances • Both Represented by Nodes linked by an • IS_A link Fido Dog IS_A Instance Object

17. Labels used in Semantic Nets • Objects and Classes • Both Represented by Nodes linked by an • Subset or a SuperSet link Dog Pets Subset Object Class

18. SuperSet links • Objects and Classes • Both Represented by Nodes linked by an • SuperSet link Dog Pets Subset Object Class Superset

19. Relationships, attributes and associations Relationships, attributes and associations Represented by a labelled link between objects etc Fido Black and white colour

20. Component Parts • Object components • Both Represented by Nodes linked by an • HAP (has as part) link • A dog has a tail Dog Tail HAP Object Object

21. Inheritance • Attributes of classes are inherited by subclasses and instances of objects • Because we know dogs have tails and Fido is a dog we know Fido has a tail since this is inherited from the parent class

22. Inheritance • A dog has a tail and Fido is a dog Dog Tail HAP ISA Object Object Fido

23. Bigger Example • Exercise • What does the following Semantic net represent

24. Building subset Purpose Habitation House No. Of Storeys subset brick Bungalow 1 Made_of ISA Colour HAP HAP Roof My House Red walls

25. Advantages of Semantic Nets • Easy to visualize – Graphical in nature – easy for humans to interpret • Expressive power equal to or exceeding that of First Order Predicate Logic • Formal definitions of semantic networks have been developed for use • Related knowledge is easily clustered – logically and physically close • Efficient in space requirements • Objects represented only once • Relationships handled by pointers • Other schemes are limited to True or False answers where as Semantic Nets are more informative and flexible • Not limited to only binary representation… can also represent action concepts

26. Disadvantages of Semantic Nets • Inheritance (particularly from multiple sources and when exceptions in inheritance are wanted) can cause problems such as conflicts • Facts placed inappropriately cause problems • No standards about node and arc values – in spite of the generic formal definitions