cs 106a, lecture 23 gcanvasand interactorsweb.stanford.edu/class/archive/cs/cs106a/cs106a... · 1....

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CS106A,Lecture23GCanvas andInteractors

suggestedreading:JavaCh.10.5-10.6

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Plan for today•Recap:Inheritance•ExtendingGCanvas•Example:Aquarium•Interactors–JButton–JLabel–JTextField

•Example:TipCalculator

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Inheritance

Inheritanceletsusrelateourvariable

typestooneanother.

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Inheritance

Employee

Programmer

KarelProgrammer

Variabletypescanseemto“inherit”fromeachother.Wedon’twanttohavetoduplicatecodeforeachone!

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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– 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);

// Operate on this canvasGObject obj = canvas.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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The init method• init isaspecialpublicmethod,likerun,thatiscalledwhenyourprogramisbeinginitialized.

• Unlikerun,however,itiscalledbefore yourprogramlaunches,lettingyoudoanyinitializationyouneed.

public class MyProgram extends GraphicsProgram {public void init() {

// executed before program launches}

public void run() {// executed after program launches

}}

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The init method• init istypicallyusedtoinitializegraphicalcomponents,suchasaddingacustomGCanvas tothescreen.

public class MyProgram extends Program {private MyCanvas canvas;public void init() {

canvas = new MyCanvas();add(canvas);

}

public void run() {canvas.addCenteredSquare(20);

}}

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Common Bugs• Whenyouareusingacustomcanvas,makesuretonotcallgetWidth orgetHeight onthecanvasuntilitisshownonscreen!

public class MyProgram extends Program {private MyCanvas canvas;public void init() {

// canvas not created yet!canvas = new MyCanvas();// canvas not added yet!add(canvas);// window not showing yet!

}

public void run() {// good to go

}}

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Example: Aquarium• Let’swriteagraphicalprogramcalledAquarium thatsimulatesfishswimmingaround.

• Todecomposeourcode,wecanmakeourownGCanvas subclass.

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Interactive ProgramsSofar,wehavelearnedabouttwowaysofmakinginteractiveprograms:• ReadinguserinputinConsolePrograms•DetectingmouseeventsinGraphicsPrograms

Today,we’regoingtolearnaboutathird:interactors.

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Interactors

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Interactors

JComponent

JButton

JTextFieldJLabel

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JButton

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JButtonimport java.awt.event.*;import javax.swing.*;

JButton button = new JButton(“Press me”);add(button, SOUTH);

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Window Regions• Ingraphicsorconsoleprograms,thewindowisdividedintofiveregions:

• TheCENTER regionistypicallywheretheactionhappens.– ConsoleProgram addsaconsolethere– GraphicsProgram putsaGCanvas there

• Otherregionsarevisibleonlyifyouaddaninteractortothemusingadd(component, REGION);

• Interactorsareautomaticallycenteredwithineachregion.

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Responding To Button ClicksTorespondtoeventsfrominteractors,wemustdothefollowing:1. CalladdActionListeners() attheendofinit,onceweare

doneaddingbuttons.ThistellsJavatoletusknowifanyofthepreviousbuttonswereclicked.

2. ImplementthepublicactionPerformedmethod.Thismethodiscalledwheneverabuttonisclicked.

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JButton Examplepublic class Interactors extends ConsoleProgram {

public void init() {JButton yayButton = new JButton("Yay");add(yayButton, SOUTH);JButton nayButton = new JButton("Nay");add(nayButton, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {... // ?

}}

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ActionEvent• TheActionEvent parametercontainsusefuleventinformation.

– UsegetSource orgetActionCommand tofigureoutwhatbuttonorcomponentwasinteractedwith.

public void actionPerformed(ActionEvent event) {String command = event.getActionCommand();if (command.equals("Save File")) {

// user clicked the Save File button...

}}

Method Descriptione.getActionCommand() atextdescriptionoftheevent

(e.g.thetextofthebuttonclicked)e.getSource() theinteractorthatgeneratedtheevent

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JButton Example

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private JButton yayButton;private JButton nayButton;public void init() {

yayButton = new JButton("Yay");add(yayButton, SOUTH);nayButton = new JButton("Nay");add(nayButton, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {if (event.getSource() == yayButton) {

println("Yay");} else if (event.getSource() == nayButton) {

println("Nay");}

}}

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}



println("Nay");}

}}

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}



println("Nay");}

}}

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}



println("Nay");}

}}

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}



println("Nay");}

}}

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}



println("Nay");}

}}

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JButton Example #2public class Interactors extends ConsoleProgram {


JButton yayButton = new JButton("Yay");add(yayButton, SOUTH);JButton nayButton = new JButton("Nay");add(nayButton, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {if (event.getActionCommand().equals("Yay")) {

println("Yay");} else if (event.getActionCommand().equals("Nay")) {

println("Nay");}

}}

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JButton Example #2public class Interactors extends ConsoleProgram {


JButton yayButton = new JButton("Yay");add(yayButton, SOUTH);JButton nayButton = new JButton("Nay");add(nayButton, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {println(event.getActionCommand());

}}

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JLabel

JLabel label = new JLabel("Hello, world!");add(label, SOUTH);

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JTextField

JTextField field = new JTextField(10);add(field, SOUTH);

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JTextField

JTextField field = new JTextField(10);add(field, SOUTH);

// Set the text in the text fieldfield.setText("Hello!");

// Get the text currently in the text fieldString text = field.getText();

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JTextField Example

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JTextField Examplepublic class Interactors extends ConsoleProgram {

private JTextField textField;public void init() {

textField = new JTextField(10);add(textField, SOUTH);JButton goButton = new JButton("Go");add(goButton, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {println(textField.getText());

}}

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Detecting ENTER PressedDetectingtheENTERkeypressedinaJTextField requiresextrawork.

JTextField field = new JTextField(10);// Tells Java to listen for ENTER on the text fieldfield.addActionListener(this);// Sets the action command (like JButtons) to “Go”field.setActionCommand("Go");add(field, SOUTH);

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Detecting ENTER PressedDetectingtheENTERkeypressedinaJTextField requiresextrawork.

JTextField field = new JTextField(10);field.addActionListener(this);field.setActionCommand("Go");add(field, SOUTH);

...public void actionPerformed(ActionEvent event) {

if (event.getActionCommand().equals(“Go”)) {...

}}

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getActionCommand

Oftentimes,atextfieldhasa“corresponding”buttonthattakesactionwiththeenteredtext.Ifwesetthetextfield’sactioncommandtobethesame asitscorrespondingbutton,wecancheckfor

bothaclickandENTERatonce!

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getActionCommandpublic void init() {

JButton button = new JButton("Go");add(button, SOUTH);JTextField field = new JTextField(10);field.addActionListener(this);field.setActionCommand("Go");add(field, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {if (event.getActionCommand().equals(“Go”)) {

...}

}

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getActionCommandpublic void init() {

JButton button = new JButton("Go");add(button, SOUTH);JTextField field = new JTextField(10);field.addActionListener(this);field.setActionCommand("Go");add(field, SOUTH);addActionListeners();

}

public void actionPerformed(ActionEvent event) {if (event.getActionCommand().equals(“Go”)) {

...}

}

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Practice: TipCalculatorLet’swriteaprogramcalledTipCalculator thatusesinteractorstocalculatethetipforabill.

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Practice: TipCalculatorLet’swriteaprogramcalledTipCalculator thatusesinteractorstocalculatethetipforabill.• Theprogramshouldcalculatetheappropriatetipdependingonthebuttontheuserclickson

• Theconsoleshouldclearwhenanewtipiscalculated(hint:useclearConsole()).

• ConvertastringintoadoubleusingDouble.parseDouble(str);

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Recap•Recap:Inheritance•ExtendingGCanvas•Example:Aquarium•Interactors:JButton,JLabel,JTextField•Example:TipCalculator

Nexttime:NameSurfer overview

cs 106a, lecture 23 gcanvasand interactorsweb.stanford.edu/class/archive/cs/cs106a/cs106a... · 1....

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