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CS106A,Lecture22MoreClasses

suggestedreading:JavaCh.6

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Learning Goals• Knowhowtodefineourownvariabletypes• Knowhowtodefinevariabletypesthatinheritfromothertypes• Beabletowriteprogramsconsistingofmultipleclasses

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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What Is A Class?

Aclassdefinesanewvariabletype.

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Classes Are Like BlueprintsiPodblueprint(class)

state:currentsongvolumebatterylifebehavior:poweron/offchangestation/songchangevolumechooserandomsong

iPod(variable)#1state:song="1,000,000Miles"volume=17batterylife=2.5hrsbehavior:poweron/offchangestation/songchangevolumechooserandomsong

iPod(variable)#2state:song="LettingYou"volume=9batterylife=3.41hrsbehavior:poweron/offchangestation/songchangevolumechooserandomsong

iPod(variable)#3state:song="Discipline"volume=24batterylife=1.8hrsbehavior:poweron/offchangestation/songchangevolumechooserandomsong

constructs

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What if…Whatifwecouldwriteaprogramlikethis:

BankAccount nickAccount = new BankAccount();nickAccount.setName(“Nick”);nickAccount.deposit(50);

BankAccount rishiAccount = new BankAccount();rishiAccount.setName(“Rishi”);rishiAccount.deposit(50);boolean success = rishiAccount.withdraw(10);if (success) {

println(“Rishi withdrew $10.”);}

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Creating A New Class1. Whatinformationisinsidethisnewvariabletype? Theseareitsprivateinstancevariables.

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Example: BankAccount// In file BankAccount.javapublic class BankAccount {

// Step 1: the data inside a BankAccountprivate String name;private double balance;

}

Each BankAccount object has its own copyof all instance variables.

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Creating A New Class1. Whatinformationisinsidethisnewvariabletype? Theseareitsinstancevariables.

2. Whatcanthisnewvariabletypedo?Theseareitspublicmethods.

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Example: BankAccountpublic class BankAccount {

// Step 1: the data inside a BankAccountprivate String name;private double balance;

// Step 2: the things a BankAccount can dopublic void deposit(double amount) {

balance += amount;}

public boolean withdraw(double amount) {if (balance >= amount) {

balance -= amount;return true;

}return false;

}}

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Defining Methods In ClassesMethodsdefinedinclassescanbecalledonaninstanceofthatclass.

Whenoneofthesemethodsexecutes,itcanreferencethatobject’scopy ofinstancevariables.

ba1.deposit(0.20);ba2.deposit(1000.00);

Thismeanscallingoneofthesemethodsondifferentobjectshasdifferenteffects.

name = "Marty"balance = 1.45deposit(amount) {

balance += amount;}

name = "Mehran"balance = 901000.00deposit(amount) {

balance += amount;}

ba1

ba2

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Getters and SettersInstancevariablesinaclassshouldalwaysbeprivate.Thisissoonlytheobjectitselfcanmodifythem,andno-oneelse.

Toallowtheclienttoreferencethem,wedefinepublicmethodsintheclassthatset aninstancevariable’svalueandget (return)aninstancevariable’svalue.Thesearecommonlyknownasgetters andsetters.

account.setName(“Nick”);String accountName = account.getName();

Gettersandsetterspreventinstancevariablesfrombeingtamperedwith.

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Example: BankAccountpublic class BankAccount {

private String name;private double balance;

...public void setName(String newName) {

if (newName.length() > 0) {name = newName;

}}

public String getName() {return name;

}}

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Creating A New Class1. Whatinformationisinsidethisnewvariabletype? Theseareitsinstancevariables.

2. Whatcanthisnewvariabletypedo?Theseareitspublicmethods.

3. Howdoyoucreateavariableofthistype?Thisistheconstructor.

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ConstructorsBankAccount ba1 = new BankAccount();

BankAccount ba2 = new BankAccount(“Nick”, 50);

The constructor is executed when a new object is created.

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Example: BankAccountpublic class BankAccount {// Step 1: the data inside a BankAccountprivate String name;private double balance;

// Step 2: the things a BankAccount can do (omitted)// Step 3: how to create a BankAccountpublic BankAccount(String accountName, double startBalance) {

name = accountName;balance = startBalance;

}

public BankAccount(String accountName) {name = accountName;balance = 0;

}}

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Using ConstructorsBankAccount ba1 =

new BankAccount("Marty", 1.25);

BankAccount ba2 =new BankAccount("Mehran", 900000.00);

• Whenyoucallaconstructor(withnew):– Javacreatesanewobjectofthatclass.– Theconstructorruns,onthatnewobject.– Thenewlycreatedobjectisreturnedtoyourprogram.

name = "Marty"balance = 1.25BankAccount(nm, bal) {

name = nm;balance = bal;

}

name = "Mehran"balance = 900000.00BankAccount(nm, bal) {

name = nm;balance = bal;

}

ba1

ba2

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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Printing Variables• Bydefault,Javadoesn'tknowhowtoprintobjects.

// ba1 is BankAccount@9e8c34BankAccount ba1 = new BankAccount("Marty", 1.25);println("ba1 is " + ba1);

// better, but cumbersome to write// ba1 is Marty with $1.25println("ba1 is " + ba1.getName() + " with $"

+ ba1.getBalance());

// desired behaviorprintln("ba1 is " + ba1); // ba1 is Marty with $1.25

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The toString MethodAspecialmethodinaclassthattellsJavahowtoconvertanobjectintoastring.

BankAccount ba1 = new BankAccount("Marty", 1.25);println("ba1 is " + ba1);

// the above code is really calling the following:println("ba1 is " + ba1.toString());

• EveryclasshasatoString,evenifitisn'tinyourcode.– Default:class'sname@ object'smemoryaddress(base16)

BankAccount@9e8c34

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The toString Methodpublic String toString() {

code that returns a Stringrepresenting this object;

}

– Methodname,return,andparametersmustmatchexactly.

– Example:// Returns a String representing this account.public String toString() {

return name + " has $" + balance;}

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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The “this” Keywordthis:Referstotheobjectonwhichamethodiscurrentlybeingcalled

BankAccount ba1 = new BankAccount();ba1.deposit(5);

// in BankAccount.javapublic void deposit(double amount) {

// for code above, “this” -> ba1...

}

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Using “this”Sometimeswewanttonameparametersthesameasinstancevariables.

public class BankAccount {private double balance;private String name;...

public void setName(String newName) {name = newName;

}}

– Here,theparametertosetName isnamednewName tobedistinctfromtheobject'sfieldname .

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Using “this”public class BankAccount {

private double balance;private String name;...

public void setName(String name) {name = name;

}}

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Using “this”Wecanuse“this”tospecifywhichoneistheinstancevariableandwhichoneisthelocalvariable.

public class BankAccount {private double balance;private String name;...

public void setName(String name) {this.name = name;

}}

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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Practice: EmployeeLet’sdefineanewvariabletypecalledEmployeethatrepresentsasingleEmployee.

WhatinformationwouldanEmployeestore?

WhatcouldanEmployeedo?

HowwouldyoucreateanewEmployeevariable?

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Plan for today•Recap:Classes•toString•this•Example: Employee•Inheritance

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Inheritance

Inheritanceletsusrelateourvariable

typestooneanother.

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Inheritance

Employee

Programmer

KarelProgrammer

Variabletypescanseemto“inherit”fromeachother.Wedon’twanttohavetoduplicatecodeforeachone!

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Example: GObjects• TheStanfordlibraryusesaninheritancehierarchyofgraphicalobjectsbasedonacommonsuperclassnamedGObject.

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Example: GObjects• GObject definesthestateandbehaviorcommontoallshapes:

contains(x, y)getColor(), setColor(color)getHeight(), getWidth(), getLocation(), setLocation(x, y)getX(), getY(), setX(x), setY(y), move(dx, dy)setVisible(visible), sendForward(), sendBackward()toString()

• Thesubclassesaddstateandbehavioruniquetothem:GLabel GLine GPolygon

get/setFont get/setStartPoint addEdgeget/setLabel get/setEndPoint addVertex

get/setFillColor... ... ..

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Using Inheritancepublic class Name extends Superclass {

– Example:

public class Programmer extends Employee {...

}

• ByextendingEmployee,thistellsJavathatProgrammer candoeverythinganEmployeecando,plusmore.

• ProgrammerautomaticallyinheritsallofthecodefromEmployee!• ThesuperclassisEmployee,thesubclass isProgrammer.

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Example: Programmerpublic class Programmer extends Employee {

private int timeCoding;...public void code() {

timeCoding += 10;}

}

...

Programmer rishi = new Programmer(“Rishi”);rishi.code(); // from Programmerrishi.promote(); // from Employee!

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Example: KarelProgrammerpublic class KarelProgrammer extends Programmer {

private int numBeepersPicked;...public void pickBeepers() {

numBeepersPicked += 2;}

}

...KarelProgrammer nick = new KarelProgrammer(“Nick”);nick.pickBeepers(); // from KarelProgrammernick.code(); // from Programmer!nick.promote(); // From Employee!

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GCanvas• AGCanvas isthecanvasareathatdisplaysallgraphicalobjectsinaGraphicsProgram.

• WhenyoucreateaGraphicsProgram,itautomaticallycreatesaGCanvas foritself,putsitonthescreen,andusesittoaddallgraphicalshapes.

• GCanvas istheonethatcontainsmethodslike:– getElementAt– add– remove– getWidth– getHeight– …

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GCanvaspublic class Graphics extends GraphicsProgram {

public void run() {// A GCanvas has been created for us!GRect rect = new GRect(50, 50);add(rect); // adds to the GCanvas!

...// Checks our GCanvas for elements!GObject obj = getElementAt(25, 25);

}}

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Extending GCanvaspublic class Graphics extends Program {

public void run() {// We have to make our own GCanvas nowMyCanvas canvas = new MyCanvas();add(canvas);

// Can’t do this anymore, because we are // not using GraphicsProgram’s provided // canvasGObject obj = getElementAt(…);

}}

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Extending GCanvaspublic class MyCanvas extends GCanvas {

public void addCenteredSquare(int size) {GRect rect = new GRect(size, size);int x = getWidth() / 2.0 –

rect.getWidth() / 2.0;int y = getHeight() / 2.0 –

rect.getHeight() / 2.0;add(rect, x, y);

}}

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Extending GCanvaspublic class Graphics extends Program {

public void run() {// We have to make our own GCanvas nowMyCanvas canvas = new MyCanvas();add(canvas);

canvas.addCenteredSquare(20);}

}

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Extending GCanvas•Sometimes,wewanttobeabletohaveallofourgraphics-relatedcodeinaseparatefile.

•Todothis,insteadofusingtheprovidedGraphicsProgram canvas,wedefineourownsubclassofGCanvas, haveourprogramextendProgram,andaddourowncanvasourselves.

•Then,allgraphics-relatedcodecangoinourGCanvas subclass.

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Recap•Classesletusdefineourownvariabletypes,withtheirowninstancevariables,methodsandconstructors.

•Wecanrelate ourvariabletypestooneanotherbyusinginheritance.Oneclasscanextend anothertoinherititsbehavior.

•WecanextendGCanvas inagraphicalprogramtodecomposeallofourgraphics-relatedcodeinoneplace.

Nexttime: InteractorsandGUIs