Chapter 2: Designing Chapter 2: Designing SoftwareSoftware

Why design?Why design?

• Every program is designedEvery program is designed– Either by intent or by defaultEither by intent or by default

• Two programs that do the same thingTwo programs that do the same thing– May have big differences inMay have big differences in

• ComplexityComplexity

• SpeedSpeed

• EfficiencyEfficiency

Why design?Why design?

• Every time you write a program, big Every time you write a program, big or small, you are a designer.or small, you are a designer.– You select variables, names, scope, You select variables, names, scope,

choose to use functions, objects, choose to use functions, objects, interface, etc.interface, etc.

• There are good and bad ways to There are good and bad ways to design programsdesign programs

Software design goalsSoftware design goals• UsabilityUsability

– it solves problems people want to solveit solves problems people want to solve

• ReliabilityReliability– It solves them correctly, without crashingIt solves them correctly, without crashing

• MaintainabilityMaintainability– Problems can be easily correctedProblems can be easily corrected

• ReusabilityReusability– Reduces cost, increases reliabilityReduces cost, increases reliability

Main approaches to software Main approaches to software designdesign

• Top-down design (TDD)Top-down design (TDD)– Breaks problems down into smaller Breaks problems down into smaller

onesones

• Object-oriented design (OOD)Object-oriented design (OOD)– Breaks problems down into objectsBreaks problems down into objects

Top-Down Design (TDD)Top-Down Design (TDD)

• Historically the most common formHistorically the most common form

• StagesStages– Problem statementProblem statement

– DecompositionDecomposition• Structure chartStructure chart

– RefinementRefinement• Separate algorithms for each moduleSeparate algorithms for each module

Example: Video rental Example: Video rental systemsystem

• Build a software system to support the operation of a Build a software system to support the operation of a video rental system. video rental system.

• The system should automate the process of renting The system should automate the process of renting tapes and receiving returned tapes, including tapes and receiving returned tapes, including calculating and printing patron bills, which may or may calculating and printing patron bills, which may or may not be done at the same time the tape is returned. not be done at the same time the tape is returned.

• It must also give the clerk access to information about It must also give the clerk access to information about the tapes, such as the number of copies on the shelf of the tapes, such as the number of copies on the shelf of any given video owned by the store. any given video owned by the store.

• It must be able to add new customers and tapes to and It must be able to add new customers and tapes to and from the database. Each patron and each copy of each from the database. Each patron and each copy of each tape are to be associated with a unique bar-coded tape are to be associated with a unique bar-coded label.label.

Structure chart for Structure chart for initial decomposition of initial decomposition of video rental systemvideo rental system

Video Rental System

Process Transactions

Process Queries

Process Modifications

Structure chart for “process transactions” Structure chart for “process transactions” refinementrefinement

Process Transactions

Rent Tape

Query Tape

Validate Patron

Update Tape

Update Patron

Query Tape

Validate Patron

Compute Bill

Update Tape

Update Patron

Update Patron

Return Tape

Pay Bill

Final decomposition of video rental systemFinal decomposition of video rental system

Process Transactions

Rent Tape

Query Tape

Validate Patron

Update Tape

Update Patron

Query Tape

Validate Patron

Compute Bill

Update Tape

Update Patron

Update Patron

Return Tape

Pay Bill

Video Rental System

Process Queries

Process Modifications

Tape Queries

Patron Queries

Add Patron

Delete Patron

Add Tape

Delete Tape

Criteria for good TDDCriteria for good TDD

• CohesionCohesion– Every part fits naturally in its sectionEvery part fits naturally in its section

• CouplingCoupling– Interconnectedness between separate Interconnectedness between separate

piecespieces

– Avoid interconnections if possibleAvoid interconnections if possible

Object Oriented Design Object Oriented Design (OOD)(OOD)

• MethodologyMethodology– Techniques for creating a designTechniques for creating a design

– Use the requirements specification to Use the requirements specification to find the objectsfind the objects in the system in the system

– Then describe the objects in a formal Then describe the objects in a formal wayway

• C++ provides an ideal languageC++ provides an ideal language

Example: Video Rental Example: Video Rental SystemSystem

• Start by looking at the real world items it Start by looking at the real world items it manipulatesmanipulates

• These are easily identifiable as ‘objects’These are easily identifiable as ‘objects’

• Other things that are ‘objects’Other things that are ‘objects’– Anything that is capable of ‘doing things’Anything that is capable of ‘doing things’

– Example: Date Example: Date • It can increase, decrease, be expressed many It can increase, decrease, be expressed many

waysways

Example: Video rental Example: Video rental systemsystem

• Build a software system to support the operation of a Build a software system to support the operation of a video rental system. video rental system.

• The system should automate the process of renting The system should automate the process of renting videovideo tapestapes and receiving returned tapes, including and receiving returned tapes, including calculating and printing patron calculating and printing patron billsbills, which may or may , which may or may not be done at the same time the tape is returned. not be done at the same time the tape is returned.

• It must also give the It must also give the clerkclerk access to information about access to information about the tapes, such as the number of copies on the shelf of the tapes, such as the number of copies on the shelf of any given video owned by the store. any given video owned by the store.

• It must be able to add new It must be able to add new customerscustomers and tapes to and and tapes to and from the database. Each patron and each copy of each from the database. Each patron and each copy of each tape are to be associated with a unique bar-coded .tape are to be associated with a unique bar-coded .

ClassesClasses

• Each type of object is called a classEach type of object is called a class

• When we ‘find the objects’ we are When we ‘find the objects’ we are really finding the classesreally finding the classes

• ExampleExample– Tape class Tape class

– Patron classPatron class

Finding classesFinding classes• One method of finding classes is to develop One method of finding classes is to develop

scenarios that walk through the problem.scenarios that walk through the problem.

• This allows us to identify objects at every step This allows us to identify objects at every step in the process.in the process.

• Example: for class Date we might considerExample: for class Date we might consider– Video rental dateVideo rental date

– Video due dateVideo due date

Video example classesVideo example classes• Identify these by walking through the scenario the Identify these by walking through the scenario the

program is to mimicprogram is to mimic

• The main actors areThe main actors are– The customerThe customer

– The video tapeThe video tape

– The transaction processThe transaction process• The consoleThe console

• The scannerThe scanner

• The printerThe printer

The video rental consoleThe video rental console

Responsibility-driven Responsibility-driven designdesign

• The responsibilities of a class areThe responsibilities of a class are– the information it maintains (data the information it maintains (data

state)state)

– the actions it performs (operations)the actions it performs (operations)

• Classes also must work togetherClasses also must work together– they collaborate with other classesthey collaborate with other classes

OOD ProceduresOOD Procedures

• Find the classesFind the classes

• Determine the responsibilities of each Determine the responsibilities of each classclass

• Determine who collaborates with each Determine who collaborates with each classclass

OOD Procedures (con’t)OOD Procedures (con’t)

• Often you realize the need for new Often you realize the need for new classes while describing current classes while describing current onesones

• If you have a class with no If you have a class with no responsibilities you discard it.responsibilities you discard it.

Video Rental System Video Rental System ClassesClasses

• TapeTape

• PatronPatron

• ConsoleConsole

• ScannerScanner

• PrinterPrinter

Class responsibilities Class responsibilities (tape)(tape)

• Class TapeClass Tape– Keep track of tape identification dataKeep track of tape identification data

– Check self outCheck self out

– Check self inCheck self in

– Answer queries about it’s locationAnswer queries about it’s location

Tape classTape class

Class Responsibilities

Tape Keep track of tape identification dataCheck self outCheck self inAnswer queries about location of self

Responsibilities (patron)Responsibilities (patron)

• Class patronClass patron– Keep track of patron identification Keep track of patron identification

numbernumber

– Update patron identification dataUpdate patron identification data

– Update list of rented tapesUpdate list of rented tapes

– Update billing informationUpdate billing information

Patron classPatron class

Class Responsibilities

Patron Keep track of patron identification dataUpdate patron identification dataUpdate list of rented tapesUpdate billing information

Responsibilities (console)Responsibilities (console)

• ConsoleConsole– Check tapes in and outCheck tapes in and out

– Find the location of tapesFind the location of tapes

– Add and remove tapesAdd and remove tapes

– Add, update and remove patronsAdd, update and remove patrons

– Update patron billing informationUpdate patron billing information

Console classConsole class

Class Responsibilities

Console Check tapes in and outFind the location of tapesFind out about patronsAdd and remove tapesAdd, update, and remove patronsUpdate patron billing information

Responsibilities (scanner)Responsibilities (scanner)

• ScannerScanner– Read a bar code and return either a Read a bar code and return either a

tape or a patrontape or a patron

Scanner classScanner class

Class Responsibilities

Scanner Read a bar code and return either a tapeor a patron

Responsibilities (printer)Responsibilities (printer)

• PrinterPrinter– Print billsPrint bills

– Print receiptsPrint receipts

Printer classPrinter class

Class Responsibilities

Printer Print billsPrint receipts

CollaboratorsCollaborators

• After all responsibilities have been After all responsibilities have been listed we can turn our attention to listed we can turn our attention to collaborationscollaborations

• There is a collaboration whenever There is a collaboration whenever one class must get information one class must get information from, or give it to another classfrom, or give it to another class

Problems with these Problems with these classes?classes?

• Class console may be doing too muchClass console may be doing too much– Should we break it up?Should we break it up?

• Class checkoutClass checkout

• Class checkin, etc.Class checkin, etc.

• Class scanner does only one thing?Class scanner does only one thing?

• Designs need to be debugged before Designs need to be debugged before being implementedbeing implemented

Which is better,TDD or Which is better,TDD or OOD?OOD?

• TDD is extensively usedTDD is extensively used– Inertia combined with hierarchical thinkingInertia combined with hierarchical thinking

• OOD provides features TDD does notOOD provides features TDD does not– Most problems are better modeled by Most problems are better modeled by

objects with data (a state) and behavior objects with data (a state) and behavior (operations)(operations)

– It takes work to truly think in an object It takes work to truly think in an object oriented modeoriented mode

Video rental system: Video rental system: Classes, responsibilities, and collaborators (Part 1 Classes, responsibilities, and collaborators (Part 1 of 2)of 2)

Class Responsibilities Collaborators

Tape Keep track of tape identification dataCheck self inCheck self outAnswer queries about location of self

Patron Keep track of patron identification dataUpdate patron identification dataUpdate list of rented tapesUpdate billing information

Tape

Console Check tapes in and outFind the location of tapesFind out about patronsAdd and remove tapesAdd, update, and remove patronsUpdate patron billing information

PatronTapeScannerPrinter

Table 2-6: Video rental system: Table 2-6: Video rental system: Classes, responsibilities, and collaborators (Part 2 Classes, responsibilities, and collaborators (Part 2 of 2)of 2)

Class Responsibilities Collaborators

Scanner Read a bar code and returneither a tape or a patron. (Thissuggests that a database thatlinks bar codes to tapes andpatrons is maintained by theScanner class.)

TapePatron

Printer Print billsPrint receipts

TapePatron