java语言程序设计基础篇(第八版)课件PPT第三章--机械工业出版报社--李娜译.ppt

Liang,IntroductiontoJavaProgramming,EighthEdition,(c)2011PearsonEducation,Inc.Allrightsreserved.0132130807,1,Chapter3Selections,2,Motivations,IfyouassignedanegativevalueforradiusinListing2.1,ComputeArea.java,theprogramwouldprintaninvalidresult.Iftheradiusisnegative,youdontwanttheprogramtocomputethearea.Howcanyoudealwiththissituation?,Liang,IntroductiontoJavaProgramming,EighthEdition,(c)2011PearsonEducation,Inc.Allrightsreserved.0132130807,3,Objectives,TodeclarebooleantypeandwriteBooleanexpressionsusingcomparisonoperators(3.2).ToprogramAdditionQuizusingBooleanexpressions(3.3).Toimplementselectioncontrolusingone-wayifstatements(3.4)ToprogramtheGuessBirthdaygameusingone-wayifstatements(3.5).Toimplementselectioncontrolusingtwo-wayifstatements(3.6).Toimplementselectioncontrolusingnestedifstatements(3.7).Toavoidcommonerrorsinifstatements(3.8).Toprogramusingselectionstatementsforavarietyofexamples(BMI,ComputeTax,SubtractionQuiz)(3.9-3.11).TogeneraterandomnumbersusingtheMath.random()method(3.9).Tocombineconditionsusinglogicaloperators(,5,ComparisonOperators,OperatorNamegreaterthan=greaterthanorequalto=equalto!=notequalto,6,Problem:ASimpleMathLearningTool,AdditionQuiz,Run,Thisexamplecreatesaprogramtoletafirstgraderpracticeadditions.Theprogramrandomlygeneratestwosingle-digitintegersnumber1andnumber2anddisplaysaquestionsuchas“Whatis7+9?”tothestudent.Afterthestudenttypestheanswer,theprogramdisplaysamessagetoindicatewhethertheansweristrueorfalse.,7,One-wayifStatements,if(boolean-expression)statement(s);,if(radius=0)area=radius*radius*PI;System.out.println(Thearea+forthecircleofradius+radius+is+area);,8,Note,9,SimpleifDemo,SimpleIfDemo,Run,Writeaprogramthatpromptstheusertoenteraninteger.Ifthenumberisamultipleof5,printHiFive.Ifthenumberisdivisibleby2,printHiEven.,10,Problem:GuessingBirthday,GuessBirthday,Theprogramcanguessyourbirthdate.Runtoseehowitworks.,11,MathematicsBasisfortheGame,19is10011inbinary.7is111inbinary.23is11101inbinary,12,TheTwo-wayifStatement,if(boolean-expression)statement(s)-for-the-true-case;elsestatement(s)-for-the-false-case;,13,if.elseExample,if(radius=0)area=radius*radius*3.14159;System.out.println(Theareaforthe“+“circleofradius+radius+is+area);elseSystem.out.println(Negativeinput);,14,MultipleAlternativeifStatements,15,Traceif-elsestatement,if(score=90.0)grade=A;elseif(score=80.0)grade=B;elseif(score=70.0)grade=C;elseif(score=60.0)grade=D;elsegrade=F;,Supposescoreis70.0,Theconditionisfalse,animation,16,Traceif-elsestatement,if(score=90.0)grade=A;elseif(score=80.0)grade=B;elseif(score=70.0)grade=C;elseif(score=60.0)grade=D;elsegrade=F;,Supposescoreis70.0,Theconditionisfalse,animation,17,Traceif-elsestatement,if(score=90.0)grade=A;elseif(score=80.0)grade=B;elseif(score=70.0)grade=C;elseif(score=60.0)grade=D;elsegrade=F;,Supposescoreis70.0,Theconditionistrue,animation,18,Traceif-elsestatement,if(score=90.0)grade=A;elseif(score=80.0)grade=B;elseif(score=70.0)grade=C;elseif(score=60.0)grade=D;elsegrade=F;,Supposescoreis70.0,gradeisC,animation,19,Traceif-elsestatement,if(score=90.0)grade=A;elseif(score=80.0)grade=B;elseif(score=70.0)grade=C;elseif(score=60.0)grade=D;elsegrade=F;,Supposescoreis70.0,Exittheifstatement,animation,20,Note,Theelseclausematchesthemostrecentifclauseinthesameblock.,21,Note,cont.,Nothingisprintedfromtheprecedingstatement.Toforcetheelseclausetomatchthefirstifclause,youmustaddapairofbraces:inti=1;intj=2;intk=3;if(ij)if(ik)System.out.println(A);elseSystem.out.println(B);ThisstatementprintsB.,22,CommonErrors,Addingasemicolonattheendofanifclauseisacommonmistake.if(radius=0);area=radius*radius*PI;System.out.println(Theareaforthecircleofradius+radius+is+area);Thismistakeishardtofind,becauseitisnotacompilationerrororaruntimeerror,itisalogicerror.Thiserroroftenoccurswhenyouusethenext-lineblockstyle.,Wrong,23,TIP,24,CAUTION,25,Problem:AnImprovedMathLearningTool,Thisexamplecreatesaprogramtoteachafirstgradechildhowtolearnsubtractions.Theprogramrandomlygeneratestwosingle-digitintegersnumber1andnumber2withnumber1number2anddisplaysaquestionsuchas“Whatis92?”tothestudent.Afterthestudenttypestheanswerintheinputdialogbox,theprogramdisplaysamessagedialogboxtoindicatewhethertheansweriscorrect.,SubtractionQuiz,26,Problem:BodyMassIndex,BodyMassIndex(BMI)isameasureofhealthonweight.Itcanbecalculatedbytakingyourweightinkilogramsanddividingbythesquareofyourheightinmeters.TheinterpretationofBMIforpeople16yearsorolderisasfollows:,ComputeBMI,27,Problem:ComputingTaxes,TheUSfederalpersonalincometaxiscalculatedbasedonthefilingstatusandtaxableincome.Therearefourfilingstatuses:singlefilers,marriedfilingjointly,marriedfilingseparately,andheadofhousehold.Thetaxratesfor2009areshownbelow.,28,Problem:ComputingTaxes,cont.,ComputeTax,if(status=0)/Computetaxforsinglefilerselseif(status=1)/Computetaxformarriedfilejointlyelseif(status=2)/Computetaxformarriedfileseparatelyelseif(status=3)/Computetaxforheadofhouseholdelse/Displaywrongstatus,29,LogicalOperators,OperatorName!notbreak;case1:computetaxesformarriedfilejointly;break;case2:computetaxesformarriedfileseparately;break;case3:computetaxesforheadofhousehold;break;default:System.out.println(Errors:invalidstatus);System.exit(0);,44,switchStatementFlowChart,45,switchStatementRules,switch(switch-expression)casevalue1:statement(s)1;break;casevalue2:statement(s)2;break;casevalueN:statement(s)N;break;default:statement(s)-for-default;,46,switchStatementRules,Thekeywordbreakisoptional,butitshouldbeusedattheendofeachcaseinordertoterminatetheremainderoftheswitchstatement.Ifthebreakstatementisnotpresent,thenextcasestatementwillbeexecuted.,switch(switch-expression)casevalue1:statement(s)1;break;casevalue2:statement(s)2;break;casevalueN:statement(s)N;break;default:statement(s)-for-default;,Thecasestatementsareexecutedinsequentialorder,buttheorderofthecases(includingthedefaultcase)doesnotmatter.However,itisgoodprogrammingstyletofollowthelogicalsequenceofthecasesandplacethedefaultcaseattheend.,47,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);,Supposechisa:,animation,48,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);,chisa:,animation,49,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);,Executethisline,animation,50,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);,Executethisline,animation,51,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);,Executethisline,animation,52,Traceswitchstatement,switch(ch)casea:System.out.println(ch);caseb:System.out.println(ch);casec:System.out.println(ch);Nextstatement;,Executenextstatement,animation,53,Traceswitchstatement,switch(ch)casea:System.out.println(ch);break;caseb:System.out.println(ch);break;casec:System.out.println(ch);,Supposechisa:,animation,54,Traceswitchstatement,switch(ch)casea:System.out.println(ch);break;caseb:System.out.println(ch);break;casec:System.out.println(ch);,chisa:,animation,55,Traceswitchstatement,switch(ch)casea:System.out.println(ch);break;caseb:System.out.println(ch);break;casec:System.out.println(ch);,Executethisline,animation,56,Traceswitchstatement,switch(ch)casea:System.out.println(ch);break;caseb:System.out.println(ch);break;casec:System.out.println(ch);,Executethisline,animation,57,Traceswitchstatement,switch(ch)casea:System.out.println(ch);break;caseb:System.out.println(ch);break;casec:System.out.println(ch);Nextstatement;,Executenextstatement,animation,58,ConditionalOperator,if(x0)y=1elsey=-1;isequivalenttoy=(x0)?1:-1;(boolean-expression)?expression1:expression2TernaryoperatorBinaryoperatorUnaryoperator,59,ConditionalOperator,if(num%2=0)System.out.println(num+“iseven”);elseSystem.out.println(num+“isodd”);System.out.println(num%2=0)?num+“iseven”:num+“isodd”