What Is Object-Orientation?

Transcription

1 Iformatio Systems Cocepts What Is Object-Orietatio? Roma Kotchakov Birkbeck, Uiversity of Lodo Based o Chapter 4 of Beett, McRobb ad Farmer: Object Orieted Systems Aalysis ad Desig Usig UML, (4th Editio), McGraw Hill,

2 You d have to be livig face dow i a moo crater ot to have heard about object-orieted programmig. Tom Swa Object-orieted programmig is a exceptioally bad idea which could oly have origiated i Califoria. Edsger Dijkstra

3 Outlie Object-Orietatio Cocepts Sectio 4.2 (pp ) Object-Orietatio Beefits Sectio 4.3 (pp ) 3

4 Object A object is a abstractio of somethig i a problem domai, reflectig the capabilities of the system to keep iformatio about it, iteract with it, or both. Coad ad Yourdo (1990) We defie a object as a cocept, abstractio, or thig with crisp boudaries ad meaig for the problem at had. Objects serve two purposes: they promote uderstadig of the real world ad provide a practical basis for computer implemetatio. Rumbaugh et al. (1991) 4

5 Object Objects have state, behaviour ad idetity. Idetity (Who am I?) each object is uique State (What do I kow?) Booch (1994) the coditios of a object at ay momet that affect how it behaves Behaviour (What ca I do?) the way i which a object respods to messages 5

6 Objects Object Idetity States Behaviour A perso A shirt Hussai Pervez My favourite buttodow white deim shirt Studyig Restig Qualified Pressed Dirty Wor A sale Sale o 0015, 15/06/02 Ivoiced Cacelled A bottle of ketchup This bottle of ketchup Usold Opeed Empty Speak Walk Read Shrik Stai Rip Ear loyalty poits Spill i trasit A coffee machie object? 6

7 Idetity!= Equality Differet objects must have differet idetities Differet objects may have exactly the same state (be equal) e.g., twi brothers, two iterchageable blue pes, etc. [Java] if (obj1 == obj2) if (obj1.equals(obj2)) tests idetity tests equality 7

8 Object Object has State Behaviour Idetity (equal idetical) BOOCH, G. (1994): Object Orieted Aalysis ad Desig with Applicatios, 2 d ed, The Bejami/Cummigs Publ

9 Class A class is a set of objects that share the same specificatios of features (attributes, operatios, liks), costraits (e.g. whe ad whether a object ca be istatiated) ad sematics OMG (2004) Moreover, The purpose of a class is to specify a classificatio of objects ad to specify the features that characterize the structure ad behaviour of those objects OMG (2004) 9

10 Class A object = A istace of some class Every object must be a istace of some class A class = A set of objects that share the same structure what iformatio it holds what liks it has to other objects behaviour what thigs it ca do 10

11 Objects ad Classes: termiology Object Class I C++ I Java Idetity Data State (values) Structure Specificatio Member Variables Fields (istace variables) Operatios Behaviour Behaviour Specificatio Member Fuctios Methods Ways of thikig about a class A factory maufacturig objects accordig to a blueprit A set that specifies what features its member objects have A template that allows us to produce ay umber of objects of a give shape 11

12 Class Class a abstractio (geeric descriptio) for a set of objects Object a istace of a class BOOCH, G. (1994): Object Orieted Aalysis ad Desig with Applicatios, 2 d ed, The Bejami/Cummigs Publ

13 Message Passig Objects collaborate to fulfil some system fuctio, ad they commuicate by sedig each other messages: A questio message asks a object for some iformatio How much is the balace? A commad message tells a object to do somethig Withdraw 100 pouds 13

14 Message Passig: Example Buyig a loaf of bread: 14

15 Ecapsulatio Layers of a oio model of a object A outer layer of operatio sigatures gives access to middle layer of operatios which ca access ier core of data Message from aother object requests a service. Operatio called oly via valid operatio sigature. Data accessed oly by object s ow operatios. A object s data is hidde (ecapsulated). 15

16 Ecapsulatio Cosider a object represetig a circle. A circle would be likely to have operatios allowig us to discover its radius, diameter, area ad perimeter. We could store ay oe of the four attributes ad calculate the other three o demad. Let's say we choose to store the diameter. Without ecapsulatio, ay programmer who was allowed to access the diameter might do so, rather tha goig via the getdiameter operatio. If, for a later versio of our software, we decided that we wated to store the radius istead, we would have to fid all the pieces of code i the system that used direct access to the diameter, so that we could correct them (ad we might itroduce faults alog the way). With ecapsulatio, there is o problem. 16

17 Ecapsulatio Object data is hidde Operatios ecapsulate maipulatio of the data BOOCH, G. (1994): Object Orieted Aalysis ad Desig with Applicatios, 2 d ed, The Bejami/Cummigs Publ

18 Geeralizatio / Specializatio Classificatio is hierarchical i ature A perso may be a employee, a customer or a supplier A employee may be paid mothly, weekly or hourly A hourly-paid employee may be a driver, a cleaer or a sales assistat. Every istace of the specific class (subclass) is also a istace of the more geeral class (superclass) A subclass is a (kid of) its superclass 18

19 Geeralizatio / Specializatio Perso More geeral (superclasses) Employee Customer Supplier Mothly-paid Weekly-paid Hourly-paid Driver Cleaer Sales Assistat More specific (subclasses) 19

20 Taxoomies Aimal More geeral (superclasses) Mammal Fish Bird Whale Dog Cat Domestic Cat Tiger More specific (subclasses) 20

21 Iheritace A subclass always iherits all the characteristics (data structure ad behaviour) of all its superclasses The defiitio of a subclass should always iclude at least oe detail ot derived from ay of its superclasses 21

22 Geeralizatio A subclass iherits the structure ad behavior of its superclass Not a good visualizatio of geeralizatio, because subclasses iherit types, ot values (a ose ot a log ose)! BOOCH, G. (1994): Object Orieted Aalysis ad Desig with Applicatios, 2 d ed, The Bejami/Cummigs Publ

23 Geeralizatio i UML Employee dateofappoitmet dateofbirth departmet employeenumber liemaager ame A superclass has geeral characteristics that are iherited by all subclasses. The symbol for geeralizatio. MothlyPaidEmployee mothlysalary HourlyPaidEmployee hourlyrate hoursworked Subclasses have specialized characteristics that are uique to each subclass. 23

24 Advatages of usig Geeralizatio Employee dateofappoitmet dateofbirth departmet employeenumber liemaager ame This ew subclass requires o chage to the existig structure. MothlyPaidEmployee mothlysalary HourlyPaidEmployee hourlyrate hoursworked WeeklyPaidEmployee weeklywage 24

25 Multiple Iheritace We may wat the Part-Time BSc Studet class to be a sub-class of both the BSc Studet class ad the `Part-Time Studet class. Perso Perso Lecturer Studet Lecturer Studet BSc Studet FdIT Studet Full-Time Studet Part-Time Studet 25

26 Geeralizatio: Exercise How shall we group these classes ito a geeralizatio hierarchy? 26

27 Polymorphism Polymorphism allows oe message to be set to objects of differet classes Sedig object eed ot kow what kid of object will receive the message Each receivig object respods appropriately, i.e., differet kids of objects may respod to the message i differet ways poly morph ic = havig may shapes 27

28 Polymorphism: Example ope 28

29 Polymorphism: Example if (x is of type 1) a = getcirclearea(x.r); else if (x is of type 2) a = getrectaglearea(x.l, x.w); else if (x is of type 3) a = gettriaglearea(x.b, x.h); a = x.getarea(); 29

30 Polymorphism: Example calculatepay for differet kids of employees Fixed mothly amout depeds oly o employee grade 2a: calculatepay() :FullTimeEmployee 2b: calculatepay() Variable mothly amout depeds o grade ad hours :MothlyPayPrit :PartTimeEmployee Pay Clerk 1: gettotalpay() 2c: calculatepay() Fixed mothly amout depeds o grade, but o pesio deductios :TemporaryEmployee 30

31 Beefits of Object-Orietatio Object-Orietatio cocepts ad techiques improve both software quality ad software productivity Abstractio, Modularity ad Reusability Evet-Drive Programmig ad GUI Programmig Model Trasitio ad Iterative/Icremetal Lifecycle 31

32 Take Home Messages Object-Orietatio Cocepts Object ad Class Ecapsulatio Geeralizatio Iheritace Polymorphism Object-Orietatio Beefits 32