Object Oriented Analysis & Design (OOAD)

1. Object Oriented Analysis & Design (OOAD)

2. OOAD • It focuses on objects where system is broken down in terms of the objects that exist within it. • Functions (behaviour) and data (state) relating to a single object are self-contained or encapsulated in one place.

3. Objects • Object is an abstraction of something in a problem domain, reflecting the capabilities of the system to keep information about it, interact with it, or both. • Objects are entities in a software system which represent instances of real-world and system entities

4. Objects

5. Object Class • Class is a description of a set of objects that share the same attributes, operations, methods, relationship and semantics. • Object classes are templates for objects. They may be used to create objects. • An object represents a particular instance of a class.

6. Term of objects • Attribute: data items that define object. • Operation: function in a class that combine to form behavior of class. • Methods: the actual implementation of procedure (the body of code that is executed in response to a request from other objects in the system).

7. Employee object & class Class Object Employee16 name: John address: M Street No.23 dateOfBirth: 02/10/65 employeeNo: 324 socialecurityNo:E342545 department: Sale manager: Employee1 salary: 2340 stauts:current taxCode: 3432 …. Eployee16.join(02/05/1997) Eployee16.retire(03/08/2005) Eployee16.changeDetail(“X Street No. 12”)

8. Encapsulation and Data Hiding • Packaging related data and operations together is called encapsulation. • Data hiding: hides the internal data from external by methods (interface).

9. Encapsulation • private attributes and methods are encapsulated within the class, they cannot be seen by clients of the class • public methods define the interface that the class provides to its clients Customer - numCustomers = 0 - MIN_BUDGET = 200 - name: String - address: String - budget: int private attributes + printNumCustomer( ): void public methods + placeOrder( ): void Customer class

10. Object communication • Objects communicate with each other by sending messages • a message is a method call from a message-sending object to a message-receiving object • a message consists of • an object reference which indicates the message receiver • a method name (corresponding to a method of the receiver), and • parameters (corresponding to the arguments of the calling method) • a message-receiving object is a server to a message-sending object, and the message-sending object is a client of the server

11. name = “Alex” address = “1 Robinson Rd” budget = 2000 placeOrder( ): void Message Passing message name = “Lawrence” employeeNo =15 commission = 200 takeOrder(“sofa”, name, address, “120799”) 199 takeOrder( ): int return value alex lawrence message lawrence.takeOrder(“sofa”, “1 Robinson Rd”, “120799”) object reference method name parameters

12. Message Passing Customer SalesPerson - numCustomers = 0 - MIN_BUDGET = 200 - name: String - address: String - budget: int - MAX_ PRICE = 200 - name: String - employeeNo: String - commission: int + printNumCustomer( ): void + takeOrder( ): void + placeOrder( ): void takeOrder alex lawrence client server

13. Inheritance • Object classes may inherit attributes and services from other object classes. • Inheritance represents the generalization of a class.

14. A generalisation hierarchy

15. Library Item Library class hierarchy Catalogue Number Acquisition date Cost Type Status Number of copies A c q u i r e ( ) C a t a l o g u e ( ) D i s p o s e ( ) I s s u e ( ) R e t u r n ( ) P u b l i s h e d i t e m R e c o r d e d i t e m T i t l e T i t l e P u b l i s h e r M e d i u m C o m p u t e r B o o k F i l m M a g a z i n e p r o g r a m D i r e c t o r A u t h o r Y e a r D a t e o f r e l e a s e V e r s i o n E d i t i o n I s s u e D i s t r i b P l a t f o r m P u b l i c a t i o n d a t e I S B N

16. User class hierarchy

17. Multiple inheritance • Rather than inheriting the attributes and services from a single parent class, a system which supports multiple inheritance allows object classes to inherit from several super-classes • Can lead to semantic conflicts where attributes/services with the same name in different super-classes have different semantics • Makes class hierarchy reorganisation more complex

18. Multiple inheritance

19. Advantages of inheritance • It is an abstraction mechanism which may be used to classify entities • It is a reuse mechanism at both the design and the programming level • The inheritance graph is a source of organisational knowledge about domains and systems

20. Problems with inheritance • Object classes are not self-contained. they cannot be understood without reference to their super-classes • Designers have a tendency to reuse the inheritance graph created during analysis. Can lead to significant inefficiency • The inheritance graphs of analysis, design and implementation have different functions and should be separately maintained

21. Inheritance and OOD • There are differing views as to whether inheritance is fundamental to OOD. • View 1. Identifying the inheritance hierarchy or network is a fundamental part of object-oriented design. Obviously this can only be implemented using an OOPL. • View 2. Inheritance is a useful implementation concept which allows reuse of attribute and operation definitions. Identifying an inheritance hierarchy at the design stage places unnecessary restrictions on the implementation • Inheritance introduces complexity and this is undesirable, especially in critical systems

22. Objects Association • Modeling an association between two classes means that there is some sort of relationship between objects of each class that may be connected. studies Student Course 0..* 1..*

23. Object aggregation • Aggregation model shows how classes which are collections are composed of other classes • Similar to the part-of relationship in semantic data models

24. Object aggregation S t u d y p a c k C o u r s e t i t l e N u m b e r Y e a r I n s t r u c t o r 1 1 1 1 0 ..* 1 ..* 1 ..* 1 ..* L e c t u r e V i d e o t a p e O H P s l i d e s A s s i g n m e n t n o t e s S l i d e s T a p e i d s . C r e d i t s T e x t 1 1 1 ..* 1 ..* E x e r c i s e s S o l u t i o n s T e x t # P r o b l e m s D e s c r i p t i o n D i a g r a m s

25. Object Cohesion & Coupling • Cohesion of a component is a measure of how well it fits together. Each operation provides functionality which allows the attributes of the object to be modified, inspected or used as a basis for service provision. • Coupling is an indication of the strength of interconnections between program units. Highly coupled systems have strong interconnections, with program units dependent on each other (shared variables, interchange control function). Loosely coupled system which are independent .

26. name getName( ) calculateArea( ) Shape radius calculateArea( ) side calculateArea( ) Circle Square Polymorphism • the ability of different objects to perform the appropriate method in response to the same message is known as polymorphism. • the selection of the appropriate method depends on the class used to create the object

27. inheritance a generic action overloading overriding Example Polymorphism class Shape { private String name; public Shape(String aName) { name=aName; } public String getName( ) { return name; } public float calculateArea( ) { return 0.0f; } } // End Shape class class Circle extends Shape { private float radius; public Circle(String aName) { super(aName); radius = 1.0f; } public Circle(String aName, float radius) { super(aName); this.radius = radius; } public float calculateArea() { return (float)3.14f*radius*radius; } } // End Circle class

28. class Square extends Shape { private float side; public Square(String aName) { super(aName); side = 1.0f; } public Square(String aName, float side) { super(aName); this.side = side; } public float calculateArea() { return (float) side*side; } } // End Square class

29. rule of subtype dynamic binding Polymorphism Example public class ShapeDemoClient { public static void main(String argv[ ]) { Shape c1 = new Circle("Circle C1"); Shape c2 = new Circle("Circle C2", 3.0f); Shape s1 = new Square("Square S1"); Shape s2 = new Square("Square S2", 3.0f); Shape shapeArray[ ] = {c1, s1, c2, s2}; for (int i = 0; i < shapeArray.length; i++) { System.out.println("The area of " + shapeArray[i].getName( ) + " is " + shapeArray[i].calculateArea( ) + " sq. cm."); } } // End main } // End ShapeDemoClient1 class

30. OO Analysis and Design OO Analysis - examines requirements from the perspective of the classes and objects found in the vocabulary of the problem domain. In other words, the world (of the system) is modelled in terms of objects and classes. OO Design - OO decomposition and a notation for depicting models of the system under development. Structures are developed whereby sets of objects collaborate to provide the behaviours that satisfy the requirements of the problem.

31. Object Oriented Analysis • Analyze the domain problem • Describe the process systems • Identify the objects • Specify attributes • Defining operations • Inter-object Communication

32. Identifying Object • Objects can be: • External Entity (e.g., other systems, devices, people) that produce or consume information to be used by system • Things (e.g., reports, displays, letters, signals) that are part of information domain for the problem • Places (e.g., book’s room) that establish the context of the problem and the overall function of the system. • Organizational units (e.g., division, group, team, department) that are relevant to an application, • Transaction (e.g., loan, take course, buy, order).

33. Example of candidate objects Just a Linemanagement wishes to increase security, both in their building and on site, without antagonizing their employees. They would also like to prevent people who are not part of the company from using the Just a Line car park. It has been decide to issue identity cards to all employees, which they are expected to wear while on the Just a Line site. The cards records the name, department and number of the member of staff, and permit access to the Just a Line car park. A barrier and a card reader are placed at the entrance to the car park. The driver of an approaching car insert his or her numbered card in the card reader, which then checks that the card number is known to the Just a Line system. If the card is recognized, the reader sends a signal to raise the barrier and the car is able to enter the car park. At the exit, there is also a barrier, which is raised when a car wishes to leave the car park. When there are no spaces in the car park a sign at the entrance display “Full” and is only switched off when a car leaves. Special visitor’s cards, which record a number and the current date, also permit access to the car park. Visitor’s cards may be sent out in advance, or collected from reception. All visitor’s cards must be returned to reception when the visitor leaves Just a Line.

34. Candidate objects: Just a Line management security building site employee people company car park card name department number member of staff access barrier card reader entrance driver car system signal exit space sign visitor reception

35. Candidate objects’ rejection • duplicates: if two or more objects are simply different names for the same thing. • irrelevant: objects which exists in the problem domain, but which are not intended. • vague: when considering words carefully it sometimes becomes clear that they do not have a price meaning and cannot be the basis of a useful in the system. • general: the meaning is too broad. • attributes: as the attribute of objects. • associations: actually represents the relationships between objects. • roles: sometimes objects referred to by the role they play in a particular part of the system.

36. Rejected Candidate objects

37. Rest Objects Car park Staff Card Visitor’s card Employee Entrance exit card reader barrier Full sign space sensor car

38. Define class attributes Full sign on:boolean switch on() switch off() Car Park capacity spaces inc.spaces() dec.spaces() space left() Barrier type: up:boolean raise() lower() Card Reader valid card nos. read card() card OK()

39. Data Dictionary Barrier: type + up type = [“Entrance”|”Exit”] up = [“true”|”false”] raise: if the barrier is not already raised, this operation takes as argument an object of the barrier class and returns an object of the same class, with the up attribute set to “true”. If the barrier is already up, the operation returns the error message “barrier is already raised” lower: if the barrier is not already lower, this operation takes as argument an object of the barrier class and returns an object of the same class, with the up attribute set to “false”. If the barrier is already down, the operation returns the error message “barrier already lowered”.

40. Objects Relationship Car Park Car Park 1 ..* 2 ..* 2 ..* Barrier Card Reader Sensor 1 .. * 1 Valid cards 1 .. * Card Visitor’s Card Staff Card

41. Object behaviour modelling • A behavioural model shows the interactions between objects to produce some particular system behaviour that is specified as a use-case • Sequence diagrams (or collaboration diagrams) in the UML are used to model interaction between objects

42. Sequence Diagram for Entrance :CarPark :Sensor :CardReader :Valid card :Barrier :Full Sign :User Full Sign off Car present Check space left Yes Card number Card number Card OK Card returned Raise Car not present lower Decrement Space Check space left Yes

43. OO Analysis and Design Object-Oriented (OO) development is very different from structured development: • Structured approach focuses on major functions within a system and on the data used by the functions. • OO approach focuses on the objects in a system and on the relationships between those objects.

44. Unlike functional decomposition, OO views a complex problem as a meaningful collection of objects that collaborate to achieve some higher level behaviour => closely mirrors how people view complex problems => using OO should make the job of developing large, complex systems more manageable.

45. Object Oriented Model

46. Structured Model

47. Comparison • OO: • Systems decomposed into collections of data objects; • function + data in one place => • System components more • independent => more resilient • to requirements and • maintenance changes. • Inheritance and polymorphism • are possible => reuse, • extension, and tailoring of • software/designs is possible. • Closely mirrors how humans • decompose and solve • complex. • Structured: • Systems decomposed into functions; functions and data modelled separately => • System components are more • dependent on each other => • requirements and • maintenance changes more • difficult • Inheritance and polymorphism • not possible => limited reuse • possible. • System components do not • map closely to real-world • entities => difficult to manage • complexity.

48. Comparison • OO: • Process allows for iterative and incremental development => • Integration of programs is • series of incremental • prototypes. • Users and developers get • important feedback • throughout development. • Testing resources distributed • more evenly. • If time is short, coding and • testing can begin before the • design is finished. • Structured: • Process less flexible and largely linear => • Integration of programs is ‘big • bang’ effect. • Users or developers provided • with little or no feedback; see • system only when it has been • completed. • Testing resources are • concentrated in the • implementation stage only. • Coding and testing cannot • begin until all previous stages • are complete.