CS 106A, Lecture 14 Events and Instance Variablesweb.stanford.edu/class/archive/cs/cs106a/cs106a.1178/lectures/...5 Plan for Today •Announcements •Review: Animation •Null •Event-driven

Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.

CS106A,Lecture14EventsandInstanceVariables

Reading:Art&ScienceofJava,Ch.10.1-10.4

2

GraphicsPrograms

Animation

HW4:Breakout

Youarehere

3

Learning Goals• LearntorespondtomouseeventsinGraphicsPrograms• Learntouseinstancevariablestostoreinformationoutsideofmethods

4

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

5

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

6

Animation• AGraphicsprogramcanbemadetoanimatewithaloopsuchas:

public void run() {...while (test) {

update the position of shapes;pause(milliseconds);

}

}

• Thebestnumberofmstopausedependsontheprogram.– mostvideogames~=50frames/sec=25mspause

7

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

8

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

9

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}

10

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!}

11

NullCallingmethodsonanobjectthatisnull willcrashyourprogram!(throwsaNullPointerException)

12

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

13

Events•event:Someexternalstimulusthatyourprogramcanrespondto.

•event-drivenprogramming:Acodingstyle(commoningraphicalprograms)whereyourcodeisexecutedinresponsetouserevents.

14

Events•Programlaunches

15

Events•Programlaunches•Mousemotion•Mouseclicking•Keyboardkeyspressed•Devicerotated•Devicemoved•GPSlocationchanged•andmore…

16

Events•Programlaunches•Mousemotion•Mouseclicking•Keyboardkeyspressed•Devicerotated•Devicemoved•GPSlocationchanged•andmore…

17

Events

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

}

18

Events

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

}

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

}

19

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

}

20

Example: ClickForDaisyimport acm.program.*;import acm.graphics.*;import java.awt.*;import java.awt.event.*; // NEW

public class ClickForDaisy extends GraphicsProgram {

// Add a Daisy image at 50, 50 on mouse clickpublic void mouseClicked(MouseEvent event) {

GImage daisy = new GImage("res/daisy.png", 50, 50);add(daisy);

}}

21

MouseEvent Objects

•AMouseEvent containsinformationabouttheeventthatjustoccurred:

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

22

Example: ClickForDaisies

23

Example: ClickForDaisiespublic class ClickForDaisies extends GraphicsProgram {

// Add a Daisy image where the user clickspublic void mouseClicked(MouseEvent event) {// Get information about the eventdouble mouseX = event.getX();double mouseY = event.getY();

// Add Daisy at the mouse locationGImage daisy = new GImage("res/daisy.png", mouseX, mouseY);add(daisy);

}}

24

Example: ClickForDaisiespublic class ClickForDaisies extends GraphicsProgram {

// Add a Daisy image where the user clickspublic void mouseClicked(MouseEvent event) {// Get information about the eventdouble mouseX = event.getX();double mouseY = event.getY();

// Add Daisy at the mouse locationGImage daisy = new GImage("res/daisy.png", mouseX, mouseY);add(daisy);

}}

25

Types of Mouse Events• Therearemanydifferenttypesofmouseevents.

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

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

26

Example: Doodler

27

Doodlerprivate static final int SIZE = 10;...

public 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);

}

28

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);

}

29

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);

}

30

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);

}

31

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);

}

32

Recap: Events1)Userperformssomeaction,likemoving/clickingthemouse.2)Thiscausesaneventtooccur.3)Javaexecutesaparticularmethodtohandlethatevent.4)Themethod'scodeupdatesthescreenappearanceinsomeway.

33

Revisiting 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?

34

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

35

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

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

– Usefulfordatathatmustpersistthroughouttheprogram,orthatcannotbestoredaslocalvariablesorparameters(eventhandlers).

– Itisbadstyletooveruseinstancevariables

DONOTUSEINSTANCEVARIABLESONHANGMAN!

36

Example: MouseTracker

37

Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

38

Putting it all together

39

Whack-A-MoleLet’suseinstancevariablesandeventstomakeWhack-A-Mole!• Amoleshouldappeareverysecondatarandomlocation,andstoponcetheuserhasgottenatleast10points.

• Iftheuserclicksamole,removeitandincreasetheirscoreby1• ThereshouldbeaGLabel intheleftcornershowingtheirscore

40

Exception• Iftheuserclicksanareawithnomole,theprogramcrashes.

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

41

Recap•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole

NextTime:MoreEvents+Memory