Relationships between Objects-Object Oriented Programming-Lecture Slides, Slides of Object Oriented Programming

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1

Relationships

between Objects

Lecture 6

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Overview

n What are different kinds of relationships between classes/ objects? n Inheritance n Association n Dependency n Aggregation n Composition n Polymorphism n Dynamic Binding

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BASIC TERMINOLOGY

n Objects

n Messages and methods

n Classes

n Inheritance

n Polymorphism

n Dynamic Binding

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Defining a Class

n When programming a class the data and responsibilities will become: n class variables i.e. values stored in the class as a whole n instance variables i.e. values stored in each instance n class methods i.e. methods the CLASS can do n instance methods i.e. methods an INSTANCE of the CLASS can do (after

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Example of a Class

n A Customer Class may have: n class variables - totalNumberOfCustomers n instance variables - customerAddress n class methods - new (how to create a new Customer) n instance methods - updateAddress

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UML Representation of a Class

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Different sorts of classes

n Abstract Class a class that can have one or more subclasses, but NOT an

n i.e. an abstract class can only be inherited from by another class, it cannot be

n Concrete Class a class that can have one or more subclasses and/or instances

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Modular Software is like Lego

n Each class is a brick and they can be fitted together in many different ways to form a computer system

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Execution of programs

n An OO program runs by the coordination of objects receiving, interpreting and responding to messages.

n During the program execution: n objects are created as needed n messages move between objects & methods are activated n objects deleted when no longer required

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Relationships between Classes

n There are two types of

relationships between different

objects/classes :

n inheritance is_type_of - inherits properties from another class n association uses_a - sends a message to another object/class

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Types of Association

n dependency depends_upon – rely upon other objects

n aggregation consists_of - built up of other objects/classes

n composition consists_of - strongly owns other objects/ classes

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Inheritance

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Inheritance

n Inheritance is a key concept of OO paradigm n Like a set of similar objects is described by a class, Inheritance can be used to relationships of similarity

n Example: MonthlyPaidEmployee and HourlyPaidEmployee

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How it works?

n The approach enables generalisation to occur – means of managing complexity n Code reuse without duplication n A language-provided mechanism for sharing the data/ methods amongst the classes and objects n Allows extension of the hierarchy n By enables creation of new classes from existing classes by only considering the differences

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Example of Inheritance

e m p lo y e e N u m b e r : i n t^ E m p lo y e e d a t e O f B i r t h : D a t e n a m e : S t r i n g o r g a n i z a t i o n : S t r i n g d e p a r t m e n t C o d e : in t p o s i t i o n C o d e : i n t d a t e O f A p p o in t m e n t : D a t e

M o n t h lyP a id E m p loy e e m o n t h l y S a la ry : flo a t

W e e k lyP a id E m p lo y e e w e e k lyW a g e : flo a t

H o u rlyP a id E m p lo y e e h o u r l y R a t e : f l o a t h o u r s W o r k e d : f l o a t

E m p lo y e e e m p lo y e e N u m b e r : in t d a t e O f B i r t h : D a t e n a m e : String o r g a n i z a t io n : String d e p a r t m e n t C o d e : in t p o s i t i o n C o d e : i n t d a t e O f A p p o i n t m e n t : D a t e

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Rules of Inheritance

n The subclass always inherits all the characteristics of its ( )

n The definition of the subclass always includes at least one detail not derived ( )

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Terminology

n HeadOfDepartment inherits from Lecturer

n HeadOfDepartment is a subclass of Lecturer

n HeadOfDepartment is a specialization of Lecturer

n HeadOfDepartment is more specialized than Lecturer

n HeadOfDepartment is a refinement of Lecturer

n Lecturer is a super class (or base class) of HeadOfDepartment

n Lecturer is a generalization of HeadOfDepartment

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Generalization vs. Inheritance

n Generalization/ specialization represents conceptual relationship between classes n Subclass/ superclass represents concrete relationship between classes n If one class is a subclass of the other, may specialization of the latter. n Conceptual gulf is NOT RECOMMENDED!

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Identifying Inheritance and its

n The key phrase for finding inheritance is X is a kind of Y n Inheritance can be: n single - each class has only one superclass to inherit from n multiple - each class can take properties

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Multiple Inheritance (1)

n Car inherits some properties from LandVehicle : n ( Data ) four wheels, red n ( Behaviour ) starts, stops n Car inherits some properties from HouseHoldItems: n ( Data ) owner, address n ( Behaviour ) getsNewOwner LandVehicle HouseHold Items

Car

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Multiple Inheritance (2)

n Multiple inheritance gives advantages especially in terms of re-usability, but has practical problems n What if both LandVehicle and HouseholdItems have different methods both called moveLocation - which would Car inherit?

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UML Representation of

Inheritance

e m p lo y e e N u m b e r : in t^ E m p lo y e e d a t e O f B ir th n a m e : S t r in g : D a t e o r g a n iz a t io n d e p a r t m e n t C o d e : S t r i n g : in t p o s i t i o n C o d e : int d a t e O f A p p o in t m e n t : D a t e

M o n t h lyP a id E m p lo y e e m o n t h l y S a la r y : f lo a t W e e k lyP a id E m p lo y e e w e e k lyW a g e : f lo a t H o u r l y P a i d E m p l o y e e ho u r ly R a t e : f l o a t ho u r s W o rk e d : f l o a t

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A Question

n Consider n HoD and Lecturer n One is the specialization of the other n Both can perform a particular service, e.g.

n It is possible that they perform the same task differently n How to implement it?

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The Answer:

Method Overriding

n Encapsulation hides the

implementation

n If the object HoD receives a message to perform an operation which both the and the Lecturer can do, then the method invoked will be the specialized version provided by the class HoD!

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Summary

n Types of classes, attributes and

n Introduction to relationships between

n Inheritance

n Inheritance…