计算机软件及应用JAVA程序设计教程第七版

MoreConditionalsandLoopsNowwecanfillinsomeadditionaldetailsregardingJavaconditionalandrepetitionstatementsChapter6focuseson:theswitchstatementtheconditionaloperatorthedolooptheforloopdrawingwiththeaidofconditionalsandloopsdialogboxesOutlineTheswitch

StatementTheConditionalOperatorThedo

StatementThefor

StatementDrawingwithLoopsandConditionalsDialogBoxesTheswitchStatementTheswitchstatementprovidesanotherwaytodecidewhichstatementtoexecutenextTheswitchstatementevaluatesanexpression,thenattemptstomatchtheresulttooneofseveralpossiblecasesEachcasecontainsavalueandalistofstatementsTheflowofcontroltransferstostatementassociatedwiththefirstcasevaluethatmatchesTheswitchStatementThegeneralsyntaxofaswitchstatementis:switch(expression){casevalue1

:

statement-list1casevalue2

:

statement-list2casevalue3:

statement-list3

case

...}switchandcasearereservedwordsIfexpressionmatchesvalue2,controljumpstohereTheswitchStatementOftenabreakstatementisusedasthelaststatementineachcase'sstatementlistAbreakstatementcausescontroltotransfertotheendoftheswitchstatementIfabreakstatementisnotused,theflowofcontrolwillcontinueintothenextcaseSometimesthismaybeappropriate,butoftenwewanttoexecuteonlythestatementsassociatedwithonecaseTheswitchStatementswitch(option){case'A':aCount++;break;

case'B':bCount++;break;case'C':cCount++;break;}Anexampleofaswitchstatement:TheswitchStatementAswitchstatementcanhaveanoptionaldefaultcaseThedefaultcasehasnoassociatedvalueandsimplyusesthereservedworddefaultIfthedefaultcaseispresent,controlwilltransfertoitifnoothercasevaluematchesIfthereisnodefaultcase,andnoothervaluematches,controlfallsthroughtothestatementaftertheswitchTheswitchStatementThetypeofaswitchexpressionmustbeintegers,characters,orenumeratedtypesAsofJava7,aswitchcanalsobeusedwithstringsYoucannotuseaswitchwithfloatingpointvaluesTheimplicitbooleanconditioninaswitchstatementisequalityYoucannotperformrelationalcheckswithaswitchstatementSeeGradeReport.java//********************************************************************//GradeReport.javaAuthor:Lewis/Loftus////Demonstratestheuseofaswitchstatement.//********************************************************************importjava.util.Scanner;publicclassGradeReport{

////Readsagradefromtheuserandprintscommentsaccordingly.//

publicstaticvoidmain(String[]args){

intgrade,category;Scannerscan=newScanner(System.in);System.out.print("Enteranumericgrade(0to100):");grade=scan.nextInt();category=grade/10;System.out.print("Thatgradeis");continuecontinue

switch(category){

case10:System.out.println("aperfectscore.Welldone.");

break;

case9:System.out.println("wellaboveaverage.Excellent.");

break;

case8:System.out.println("aboveaverage.Nicejob.");

break;

case7:System.out.println("average.");

break;

case6:System.out.println("belowaverage.Youshouldseethe");System.out.println("instructortoclarifythematerial"+"presentedinclass.");

break;

default:System.out.println("notpassing.");}}}continue

switch(category){

case10:System.out.println("aperfectscore.Welldone.");

break;

case9:System.out.println("wellaboveaverage.Excellent.");

break;

case8:System.out.println("aboveaverage.Nicejob.");

break;

case7:System.out.println("average.");

break;

case6:System.out.println("belowaverage.Youshouldseethe");System.out.println("instructortoclarifythematerial"+"presentedinclass.");

break;

default:System.out.println("notpassing.");}}}SampleRunEnteranumericgrade(0to100):91Thatgradeiswellaboveaverage.Excellent.OutlineTheswitch

StatementTheConditionalOperatorThedo

StatementThefor

StatementDrawingwithLoopsandConditionalsDialogBoxesTheConditionalOperatorTheconditionaloperatorevaluatestooneoftwoexpressionsbasedonabooleanconditionItssyntaxis:condition?expression1:expression2Ifthecondition

istrue,expression1

isevaluated;ifitisfalse,expression2

isevaluatedThevalueoftheentireconditionaloperatoristhevalueoftheselectedexpressionTheConditionalOperatorTheconditionaloperatorissimilartoanif-elsestatement,exceptthatitisanexpressionthatreturnsavalueForexample: larger=((num1>num2)?num1:num2);Ifnum1isgreaterthannum2,thennum1isassignedtolarger;otherwise,num2isassignedtolargerTheconditionaloperatoristernarybecauseitrequiresthreeoperandsTheConditionalOperatorAnotherexample:Ifcount

equals1,the"Dime"isprintedIfcountisanythingotherthan1,then"Dimes"isprintedSystem.out.println("Yourchangeis"+count+((count==1)?"Dime":"Dimes"));QuickCheckExpressthefollowinglogicinasuccinctmannerusingtheconditionaloperator.if(val<=10)System.out.println("Itisnotgreaterthan10.");elseSystem.out.println("Itisgreaterthan10.");QuickCheckExpressthefollowinglogicinasuccinctmannerusingtheconditionaloperator.if(val<=10)System.out.println("Itisnotgreaterthan10.");elseSystem.out.println("Itisgreaterthan10.");System.out.println("Itis"+((val<=10)?"not":"")+"greaterthan10.");OutlineTheswitch

StatementTheConditionalOperatorThedo

StatementThefor

StatementDrawingwithLoopsandConditionalsDialogBoxesThedoStatementAdostatementhasthefollowingsyntax: do {

statement-list; } while(condition);

Thestatement-list

isexecutedonceinitially,andthenthecondition

isevaluatedThestatementisexecutedrepeatedlyuntiltheconditionbecomesfalseLogicofadoLooptrueconditionevaluatedstatementfalseThedoStatementAnexampleofadoloop:Thebodyofado

loopexecutesatleastonceSeeReverseNumber.javaintcount=0;do{count++;System.out.println(count);}while(count<5);//********************************************************************//ReverseNumber.javaAuthor:Lewis/Loftus////Demonstratestheuseofadoloop.//********************************************************************importjava.util.Scanner;publicclassReverseNumber{

////Reversesthedigitsofanintegermathematically.//

publicstaticvoidmain(String[]args){

intnumber,lastDigit,reverse=0;Scannerscan=newScanner(System.in);continuecontinue

System.out.print("Enterapositiveinteger:");number=scan.nextInt();

do{lastDigit=number%10;reverse=(reverse*10)+lastDigit;number=number/10;}

while(number>0);System.out.println("Thatnumberreversedis"+reverse);}}continue

System.out.print("Enterapositiveinteger:");number=scan.nextInt();

do{lastDigit=number%10;reverse=(reverse*10)+lastDigit;number=number/10;}

while(number>0);System.out.println("Thatnumberreversedis"+reverse);}}SampleRunEnterapositiveinteger:2896Thatnumberreversedis6982ComparingwhileanddostatementtruefalseconditionevaluatedThewhileLooptrueconditionevaluatedstatementfalseThedoLoopOutlineTheswitch

StatementTheConditionalOperatorThedo

StatementThefor

StatementDrawingwithLoopsandConditionalsDialogBoxesTheforStatementAforstatementhasthefollowingsyntax:for(initialization;condition;increment)

statement;TheinitializationisexecutedoncebeforetheloopbeginsThestatement

isexecuteduntiltheconditionbecomesfalseTheincrement

portionisexecutedattheendofeachiterationLogicofaforloopstatementtrueconditionevaluatedfalseincrementinitializationTheforStatementAforloopisfunctionallyequivalenttothefollowingwhileloopstructure:initialization;while(condition){

statement;

increment;}TheforStatementAnexampleofaforloop: for(intcount=1;count<=5;count++) System.out.println(count);TheinitializationsectioncanbeusedtodeclareavariableLikeawhileloop,theconditionofaforloopistestedpriortoexecutingtheloopbodyTherefore,thebodyofaforloopwillexecutezeroormoretimesTheforStatementTheincrementsectioncanperformanycalculation: for(intnum=100;num>0;num-=5) System.out.println(num);AforloopiswellsuitedforexecutingstatementsaspecificnumberoftimesthatcanbecalculatedordeterminedinadvanceSeeMultiples.javaSeeStars.java//********************************************************************//Multiples.javaAuthor:Lewis/Loftus////Demonstratestheuseofaforloop.//********************************************************************importjava.util.Scanner;publicclassMultiples{

////Printsmultiplesofauser-specifiednumberuptoauser-//specifiedlimit.//

publicstaticvoidmain(String[]args){

finalintPER_LINE=5;

intvalue,limit,mult,count=0;Scannerscan=newScanner(System.in);System.out.print("Enterapositivevalue:");value=scan.nextInt();continuecontinue

System.out.print("Enteranupperlimit:");limit=scan.nextInt();System.out.println();System.out.println("Themultiplesof"+value+"between"+value+"and"+limit+"(inclusive)are:");

for(mult=value;mult<=limit;mult+=value){System.out.print(mult+"\t");//Printaspecificnumberofvaluesperlineofoutputcount++;

if(count%PER_LINE==0)System.out.println();}}}continue

System.out.print("Enteranupperlimit:");limit=scan.nextInt();System.out.println();System.out.println("Themultiplesof"+value+"between"+value+"and"+limit+"(inclusive)are:");

for(mult=value;mult<=limit;mult+=value){System.out.print(mult+"\t");//Printaspecificnumberofvaluesperlineofoutputcount++;

if(count%PER_LINE==0)System.out.println();}}}SampleRunEnterapositivevalue:7Enteranupperlimit:400Themultiplesof7between7and400(inclusive)are:7 14 21 28 35 42 49 56 63 70 77 84 91 98 105 112 119 126 133 140 147 154 161 168 175 182 189 196 203 210 217 224 231 238 245 252 259 266 273 280 287 294 301 308 315 322 329 336 343 350 357 364 371 378 385 392 399//********************************************************************//Stars.javaAuthor:Lewis/Loftus////Demonstratestheuseofnestedforloops.//********************************************************************publicclassStars{

////Printsatriangleshapeusingasterisk(star)characters.//

publicstaticvoidmain(String[]args){

finalintMAX_ROWS=10;

for(introw=1;row<=MAX_ROWS;row++){

for(intstar=1;star<=row;star++)System.out.print("*");System.out.println();}}}//********************************************************************//Stars.javaAuthor:Lewis/Loftus////Demonstratestheuseofnestedforloops.//********************************************************************publicclassStars{

////Printsatriangleshapeusingasterisk(star)characters.//

publicstaticvoidmain(String[]args){

finalintMAX_ROWS=10;

for(introw=1;row<=MAX_ROWS;row++){

for(intstar=1;star<=row;star++)System.out.print("*");System.out.println();}}}Output*******************************************************QuickCheckWriteacodefragmentthatrollsadie100timesandcountsthenumberoftimesa3comesup.QuickCheckWriteacodefragmentthatrollsadie100timesandcountsthenumberoftimesa3comesup.Diedie=newDie();intcount=0;for(intnum=1;num<=100;num++)if(die.roll()==3)count++;Sytem.out.println(count);TheforStatementEachexpressionintheheaderofaforloopisoptionalIftheinitializationisleftout,noinitializationisperformedIftheconditionisleftout,itisalwaysconsideredtobetrue,andthereforecreatesaninfiniteloopIftheincrementisleftout,noincrementoperationisperformedFor-eachLoopsAvariantoftheforloopsimplifiestherepetitiveprocessingofitemsinaniteratorForexample,supposebookListisanArrayList<Book>

objectThefollowingloopwillprinteachbook: for(BookmyBook:bookList) System.out.println(myBook);Thisversionofaforloopisoftencalledafor-eachloopFor-eachLoopsAfor-eachloopcanbeusedonanyobjectthatimplementstheIterable

interfaceIteliminatestheneedtoretrieveaniteratorandcallthehasNextandnextmethodsexplicitlyItalsowillbehelpfulwhenprocessingarrays,whicharediscussedinChapter8QuickCheckWriteafor-eachloopthatprintsalloftheStudentobjectsinanArrayList<Student>objectcalledroster.QuickCheckWriteafor-eachloopthatprintsalloftheStudentobjectsinanArrayList<Student>objectcalledroster.for(Studentstudent:roster)System.out.println(student);OutlineTheswitch

StatementTheConditionalOperatorThedo

StatementThefor

StatementDrawingwithLoopsandConditionalsDialogBoxesDrawingTechniquesConditionalsandloopsenhanceourabilitytogenerateinterestinggraphicsSeeBullseye.javaSeeBullseyePanel.javaSeeBoxes.javaSeeBoxesPanel.java//********************************************************************//Bullseye.javaAuthor:Lewis/Loftus////Demonstratestheuseofloopstodraw.//********************************************************************importjavax.swing.JFrame;publicclassBullseye{

////Createsthemainframeoftheprogram.//

publicstaticvoidmain(String[]args){JFrameframe=newJFrame("Bullseye");frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);BullseyePanelpanel=newBullseyePanel();frame.getContentPane().add(panel);frame.pack();frame.setVisible(true);}}//********************************************************************//Bullseye.javaAuthor:Lewis/Loftus////Demonstratestheuseofloopstodraw.//********************************************************************importjavax.swing.JFrame;publicclassBullseye{

////Createsthemainframeoftheprogram.//

publicstaticvoidmain(String[]args){JFrameframe=newJFrame("Bullseye");frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);BullseyePanelpanel=newBullseyePanel();frame.getContentPane().add(panel);frame.pack();frame.setVisible(true);}}//********************************************************************//BullseyePanel.javaAuthor:Lewis/Loftus////Demonstratestheuseofconditionalsandloopstoguidedrawing.//********************************************************************importjavax.swing.JPanel;importjava.awt.*;publicclassBullseyePanelextendsJPanel{

privatefinalintMAX_WIDTH=300,NUM_RINGS=5,RING_WIDTH=25;

////Setsupthebullseyepanel.//

publicBullseyePanel(){setBackground(Color.cyan);setPreferredSize(newDimension(300,300));

}continuecontinue////Paintsabullseyetarget.//

publicvoidpaintComponent(Graphicspage){

super.paintComponent(page);

intx=0,y=0,diameter=MAX_WIDTH;page.setColor(Color.white);

for(intcount=0;count<NUM_RINGS;count++){

if(page.getColor()==Color.black)//alternatecolorspage.setColor(Color.white);

elsepage.setColor(Color.black);page.fillOval(x,y,diameter,diameter);diameter-=(2*RING_WIDTH);x+=RING_WIDTH;y+=RING_WIDTH;}//Drawtheredbullseyeinthecenterpage.setColor(Color.red);page.fillOval(x,y,diameter,diameter);}}//********************************************************************//Boxes.javaAuthor:Lewis/Loftus////Demonstratestheuseofloopstodraw.//********************************************************************importjavax.swing.JFrame;publicclassBoxes{

////Createsthemainframeoftheprogram.//

publicstaticvoidmain(String[]args){JFrameframe=newJFrame("Boxes");frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);BoxesPanelpanel=newBoxesPanel();frame.getContentPane().add(panel);frame.pack();frame.setVisible(true);}}//********************************************************************//Boxes.javaAuthor:Lewis/Loftus////Demonstratestheuseofloopstodraw.//********************************************************************importjavax.swing.JFrame;publicclassBoxes{

////Createsthemainframeoftheprogram.//

publicstaticvoidmain(String[]args){JFrameframe=newJFrame("Boxes");frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);BoxesPanelpanel=newBoxesPanel();frame.getContentPane().add(panel);frame.pack();frame.setVisible(true);}}//********************************************************************//BoxesPanel.javaAuthor:Lewis/Loftus////Demonstratestheuseofconditionalsandloopstoguidedrawing.//********************************************************************importjavax.swing.JPanel;importjava.awt.*;importjava.util.Random;publicclassBoxesPanelextendsJPanel{

privatefinalintNUM_BOXES=50,THICKNESS=5,MAX_SIDE=50;

privatefinalintMAX_X=350,MAX_Y=250;

privateRandomgenerator;

////Setsupthedrawingpanel.//

publicBoxesPanel(){generator=newRandom();setBackground(Color.black);setPreferredSize(newDimension(400,300));

}continuecontinue////Paintsboxesofrandomwidthandheightinarandomlocation.//Narroworshortboxesarehighlightedwithafillcolor.//

publicvoidpaintComponent(Graphicspage){

super.paintComponent(page);

intx,y,width,height;

for(intcount=0;count<NUM_BOXES;count++){x=generator.nextInt(MAX_X)+1;y=generator.nextInt(MAX_Y)+1;width=generator.nextInt(MAX_SIDE)+1;height=generator.nextInt(MAX_SIDE)+1;continuecontinue

if(width<=THICKNESS)//checkfornarrowbox{page.setColor(Color.yellow);page.fillRect(x,y,width,height);}

else

if(height<=THICKNESS)//checkforshortbox{page.setColor(Color.green);page.fillRect(x,y,width,height);}

else