Nothing Stays the Same Chapter 5 HeadFirst OOAD Tom Perkins - NTPCUG.

Nothing Stays the Same

Chapter 5

HeadFirst OOAD

Tom Perkins - NTPCUG

Inventory

addGuitar(string,double,GuitarSpec) getGuitar(string):Guitar search(GuitarSpec):Guitar[*]

GuitarSpecGuitar

serialNumber:stringprice:double

getSerialNumber():stringgetPrice():doublesetPrice():doublegetSpec():GuitarSpec

TypeBuilder Wood

model:stringnumStrings:int

getBuilder():BuildergetModel():StringgetType():TypegetBackWood():WoodgetTopWood():WoodgetNumStrings():intmatches(GuitarSpec):bool

toString():string toString():string toString:string

inventory *spec 1

builder 1type 1 topWood 1

backWood 1

Let’s Sell Mandolins!

Design Decision

• Guitar and Mandolin have many shared properties

• Create an Instrument class to store common properties and methods – abstract Base class

• Need new concrete classes Mandolin and MandolinSpec to handle mandolins

• Mandolin and Guitar will become “inherited classes”, subclasses, derived classes

Inherited Classes

• A new class, retains all members and functionality of a previously defined class

• You can add new members and functionality to new class

• Derived class extends base class

• Override functionality of base class

Public Class Truck

End Class

Public Class PickupTruck

Inherits Truck

End Class

public class Truck{}

public class PickupTruck : Truck {}

Abstract Classes

• Used to provide some invariant functionality in a class

• Cannot be instantiated – cannot create an object from an abstract class

• Some members can be overridden in inherited classes

Public MustInherit MyAbstractClass

End Class

public abstract MyAbstractClass {}

Abstract Classes

• Placeholders for actual implementation classes

• The abstract class defines the behavior; the subclasses implement that behavior

Guitar

serialNumber:stringprice:double

getSerialNumber():stringgetPrice():doublesetPrice():doublegetSpec():GuitarSpec

Mandolin

getSpec():MandolinSpec

Instrument

serialNumber:stringprice:double

getSerialNumber():stringgetPrice():doublesetPrice():doublegetSpec():GuitarSpec

Abstract class

Attributes common to Mandolin and Guitar moved to Instrument

Can’t create an instance of Instrument

Abstract class defines some basic behavior

Subclasses implement behavior

GuitarSpec

builder:Buildermodel:stringtype:TypebackWood:WoodtopWood:WoodnumStrings:int

getBuilder():BuildergetModel():StringgetType():TypegetBackWood():WoodgetTopWood():WoodgetNumStrings():intmatches(GuitarSpec):bool

MandolinSpec

builder:Buildermodel:stringtype:Typestyle:StylebackWood:WoodtopWood:WoodnumStrings:int

getBuilder():BuildergetModel():StringgetType():TypegetStyle():StylegetBackWood():WoodgetTopWood():WoodgetNumStrings():intmatches(MandolinSpec):bool

Style

toString():string

Inventory

addInstrument(string,double,GuitarSpec) get(string):Guitar search(GuitarSpec):Guitar[*]Search(MandolinSpec):Mandolin[*]

Instrument

serialNumber:stringprice:double

getSerialNumber():stringgetPrice():doublesetPrice():doublegetSpec():GuitarSpec

Mandolin Guitar

Full Diagram, Part I

InstrumentSpec

model:string

getBuilder():stringgetModel():stringgetType():TypegetBackWood():WoodgetTopWood():Woodmatches(InstrumentSpec):boolean

Mandolin

getNumStrings():Intmatches(GuitarSpec):bool

Guitar

Type

Builder

Wood

toString():string

toString():string

toString:string

builder 1

type 1

topWood 1

backWood 1

getStyle():Stylematches(MandolynSpec):bool

StyletoString():string

Full Diagram, Part II

RicksGuitarsAndMandolins

Rick goes Bonkers!

Has Rick’s application become your career?

• For each added instrument, we have to create:– Instrument class– Create a new subclass for Instrument Spec

(BanjoSpec, FiddleSpec, etc)– Write a new search method for that instrument

in the Inventory class …

• “What a revolting development this is!” (William Bendix in The Life of Riley)

Some OO Principles

• “Code to an Interface, not an implementation”

• Encapsulation– Eliminate duplicate code– Encapsulate what varies (less probablity of

change)

• Cohesion – A class should do only one thing!

Toward one search Method …

public ArrayList search(GuitarSpec searchSpec) { ArrayList matchingGuitars = new ArrayList(); foreach (Instrument g in inventory) { if (g.getSpec().matches(searchSpec)) matchingGuitars.Add(g); } return matchingGuitars; } public ArrayList search(MandolinSpec searchSpec) { ArrayList matchingMandolins = new ArrayList(); foreach (Instrument g in inventory) { if (g.getSpec().matches(searchSpec)) matchingMandolins.Add(g); } return matchingMandolins; }

Try making InstrumentSpec a

concrete class

Improved (single) Search method public ArrayList search(InstrumentSpec searchSpec)

{

ArrayList matchingInstruments = new ArrayList();

foreach (Instrument g in inventory)

{

if (g.getSpec().matches(searchSpec))

matchingInstruments.Add(g);

}

return matchingInstruments;

}

See RicksConcreteInstrumentSpec.sln

Uses concrete InstrumentSpec, single search method

“What? A minor flaw in my design? Impossible!”

• It’s hard to change your own designs• Code once, look twice

– “Measure twice, saw once!”– Keep looking at your design when you run

into problems

• Peer programming• Design is iterative – not a single shot!• Major problem:

Modified DesignIndividual

InstrumentSpec

Classes

InstrumentSpec

properties:HashTablebuilder: Buildermodel:stringtype:InstrumentTypebackWood:WoodtopWood:Wood

getProperty(string):ObjectgetProperties():HashTablegetBuilder():stringgetModel():stringgetType():TypegetBackWood():WoodgetTopWood():Woodmatches(InstrumentSpec):boolean

Code Walkthru and Demo

Useful Links

• Good discussion of constructors in C#: http://www.yoda.arachsys.com/csharp/constructors.html

• Java to C# conversion – www.25hoursaday.com/CsharpVsJava.html