1. 1 CS 501 Spring 2007 CS 501: Software Engineering Lecture 16 Object Oriented Design 2

2. 2 CS 501 Spring 2007 Administration Quiz 3: Change of date The date of Quiz 3 has been changed to Thursday, March 29. If this change of date causes schedule problems, please send email to wya@cs.cornell.edu.

3. 3 CS 501 Spring 2007 Administration Third presentation and report next week Sign up now.

4. 4 CS 501 Spring 2007 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

5. 5 CS 501 Spring 2007 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.

6. 6 CS 501 Spring 2007 State Diagram: Notation State diagram for class Book in a library system not borrowable returned() borrowable borrowed()[not last copy] borrowed()[last copy] guard expression

7. 7 CS 501 Spring 2007 Notation for Classes and Objects ClassesObjects AnyClass attribute1 attribute2 operation1() operation2() AnyClass or anObject:AnyClass :AnyClass anObject The names of objects are underlined. or

8. 8 CS 501 Spring 2007 Notation: Active Class EventManager eventlist suspend() flush() An active class is a class whose objects own one or more processes or threads and therefore can initiate control activity.

9. 9 CS 501 Spring 2007 Interaction: Bouncing Ball Diagrams Example: execution of http://www.cs.cornell.edu/ Client Server(s) domain name service TCP connection HTTP get

10. 10 CS 501 Spring 2007 Notation: Interaction display An interaction is a behavior that comprises a set of messages exchanged among a set of objects within a particular context to accomplish a specific purpose.

11. 11 CS 501 Spring 2007 Actions on Objects call return send create destroy returnCopy(c) okToBorrow()local status notifyReturn(b)asynchronous signal > stereotypes

12. 12 CS 501 Spring 2007 Sequence Diagram: Borrow Copy of a Book BookBorrower libMem: LibraryMember theCopy:Copy theBook:Book borrow(theCopy) okToBorrow borrow

13. 13 CS 501 Spring 2007 Sequence Diagram: Change in Cornell Program Cornellian :MEngStudent 1 : getName() sequence numbers added to messages :PhDStudent 1.1 : name 2: > PhDStudent(name) 3: >

14. 14 CS 501 Spring 2007 Sequence Diagram: Painting Mechanism :Thread:Toolkit:ComponentPeertarget:HelloWorld run callbackLoop handleExpose paint

15. 15 CS 501 Spring 2007 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.

16. 16 CS 501 Spring 2007 Activity Diagram: Notation Release work order Assign tasksReschedule [materials not ready] [materials ready] guard expression branch

17. 17 CS 501 Spring 2007 Activity Diagram: Parallel Activities DecompressStream audioStream video fork join start state stop state

18. 18 CS 501 Spring 2007 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.

19. 19 CS 501 Spring 2007 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

20. 20 CS 501 Spring 2007 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

21. 21 CS 501 Spring 2007 Software Reuse Examples (Tools) Standard functions mathematical methods formatting User interface toolkits (e.g. Quickdraw) class libraries, (e.g., Swing)

22. 22 CS 501 Spring 2007 Software Reuse (Application Packages) Application 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

23. 23 CS 501 Spring 2007 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.

24. 24 CS 501 Spring 2007 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.

25. 25 CS 501 Spring 2007 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.

26. 26 CS 501 Spring 2007 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.

27. 27 CS 501 Spring 2007 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

28. 28 CS 501 Spring 2007 Design for Reuse: Inheritance and Abstract Classes 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

29. 29 CS 501 Spring 2007 Design for Reuse: Implementation and Specification Inheritance 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.

30. 30 CS 501 Spring 2007 Design for Reuse: Specification Inheritance 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.

31. 31 CS 501 Spring 2007 Design for Reuse: Delegation 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. The discussion of design for reuse draws from the book by Bruegge and Dutoit in the readings. *