CS 501: Software Engineering

1. CS 501: Software Engineering Lecture 17 Object Oriented Design 3

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3. Quiz 3 A Mail Order System manages orders received by a mail order company. Most orders are received by mail and are input into the system for over-night processing. In addition, during the daytime, customers may call by telephone to a customer service clerk who records orders, which are also processed over-night. During the over-night processing, payments received are recorded and bills are sent out to customers. Question 1: System Design (a) What is a suitable architectural style for the Mail Order System? (b) Draw an overview diagram of the system architecture using UML notation.

4. Mailing and reports Customer services Data input and validation Quiz 3, Question 1Architectural Style: Master File Update data check mail orders Master file update telephone orders Red parts of the diagram are optional From lecture 14, slide 5

5. Quiz 3, Question 2 Question 2: Errors (a) When orders are received by mail it is possible for the customer to supply an incorrect part number. How would you identify these errors and handle them? When orders are called in by telephone it is possible for the customer to supply an incorrect part number. How would you identify these errors and handle them? In designing the system architecture the goal is to minimize staff costs while giving good service to customers.

6. Mailing and reports Data input and validation Quiz 3, Question 2Errors in mail orders data check mail orders Master file update

7. Quiz 3, Question 2Errors in mail orders Data entry clerk types in data from mail order. Before submitting order it is checked against master file for valid parts numbers, prices, etc. If there is an error that the clerk cannot correct, an error transaction is created. The master file update creates a report on all transactions for each customer, which is passed to the mailing subsystem. Customer receives mail that identifies the error. No staff time is needed except for data entry clerk.

8. Customer services Quiz 3, Question 2Error in telephone order Master file update telephone orders

9. Quiz 3, Question 2Errors in telephone order Customer service clerk interviews customer and types in order while on the telephone. While talking to the customer, the error is checked against the master file for valid parts numbers, prices, etc. The clerk reads the order back to the customer, indicating any errors. The customer corrects errors and agrees to the order. The customer service subsystem creates an order transaction and passes it to the data input subsystem. No staff time is needed except for customer service clerk.

10. Modeling Dynamic Aspects of Systems Interaction diagrams: set of objects and their relationships including messages that may be dispatched among them • Sequence diagrams: time ordering of messages • Collaboration diagrams: structural organization of objects that send and receive messages Activity diagram: flow chart showing flow of control from activity to activity Statechart diagram:models a state machine

11. Notation: Statechart diagrams Waiting A state machine is a behavior that specifies the sequence of states an object or an interaction goes through during its lifetime in response to events.

12. State Diagram: Notation returned() returned() not borrowable borrowable borrowed()[last copy] guard expression borrowed()[not last copy] State diagram for class Book in a library system

13. Notation for Classes and Objects Classes Objects anObject:AnyClass AnyClass attribute1 attribute2 operation1() operation2() or :AnyClass or anObject or The names of objects are underlined. AnyClass

14. Notation: Active Class EventManager eventlist suspend() flush() An active classis a class whose objects own one or more processes or threads and therefore can initiate control activity.

15. Interaction: Bouncing Ball Diagrams Example: execution of http://www.cs.cornell.edu/ domain name service TCP connection HTTP get Client Server(s)

16. Notation: Interaction display An interaction is a behavior that comprises a set of messagesexchanged among a set of objects within a particular context to accomplish a specific purpose.

17. Actions on Objects returnCopy(c) call return send create destroy okToBorrow() local status notifyReturn(b) asynchronous signal <<create>> stereotypes <<destroy>>

18. Sequence Diagram: Borrow Copy of a Book libMem: LibraryMember theBook:Book BookBorrower theCopy:Copy borrow(theCopy) okToBorrow borrow borrow

19. Sequence Diagram: Change in Cornell Program :MEngStudent Cornellian 1 : getName() 1.1 : name 2: <<create>> PhDStudent(name) :PhDStudent 3: <<destroy>> sequence numbers added to messages

20. Sequence Diagram: Painting Mechanism :Thread :Toolkit :ComponentPeer target:HelloWorld run run callbackLoop handleExpose paint

21. Other Diagrams in UML • Activity diagram is a statechart diagram that shows the flow from activity to activity within a system. • Component diagram shows the organization and dependencies among a set of components. • Deployment diagram shows the configuration of processing nodes and the components that live on them.

22. Release work order Reschedule Assign tasks Activity Diagram: Notation branch [materials not ready] [materials ready] guard expression

23. Decompress Stream audio Stream video Activity Diagram: Parallel Activities start state fork join stop state

24. From Candidate Classes to Completed Design Methods used to move to final design: Reuse: Wherever possible use existing components, or class libraries. They may need modification. Restructuring: Change the design to improve, understandability, maintainability, etc. Techniques include merging similar classes, splitting complex classes, etc. Optimization: Ensure that the system meets anticipated performance requirements, e.g., by changed algorithms or restructuring. Completion: Fill all gaps, specify interfaces, etc.

25. Software Reuse Better software at lower cost Potential benefits of reuse: • Reduce development time and cost • Improved reliability of mature components • Shared maintenance cost Potential disadvantages of reuse: • Difficulty in finding appropriate components • Components may be a poor fit for application

26. Software Reuse: Examples Software developers rely heavily on software components provided by others System software • device drivers • file systems • exception handling • network protocols Subsystems • database management systems • firewalls • web servers

27. Software Reuse Examples (Tools) Standard functions • mathematical methods • formatting User interface • toolkits (e.g. Quickdraw) • class libraries, (e.g., Swing)

28. Software Reuse (Application Packages) Package supports a standard application (e.g., payroll) Functionality can be enhanced by: => configuration parameters (e.g., table driven) => extensibility at defined interfaces => custom written source code extensions

29. Design for Reuse The software design should anticipate possible changes in the system over its life-cycle. New vendor or new technology Components are replaced because its supplier goes out of business, ceases to provide adequate support, increases its price, etc., or because better software from another sources provides better functionality, support, pricing, etc. This can apply to either open-source or vendor-supplied components.

30. Design for Reuse New implementation The original implementation may be problematic, e.g., poor performance, inadequate back-up and recovery, difficult to trouble-shoot, or unable to support growth and new features added to the system. Example. The portal nsdl.org was originally implemented using uPortal. This did not support important extensions that were requested and proved awkward to maintain. It was reimplemented using PHP/MySQL.

31. Design for Reuse Additions to the requirements When a system goes into production, it is usual to reveal both weaknesses and opportunities for extra functionality and enhancement to the user interface design. For example, in a data-intensive system it is almost certain that there will be requests for extra reports and ways of viewing the data. Requests for enhancements are often the sign of a successful system. Clients recognize latent possibilities.

32. Design for Reuse Changes in the application domain Most application domains change continually, e.g., because of business opportunities, external changes (such as new laws), mergers and take-overs, new groups of users, etc., etc., It is rarely feasible to implement a completely new system when the application domain changes. Therefore existing systems must be modified. This may involve extensive restructuring.

33. Reuse and Object Oriented Languages Example: Java Java is a relatively straightforward language with a very rich set of class hierarchies. • Java programs derive much of their functionality from standard classes • Learning and understanding the classes is difficult. • Experienced Java programmers can write complex systems quickly • Inexperienced Java programmers write inelegant and buggy programs

34. Classes can be defined in terms of other classes using inheritance. The generalization class is called the superclass and the specialization is called the subclass. If the inheritance relationship serves only to model shared attributes and operations, i.e., the generalization is not intended to be implemented, the class is called an abstract class Design for Reuse: Inheritance and Abstract Classes

35. Implementation Inheritance Developers reuse code quickly by subclassing an existing class and refining its behavior. Is not good for reuse. Specification Inheritance The classification of concepts into type hierarchies, so that an object from a specified class can be replaced by an object from one of its subclasses. Design for Reuse: Implementation and Specification Inheritance

36. Liskov Substitution Principle (strict inheritance) If an object of type S can be substituted in all the places where an object of type T is expected, then S is a subtype of T. Interpretation The Liskov Substitution Principle means that if all classes are subtypes of their superclasses, all inheritance relationships are specification inheritance relationships. New subclasses of T can be added without modifying the methods of T. This leads to an extensible system. Design for Reuse: Specification Inheritance

37. Delegation A class is said to delegate to another class if it implements an operation by resending a message to another class. Delegation is an alternative to implementation inheritance that should be used when reuse is anticipated. Design for Reuse: Delegation The discussion of design for reuse draws from the book by Bruegge and Dutoit in the readings. *