cs 106a, lecture 15 events and memory -...

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CS106A,Lecture15EventsandMemory

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GraphicsPrograms

Animation

HW4:Breakout

Youarehere

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Plan for Today•Announcements•Review:Null,EventsandInstanceVariables•ExtraPractice:Googley Eyes•ABooleanAside•Memory•RevisitingWhack-A-Mole

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NullNullisaspecialvariablevaluethatobjects canhavethatmeans“nothing”.Primitives cannotbenull.

Ifamethodreturnsanobject,itcanreturnnull tosignify“nothing”.(justsayreturn null;)

// may be a GObject, or null if nothing at (x, y)GObject maybeAnObject = getElementAt(x, y);

Objectshavethevaluenull beforebeinginitialized.

Scanner myScanner; // initially null

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NullYoucancheckifsomethingisnullusing==and!=.

// may be a GObject, or null if nothing at (x, y)GObject maybeAnObject = getElementAt(x, y);if (maybeAnObject != null) {

// do something with maybeAnObject} else {

// null – nothing at that location}

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NullCallingmethodsonanobjectthatisnull willcrashyourprogram!

// may be a GObject, or null if nothing at (x, y)GObject maybeAnObject = getElementAt(x, y);if (maybeAnObject != null) {

int x = maybeAnObject.getX(); // OK} else {

int x = maybeAnObject.getX(); // CRASH!}

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Events•event:Someexternalstimulusthatyourprogramcanrespondto.

•event-drivenprogramming:Acodingstyle(commoningraphicalprograms)whereyourcodeisexecutedinresponsetouserevents.

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Events

public void run() {// Java runs this when program launches

}

public void mouseClicked(MouseEvent event) {// Java runs this when mouse is clicked

}

public void mouseMoved(MouseEvent event) {// Java runs this when mouse is moved

}

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Types of Mouse Events• Therearemanydifferenttypesofmouseevents.

– Eachtakestheform:public void eventMethodName(MouseEvent event) { ...

Method DescriptionmouseMoved mousecursormovesmouseDragged mousecursormoveswhilebuttonishelddownmousePressed mousebuttonispresseddownmouseReleased mousebuttonisliftedupmouseClicked mousebuttonispressedandthenreleasedmouseEntered mousecursorentersyourprogram'swindowmouseExited mousecursorleavesyourprogram'swindow

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MouseEvent Objects

•AMouseEvent containsinformationabouttheeventthatjustoccurred:

Method Descriptione.getX() thex-coordinateofmousecursorinthewindowe.getY() they-coordinateofmousecursorinthewindow

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Example: ClickForDaisies

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Example: Doodler

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Doodlerpublic void mouseDragged(MouseEvent event) {

double mouseX = event.getX();double mouseY = event.getY();double rectX = mouseX – SIZE / 2.0;double rectY = mouseY – SIZE / 2.0;GRect rect = new GRect(rectX, rectY, SIZE, SIZE);rect.setFilled(true);add(rect);

}

Whatifwewantedthesame GRect totrackthemouse,insteadofmakinganewoneeachtime?

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Instance Variablesprivate type name; // declared outside of any method

• Instancevariable:Avariablethatlivesoutsideofanymethod.– Thescope ofaninstancevariableisthroughoutanentirefile(class).

– Usefulfordatathatmustpersistthroughouttheprogram,orthatcannotbestoredaslocalvariablesorparameters(eventhandlers).

– Itisbadstyletooveruseinstancevariables

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Example: MouseTracker

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Whack-A-MoleWeusedinstancevariablesandeventstomakeWhack-A-Mole!• Amoleshouldappeareverysecondatarandomlocation,andstoponcetheuserhasgottenatleast10points.

• Iftheuserclicksamole,removeitandincreasetheirscoreby1• ThereshouldbeaGLabel intheleftcornershowingtheirscore

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Exception• Iftheuserclicksanareawithnomole,theprogramcrashes.

– AprogramcrashinJavaiscalledanexception.– Whenyougetanexception,Eclipseshowsrederrortext.– Theerrortextshowsthelinenumberwheretheerroroccurred.– Whydidthiserrorhappen?– Howcanweavoidthis?

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Whack-A-Null?public void mouseClicked(MouseEvent event) {

double mouseX = event.getX();double mouseY = event.getY();GObject mole = getElementAt(mouseX, mouseY);

remove(mole);score++;scoreLabel.setText("Score: " + score);

}

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Whack-A-Null?public void mouseClicked(MouseEvent event) {


remove(mole);score++;scoreLabel.setText("Score: " + score);

}

Problem:molemaybenulliftheuserdoesn’tclickonamole!RemovingnullwillcrashL

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Whack-A-Mole!public void mouseClicked(MouseEvent event) {


if (mole != null) {remove(mole);score++;scoreLabel.setText("Score: " + score);

}}

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Revisiting DribbleCastleWewroteaprogramcalledDribbleCastle thatsimulateswetsandfallingandformingcoolshapes.

Butweleftoutonepart:howdowestoptheanimationifonedribblecollideswithanother?

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DribbleCastleprivate void dropOneDribble() {

GOval dribble = makeDribble();add(dribble);

while (!hasHitBottom(dribble) && !hasHitSomethingElse(dribble)) {

dribble.move(0, Y_VELOCITY);pause(PAUSE_TIME);

}}

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}}

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}}

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hasHitSomethingElseprivate boolean hasHitSomethingElse(GOval dribble) {double checkX = dribble.getX()

+ dribble.getWidth() / 2.0;double checkY = dribble.getY()

+ dribble.getHeight() + 1;GObject hit = getElementAt(checkX, checkY);return hit != null;

}

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hasHitSomethingElseprivate boolean hasHitSomethingElse(GOval dribble) {double checkX = dribble.getX()

+ dribble.getWidth() / 2.0;double checkY = dribble.getY()

+ dribble.getHeight() + 1;GObject hit = getElementAt(checkX, checkY);return hit != null;

} NeedtocheckoutsideoftheGOval itself!

Otherwise,getElementAt mayreturnthedribble.

Checkifsomethingishere

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Extra Practice: GoogleyEyesLet’swriteaprogramcalledGoogleyEyes thatdrawseyeswhosepupilsfollowtheuser’smouseverticallyasitmovesaround.

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A Boolean AsideThereisonehelpfulpropertyofhowJavaevaluatesbooleanexpressions,calledshort-circuitevaluation.

Whenevaluatingboolean expressions,Javaonlyevaluatesasmuchoftheexpressionasitneedstoinordertoevaluateittobetrueorfalse.

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A Boolean AsideString str = readLine("? ");

if (str.length() > 0 && str.charAt(0) == 'A') {...

}

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A Boolean AsideString str = readLine("? "); // what about “”!


}

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}

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}

Javaonlyexecutesthisifthefirstpartistrue!Thismeansitnevercrashes.

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A Boolean AsideGObject obj = getElementAt(x, y);

if (obj == null || obj.getX() == 0) {...

}

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A Boolean AsideGObject obj = getElementAt(x, y); // what about null!


}

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}

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}

Javaonlyexecutesthisifthefirstpartisfalse!Thismeansitnevercrashes.

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By Chris Piech

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By Chris PiechNick Troccoli

origin


Chapter 1: Birth


Once upon a time…

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…a variable x was born!

int x;

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…a variable x was born!

int x;

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x was a primitive variable…

int x;Aww…!

It’ssocuuuute!

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…and its parents loved it very much.

int x;Weshouldgiveit….value27!

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x = 27;Weshouldgiveit….value27!

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A few years later, the parents decided to have another variable.

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…and a variable rect was born!

GRect rect;

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rect was an object variable…

GRect rect;Who’sacute

GRect???It’ssosquare!

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GRect rect;Weshould

makeit….abig,strongGRect!

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GRect rect = new Grect(0, 0, 50, 50);

Weshouldmakeit….abig,strongGRect!

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Thatboxisn’tbigenoughto

storeeverything

aboutaGRect!

…but rect’s box was not big enough for an object!

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…so they stored the information in a bigger box somewhere else.


Seelocation

5

x=0,y=0width=50height=50...


Chapter 2: Friends

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…x makes a new friend!

int y = 27;

27 27x y

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int y = 27;

27 27x y

Hi!I’my.

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int y = 27;

27 27x y

Wehavethesamevalue!

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int y = 27;

27 27x y

*blush*

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They can use == to compare values.

if (x == y) { // true!...

}

27x y

27

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They can use == to compare values.

if (x == y) { // true!...

}

27x y

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❤


See “A Variable Love Story” for more...

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rect also makes a new friend!

GRect r = new Grect(0, 0, 50, 50);

rect r

Seelocation5

Seelocation24

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Rect also makes a new friend!


rect r

Seelocation5

Seelocation24

Hi!I’mr.

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rect r

Seelocation5

Seelocation24

Itlookslikewehavethesamesize,

location,everything!

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rect r

Seelocation5

Seelocation24

*blush*


But when they use == to compare values...

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…something goes wrong.

if (rect == r) { // false!...

}

rect rSee

location5

Seelocation

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}

rect rSee

location5

Seelocation

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…but…butIthoughtwehadsomuchin

common!

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}

rect rSee

location5

Seelocation

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💔

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}

rect rSee

location5

Seelocation

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Yousee,==compareswhatisineachvariable’s

box.

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}

rect rSee

location5

Seelocation

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Primitivesstoretheiractualvalueintheirbox.

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}

rect rSee

location5

Seelocation

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Butobjectsstorethelocation wherealltheir

informationlives.

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}

rect rSee

location5

Seelocation

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Thismeans==onobjectscomparestheirlocations,whichisnotwhat

wewanthere!


Chapter 3: Twins

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One day, they wanted a twin.

x

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Wow,whatanawesomeint!Let’smakeanotherone.

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int x2 = x;

One day, they wanted a twin.Wow,whatan

awesomeint!Let’smakeanotherone.

x

27

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int x2 = x;

…so x2 was born!

x

27x2

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Wow,whatanawesomeint!Let’smakeanotherone.

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int x2 = x;x2 += 2;

…so x2 was born!

x

27x2

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Butlet’sincrementthisoneby2.

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int x2 = x;x2 += 2;

…so x2 was born!

x

27x2

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Cool,I’m29now!

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int x2 = x;x2 += 2;

…so x2 was born!

x

27x2

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AndI’mstill27!

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Then, they wanted a twin for rect, too.

rectSee

location5

Wow,whatanawesomeGRect!Let’smakeanother

one.

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…so rect2 was born!

rectSee

location5


one.

GRect rect2 = rect;

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rectSee

location5

GRect rect2 = rect;

rect2See

location5


one.

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GRect rect2 = rect;rect2.setColor(Color.BLUE);


rectSee

location5

rect2See

location5

Butlet’smakethisoneBLUE.

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…so they went to location 5 and changed the color to blue.

x=0,y=0width=50height=50Color=BLUE...

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But something went wrong…

rectSee

location5

rect2See

location5

Cool,I’mblue!

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rectSee

location5

rect2See

location5

Hey,whyamIblueTOO?

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rectSee

location5

rect2See

location5

Yousee,whenyousetonevariableequaltoanother,thevalueinits

box iscopiedover.

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rectSee

location5

rect2See

location5

Forprimitivevariables,thisjustmeanswecopytheirvalue.

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rectSee

location5

rect2See

location5

Butforobjects,wecopyitslocation instead.

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rectSee

location5

rect2See

location5

Therefore,bothrect andrect2thinktheirinformationisat

location5.

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rectSee

location5

rect2See

location5

Whenyouchangeinformationforanobject,yougotoitslocationfirst,andthenupdate

theinformation.

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rectSee

location5

rect2See

location5

Sincerect andrect2referencethesamelocation,whenyouchangeinformationfor

oneofthem,itchangesforboth!


Chapter 4: Leaving the Nest

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college(x);

X grew up, and went to college.

x

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Byeeveryone!I’mgoingtocollege.

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college(x);


x

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We’llmissyou,honey!Don’tforgettocall!Don’tstayuptoolate!Watchoutforroguebicyclists!

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private void college(int num) {...

}


x

27num

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run college

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private void college(int num) {num = 45;

}

x had an amazing college experience…

x

27num

45

run college

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private void college(int num) {num = 45;

}

...but ultimately returned home the same.

x

27num

45

run college

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college(x);println(x);

…but ultimately returned home the same.

x

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I’mbaaaaack!AndI’mstill27.

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college(x);println(x);

…but ultimately returned home the same.

x

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I’mbaaaaack!AndI’mstill27.Ohhhh honey!You’ll

alwaysbeourlittle27.

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college2(rect);

rect grew up too, and also went to college.

rectSee

location5


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college2(rect);


rectSee

location5


Notyoutoo,rect!Weareemptynesters

now…

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private void college2(GRect box) {...

}


run college2

rectSee

location5

boxSee

location5

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private void college2(GRect box) {box.setColor(Color.YELLOW);

}

rect also had an amazing college experience…

run college2

rectSee

location5

boxSee

location5

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rect also had an amazing college experience…

x=0,y=0width=50height=50Color=YELLOW...

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}

…but it returned home different.

run college2

rectSee

location5

boxSee

location5

I’myellow!

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}


run college2

rectSee

location5

boxSee

location5

I’myellow!

Hey,whatgives!I’myellow

nowtoo!

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college(rect);add(rect);


rectSee

location5

I’mbaaaaack!ButnowI’myellow…

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rectSee

location5



I’mbaaaaack!ButnowI’myellow…Ohmygosh….what

happened toyou??!

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rectSee

location5



Yousee,whenavariableispassedasaparameter,thevalueinsideitsbox iscopied

andgiventothemethod.

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rectSee

location5



Forprimitives,wemakeacopyoftheiractualvalue.Therefore,changestoaparameterdo

notaffecttheoriginal.

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rectSee

location5



However,forobjectswemakeacopyoftheirlocation.Sochangestotheparameterdo

affecttheoriginal!


The End

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Primitives vs. Objects• Primitivesstoretheiractualvalue intheirvariablebox.Youcancomparevalueswith==and!=,andtheoriginaldoesnotchangewhenpassedasaparameterandchanged.

• Objects storetheirlocation intheirvariablebox.Youcan’tcomparepropertiesofanobjectvia==,andtheoriginaldoeschangewhenpassedasaparameterandchanged.

• Primitivesarepassedbyvalue,Objectsarepassedbyreference


Practice – Chapter 2: Friends

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Practicepublic void run() {

int x = 5;int y = 5;println(x == y);

}

Doesthisprintouttrue orfalse?

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Practice

Doesthisprintouttrue orfalse?


Practice – Chapter 3: Twins

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int x = 5;int y = x;y += 2;println(y);println(x);

}

Whatisprintedout?

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GOval oval = new GOval(0, 0, 50, 50);GOval oval2 = oval1;oval2.setColor(Color.BLUE);add(oval2);add(oval);

}

Whatcolorareovalandoval2?


Practice – Chapter 4: Leaving the Nest

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int x = 2;addFive(x);println(x);

}

private void addFive(int num) {num += 5;

}

Whatisprintedout?

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GRect rect = new GRect(0, 0, 50, 50);makeBlue(rect);add(rect);

}

private void makeBlue(GRect box) {box.setColor(Color.BLUE);

}

Whatcolorisrect?

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Primitives vs. Objects

Primitivesstoretheiractualvalues.Objects storetheir

locations.

Thisaffectshowyourcodebehaves!

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getElementAtgetElementAt returnsareference toanobject.

GRect rect = new GRect(0, 0, 50, 50);add(rect);...

GObject obj = getElementAt(25, 25);println(obj == rect);

Thisprintstrue!

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Revisiting Whack-A-MoleLet’srevisitourWhack-A-Moleprogramtoaddsomenewfunctionalityusingwhatwejustlearned.

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Recap•Announcements•Review:Null,EventsandInstanceVariables•ExtraPractice:Googley Eyes•ABooleanAside•Memory•RevisitingWhack-A-Mole

Goodluckonthemidterm!

cs 106a, lecture 15 events and memory -...

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