Theaimsofthisunitaretointroduce:
Geog5561MIntroductiontoJavaProgrammingUnit3Notes
Operators,VariablesandControlStatements
operatorprecedence variabletypecasting arangeofcontrolstatements variableandObjectscope Exceptions
Oncompletionofthisunityoushouldbe:
abletoapplyoperatorprecedenceandvariablecasting familiarwitharangeofcontrolstatementsandknowwhentoapplythem abletoprogramasimpleGISinJavausingBlueJ.
Contents
Geog5561MIntroductiontoJavaProgrammingUnit3Notes ................................................................ 1 Contents ........................................................................................................................................ 1 1Introduction ......................................................................................................................................... 1 2Variables ............................................................................................................................................. 1 2.1OperatorPrecedence .................................................................................................................. 1 2.2Casting ........................................................................................................................................ 1 2.3TheNullValue ............................................................................................................................. 4 2.4FinalVariables ............................................................................................................................. 5 3ControlStatements ............................................................................................................................. 6 3.1Theif/elseif/elseLadder .......................................................................................................... 6 3.2TheswitchStatement .................................................................................................................. 7 DownloadSalon.javafromthemodulewebsiteforthisunit. .............................................................. 8 3.3forLoopsRevisited ...................................................................................................................... 9 3.4whileLoops .................................................................................................................................. 9 3.5OtherControlStatements .......................................................................................................... 10 3.5.1Break .................................................................................................................................. 10 3.5.2Continue ............................................................................................................................. 11 4Exceptions ........................................................................................................................................ 11 5Tasks ................................................................................................................................................ 14
1Introduction
Inthisunitwewillexaminevariables,operatorsandcontrolstatementsinmoredetail.
2Variables
IfyourememberfromUnit1,variablesarethingsthatwewanttodosomethingto,suchas data,words,numbers,etc.Weusedtheradiusoftheearthasanexampleofaninteger variabletoillustratehowstatementsworked.Inthissectionwelookatoperatorsandtheir order of precedence as well casting and some assumptions that Java makes about variables. 2.1OperatorPrecedence Operatorshaveadefiniteorderinwhichthingsaredone.Supposeyouwishtoadd6and6 togetherandthendividetheresultby3.YoumightwritethisinJavaasfollows:
int i = 6 + 6 / 3;
Theresultwillbe8forthevariable i andnot4,asyoumightexpect.Thisisbecausea divisionoperator(/)hasprecedenceovertheadditionoperator(+)sothedivisioniscarried outfirst,yielding2,whichisthenaddedto6,yielding8.Thisisanexampleofanambiguous statement.Itisalwaysbesttouseparenthesestomaketheorderofoperationsclear.To performtheadditionfirst,youwouldwritethestatementasfollows:
int i = (6 + 6) / 3;
Fulldetailsoftheprecedenceofdifferentoperatorscanbefoundat:
http://java.sun.com/docs/books/tutorial/java/nutsandbolts/expressions.html
Evaluatethefollowingexpressionsusingtherulesofoperatorprecedencefoundinthe websiteabove: 1. i=5+6*9 2. i=(82)/3 3. i=6+(5+(74)/(2+1))*2
2.2Casting Castingistheconversionofonetypeofvariabletoanother.Castingcanbedoneinvisibly withsomesimpletypes,forexample:
int m = 3;
givesi=3.0 ReferbacktoAppendixAinUnit1ifyouneedtorefreshyourmemoryonwhatconstitutes anint,float,double,etc.
double i = m;
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Thisisusuallypossiblewithsimpledatatypeswherethereisnolossofprecision. Inaddition,thiskindofimplicitcastingisaveryimportantfeatureofinheritance.Oneofthe great things about Subclasses is that, because they contain everything their Superclass does,youcantreatthemasiftheyaretheSuperclass. For example, say you have a Button Class that expects to be passed an Object implementing the ActionListener Interfaceso theObject candostuff whentheButtonis pushed.HerestheActionListenerInterface:
public interface ActionListener { public void actionPerformed(); }
Herestheclass/methodthatmighttakeinsuchanObject:
class Button { ActionListener myListener; public void addActionListener (ActionListener a) { myListener = a; } }
NowbecauseSubclassescanbetreatedliketheirSuperclasses,wecanmakeaClassthat implementstheActionListenerInterfaceandmakeanObjectoutofit.HeressuchaClass:
Class OurButtonListener implements ActionListener { public void actionPerformed() { do stuff } public void newMethod() { do new stuff } }
WecanthenpassthisObjecttotheButtonsaddActionListenermethodanditllswallowit, eventhoughithasthenewMethodinit,whichisntspecifiedbytheActionListenerInterface! ItsnotbotheredaboutanyotherstuffwemighthaveputintheClass,aslongasithasthe methodstipulatedbyActionListener.
OurButtonListener b = new OurButtonListener(); button.addActionListener(b);
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So you can see that again, like the simple types, as long as we dont lose any useful information(numericalprecisioninthecaseofsimpletypes,usefulmethodsinthecaseof Objects)thecastingwillbedoneinvisiblybythesystem. However,forsomeconversionsyouwillneedtodoanexplicitcast.Thesyntaxforexplicit castingis:
(target type) variable;
HereisanexamplewherewehaveexplicitlycastanintegervariablecallednumberOfCars toabyte:
int numberOfCars = 150; byte number; number = (byte)numberOfCars;
Javaalwaysassumesthatanynumberwithadecimalpointisadouble.Ifyouwanttomake itafloat,youhavetobeexplicitasfollows:
float f = (float)2.5; OR float f = 2.5f;
EquallyJavaassumesthatnumberswithoutdecimalplacesareintegers:
double d = (double)3; OR double d = 3d;
ImportantNote:Thiscausesproblemsinmaths,asroundingtointegerswilloccurifan equation contains any integer. It is critical you realise this if you intend to do any mathematics.Tostopthishappening,alwaysgivefigureswithdecimalpoints,e.g.:
double d = 3.7 / 2.0; not double d = 3.7 / 2;
WhenitcomestoObjects,therearealsocircumstanceswhereanexplicitcastisnecessary. ThisisusuallywhenyouvebeentreatinganObjectasitsSuperclassbutyoudliketogetat itsSubclassmethods.Forexample,letsreturntoourButtonClass.Sayitwantstogetat the newMethod of the OurButtonListener Class Object we passed into it using the AddActionListener Method. Remember that the Button thinks the Object is of the ActionListenerClass,soitcantgetattheseMethodsyet.Thewaytodothisistocastthe supposedActionListenerObjectintoitsproperClass,thus:
class Button { ActionListener myListener;
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public void addActionListener (ActionListener a) { myListener = a; OurButtonListener obl = (OurButtonListener) a; obl.newMethod(); } }
Just using a.newMethod() isnt allowed because the argument specified by AddActionListener is an ActionListener, which doesnt have a newMethod. The Object pretending to be an ActionListener has the method in it, but only an explicit cast will persuadeittorevealitstruecolours. Note:someconversionsareactuallytoohardforthesystemtodousingeventhistechnique, forexample,betweenaStringandanint.FortunatelytherearesomeClassesthatcome aspartoftheJVMthatcanhelpwiththis.WellbelookingattheseinalaterUnit. 2.3TheNullValue If we declare a nonsimple variable label (i.e. an Object label) but dont initialise it by attachingittoanObject,thecomputerwillnotknowattheoutsethowlargeitshouldbe. Thus,itcannotsetasideanabsolutechunkofmemory.Instead,itsetsasideasmallchunk andtellsitselfthatthisspacewillbeenlargedinthefuture.Inthesmallchunkofmemory thatitputsaside,itplacesatemporaryvalue,calledthenullvalue.Thenullvalueisvery importantwhenweconsiderissuesofscope.Letslookataproblemcase. SaywewanttouseanifstatementtosetupanObject.Wecouldtrysomethinglikethis:
if (needPoint == true) { Point point1 = new Point(); } point1.setX(10);
However,thescopeofthe point1 variableistheblockitisdeclaredin,sothecompiler wontrecognisethevariableinthelastline.Analternativewouldseemtobe:
Point point1; if (needPoint = true) { point1 = new Point(); } point1.setX(10);
However,thecompilerwillrealisethattheresachancethatpoint1hasntbeeninitialised, anditllrefusetocompile.Theactualsolutionistousethenullvalueexplicitly,likethis
Point point1 = null;
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if (needPoint = true) { point1 = new Point(); } if (point1 != null) point.setX(10);
2.4FinalVariables InthelastUnitwelookedatusingInterfacestoforcepeopletoputincertainmethods.What wedidntsayisthatyoucanalsoputvariablesintoInterfaces.Hereisanexample
public interface Point { int ORIGIN_X = 0; int ORIGIN_Y = 0; int xDistanceToOrigin (int x) ; } class Land implements Point { int xDistanceToOrigin (int presentX) { return presentX - ORIGIN_X; } }
VariablesdeclaredinanInterfacecanbeusedinimplementingClasses,buttheirvaluesare fixed.Ifyouvedoneanyprogramminginotherlanguages,youllknowthesevariablesas constants. Because theyre special, they usually get written in ALL_CAPITALS. Theyre usefulfordefiningthingslikepiorethatarentusefullychanged.Youcanactuallydothis innormalClassesbyusingthefinalkeyword,thus:
final int PI = 3.14; final String CONSTANT_OF_LIFE_1 = Death; final String CONSTANT_OF_LIFE_2 = Taxes;
NotethatyoudonotneedtoimplementtheInterfacevariablestousethem. Hereisanother example:
public interface City { String ROME = Paris of the South; String PARIS = Rome of the North; } class Point implements City { void getAlternativeName () {
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System.out.println(Roma, + ROME); } }
YoucanalsodeclareClassesandMethodsasfinal.Classesdeclaredasfinalcannot beinheritedandMethodsdeclaredas finalcannotbeoverridden.Aswesawinthelast Unit, overriding happens when you call a Method in the Subclass the same thing as a MethodintheSuperclassandtheargumentsarethesame.InthissituationtheSubclass MethodwillbeusedsotheSuperclassisoverridden.Thecompilerwontletyouhavea SubclassMethodthesameasaSuperclassmethodiftheMethodisfinal. Wewillnowmoveontolookingatcontrolstatementsinmoredetail.
3ControlStatements
IfyourememberfromUnit1,welookedat2maintypesofcontrolstatements:if/elseand for.Inthissectionweexpandtherangeofcontrolstatementsthatareavailable. 3.1Theif/else if/elseLadder Toremindyou,asimpleif/elsestatementcantakeoneofthefollowing4forms: if (condition) statement; if (condition){statements;} if (condition) statement; else statement; if (condition){statements;} else {statements;}
However,itispossibletohavemorecomplicatedstructuressuchasthoseshownbelow:
if (condition) statement or {statements;} else if (condition) statement or {statements;} else if (condition) statement or {statements;} else statement or {statements;}
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Thisisknownastheif/else if/elseladder.Notethatyouneedmultipleifteststo reachthedefaultelseatthebottomoftheladder.Thisstructureworkswellwhenthereisa higherlikelihoodthatsomechoiceswillhappenthanothers.Youcanputtheconditionsin order of this likelihood, and, chances are, the computer will only have to spend time assessing one or two of them. But if what all the conditions have an equal chance of occurring?Inthiscasethecomputermayhavetoassesseachcondition.Thiscantakea relativelylongtime.Undertheseconditionsitmaybebettertouseaswitchstatement.
3.2TheswitchStatement The switch statement takes a little longer torunthan a single if/else statement, but provides an efficient if /else if / else ladder replacement as it only requires one conditionassessment.Thesyntaxisasfollows:
switch (variable) { case value1: statement1; break; case value2: statement2; break; case value3: statement3; break; default: statement4; }
Forexample:
int day = 2; switch (day) { case (1): System.out.println (Today is Saturday!); break; case (2): System.out.println (Today is Sunday!); break; default:
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System.out.println(The rest are work days!); }
Thevalueofthenamedvariabledetermines whichblock ofstatementsis executed.For example,ifthevariableequalsvalue2,statement2willberun,andthenthecomputerwill jumptothelastbracketatthebottomoftheswitchblock. Thebreakstatementattheendofeachcaseisoptional.Ifyouweretoleaveoneout,the computerwouldnotexittheswitchblockbutwould,instead,justevaluatethesubsequent case statements. For example, if the break in the value2 block was missing and the variablewasequaltovalue2,statement2andstatement3wouldberun.Itsprettyrare youdwanttodothis.Oneexamplemightbeifyouhadanelectronicmenuthatshowedall themealsfortheremainingdaysinaweek.Youcouldusethecurrentdayvalue(between1 and7)toskipthedaysoftheweekthathavealreadypassed,andthenrunthroughthe remainingdayscasestoprintwhatwascomingup. Notethatthevaluesthemselvesaftereachcasestatementcannotbevariables.Notealso thatallthevaluesmustbedifferentandthestatementsdonoteachneedtobeinsidetheir ownblock.Theswitchblockistheblocktheyrunin.
DownloadSalon.javafromthemodulewebsiteforthisunit. Salon.java offers some advice on a number of haircuts. The user has to hardwire a descriptionoftheirhairintothecode. Replacetheif/else if/elseladderinthiscodewithaswitchstatement.
Finally,youmightseeifstatementsassigningvaluestovariableslikethis: variableName=(condition)?value1:value2; forexample:
int value = (w * h > 255) ? 255 : w * h; ?: is a ternary operator. The above means if w times h is greater than 255, make value 255, otherwisemakeitwtimesh. Thismight seem likea ratherunobviouswayof writing an if statement. It is. However, it is also compactandelegant.?:isrealJavakungfu;useitandyoullmarkyourselfoutasjustthatlittlebit morespecial.
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3.3forLoopsRevisited IfyourecallfromUnit1,theformoftheforloopisasfollows:
for (variable initialization; condition; increment) {statements;}
Itisusualtodeclarethevariable,i.e.thecounter,intheforstatement. Notethatnoneoftheexpressionsinthestatementarecompulsory.Thusthefollowingis perfectlylegitimate: int a = 5;
for ( ; done != true ; ) { if (a = 10) done = true; a++; }
Asithappens,youdonotevenneedthecondition,ifyouwanttocreateaninfiniteloop,e.g.: int a = 5;
for ( ; ; ) { a++; }
Aninfiniteloopisdangerousbecauseyouwillneverbeabletoexitbutitispossibleto createone.
Youcanalsoseparatemultiplevariablesandincrementswithacomma,asshownbelow:
for (int a=0, int b=10; a > b; a = a*2, b++) { System.out.println(a = + a + b = + b); }
3.4whileLoops Awhileloopisamoreeffectiveloopthantheforloopwhereyoudontneedtoincrement avariableandjustwantacondition.Thesyntaxforthisis: Example:
while (condition) { statements; } } while (found != true) { found = isFileFound();
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The statement(s) enclosed in the block will only run if the condition, which is assessed beforetheblockisrun,istrue. Itisalsopossibletoplacethewhile conditionattheendoftheblock,inwhichcaseyou precedetheblockwithado:
do { statements; } while (condition)
Inthiscase,thestatementsarerunoncewhetherornottheconditionistrueandthenonly runagainwhiletheconditionremainstrue. 3.5OtherControlStatements IfyouhaveevercodedinanoldcomputerlanguagelikeBASICyouwillhaveusedthegoto statement,aterriblestatementthatletsyoujumparoundintheprogramtodifferentcode linenumbers.Thisresultedincodethatwasverydifficulttofollow.Thereisagotokeyword inJava,butitsheldinverypoorregard. TherearesaferequivalentsinJavaintheformofthebreakandcontinuekeywords.These canbeequallymisused,buttherearesomelegitimateusesforthem.Wevealreadyseen oneuseforbreakintheswitchstatement. 3.5.1Break Abreakstatementexitsthecurrentblockandstartsthecoderunningagainafterthelast bracket:
break;
Agooduseforthisisinsearchingroutines.Sayyouwanttosearchthroughalonglistof things,gettingthenextitemacrossthenetworkonceyouvelookedatthecurrentitemand decideditsnottheoneyourelookingfor.Inthiscase,youdontwanttorunthecodethat getsthenextitemifyoufindtheoneyouwant.Atthispointifthenbecomesusefultohave somethinglike if (item == lookedFor) break; Amoredeadlybreakstatementallowsyoutojumptotheendofanamedblockofcode providedtheblockyouareinisnestedinsidethenamedblockatthetime.Younamea blockbyprecedingitsfirstbracketwithanameandacolon.Forexample:
boolean t = true; blockOne: { blockTwo: { blockThree: { if (t == true) break blockTwo; Geog5561MIntroductiontoJavaProgrammingUnit3 10
System.out.println(This is not printed); } System.out.println(This is not printed); } System.out.println(This is printed); }
However,thisisveryhardtounderstandincomplexprograms,andisregardedbymost programmersasanextremelyembarrassingadmissionofincompetence. 3.5.2Continue A continue statement skips the rest of a loop and goes back to the loops conditional statement,orinthecaseofaforloop,theincrementstatement:
continue;
Thefollowingcodeskipsbacktothebeginningoftheforloopforevennumbers,butprints outoddnumbers:
for (int i = 0; i < 20; i++) { if (i%2 == 0) continue; System.out.println(i + is an odd number); }
Aswithbreak,youcanjumptoanarbitrarylabelledblockbyputtingthelabelnameafter thecontinue,providedyouarenestedwithinthatblock.Itsveryunlikelyyoulleverhave tousecontinue.
4Exceptions
Thereisonefinalimportantflowcontrolstatementweneedtounderstand,andthatisthe trycatchblocks.Thetrycatchblocksareusedunderaveryspecialcircumstance;theyare usedwheneverythinggoeswrong.Tounderstandhowtheywork,letslookatwhathappens whenaprogrambreaks. Thereareawholesetofprogrammingproblemsthatareessentiallyunpredictable.These areknownasruntimeexceptions(anexceptionistheJavatermforaproblemwiththe programwhichstopsitworkingwhileitisrunning;thetermerrorisspecificallyreservedfor problems withtheJVM andsomeother issues deep intheJava system). For example, runtime exceptions are generated where the user tries to do something impossible, like divideanumberbyzero.Whenaruntimeexceptionoccurs,theJVMgeneratesanobject representing it of type RuntimeException and throws it into the code at the point the problemarose.Thisobjectlooksaroundthecurrentmethodcodeforsomethingtodealwith it.Ifitcantfindanything,itmovestowhereverthemethodwascalledfromanddoesthe same.Itkeepsdoingthisuntilitreachesthemainmethod,whereitthenjumpstotheJVM. The JVM can cope with RuntimeExceptions, but not very elegantly it crashes the programandprintsoutthe RuntimeExceptionsstacktracethisisarecordofallthe codetheExceptionhasbouncedthrough.
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Obviously,ifyouareauser,thisisntideal.Anyonewhohashadaprogramcrashonthem will know ...