This document is copyright (C) Stanford Computer Science and Marty Stepp, licensed under Creative Commons Attribution 2.5 License. All rights reserved. Based on slides created by Keith Schwarz, Mehran Sahami, Eric Roberts, Stuart Reges, and others. CS 106A, Lecture 14 Events and Instance Variables Reading: Art & Science of Java, Ch. 10.1-10.4
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Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.
CS106A,Lecture14EventsandInstanceVariables
Reading:Art&ScienceofJava,Ch.10.1-10.4
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GraphicsPrograms
Animation
HW4:Breakout
Youarehere
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Learning Goals• LearntorespondtomouseeventsinGraphicsPrograms• Learntouseinstancevariablestostoreinformationoutsideofmethods
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
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Animation• AGraphicsprogramcanbemadetoanimatewithaloopsuchas:
public void run() {...while (test) {
update the position of shapes;pause(milliseconds);
}
}
• Thebestnumberofmstopausedependsontheprogram.– mostvideogames~=50frames/sec=25mspause
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-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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NullCallingmethodsonanobjectthatisnull willcrashyourprogram!(throwsaNullPointerException)
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
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Events•event:Someexternalstimulusthatyourprogramcanrespondto.
•event-drivenprogramming:Acodingstyle(commoningraphicalprograms)whereyourcodeisexecutedinresponsetouserevents.
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Events•Programlaunches
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Events•Programlaunches•Mousemotion•Mouseclicking•Keyboardkeyspressed•Devicerotated•Devicemoved•GPSlocationchanged•andmore…
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Events•Programlaunches•Mousemotion•Mouseclicking•Keyboardkeyspressed•Devicerotated•Devicemoved•GPSlocationchanged•andmore…
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Events
public void run() {// Java runs this when program launches
}
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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
}
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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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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);
}}
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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: 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);
}}
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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);
}}
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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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Example: Doodler
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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);
}
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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);
}
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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);
}
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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);
}
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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);
}
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Recap: Events1)Userperformssomeaction,likemoving/clickingthemouse.2)Thiscausesaneventtooccur.3)Javaexecutesaparticularmethodtohandlethatevent.4)Themethod'scodeupdatesthescreenappearanceinsomeway.
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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?
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
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Instance Variablesprivate type name; // declared outside of any method
• Instancevariable:Avariablethatlivesoutsideofanymethod.– Thescope ofaninstancevariableisthroughoutanentirefile(class).
– Usefulfordatathatmustpersistthroughouttheprogram,orthatcannotbestoredaslocalvariablesorparameters(eventhandlers).
– Itisbadstyletooveruseinstancevariables
DONOTUSEINSTANCEVARIABLESONHANGMAN!
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Example: MouseTracker
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Plan for Today•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
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Putting it all together
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Whack-A-MoleLet’suseinstancevariablesandeventstomakeWhack-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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Recap•Announcements•Review:Animation•Null•Event-drivenprogramming(withDaisy!)•InstanceVariables•Whack-A-Mole
NextTime:MoreEvents+Memory
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