JAVA程序设计教程第七版第6章.ppt

Chapter6MoreConditionalsandLoops JavaSoftwareSolutionsFoundationsofProgramDesignSeventhEdition MoreConditionalsandLoops NowwecanfillinsomeadditionaldetailsregardingJavaconditionalandrepetitionstatementsChapter6focuseson theswitchstatementtheconditionaloperatorthedolooptheforloopdrawingwiththeaidofconditionalsandloopsdialogboxes Outline TheswitchStatementTheConditionalOperatorThedoStatementTheforStatementDrawingwithLoopsandConditionalsDialogBoxes TheswitchStatement TheswitchstatementprovidesanotherwaytodecidewhichstatementtoexecutenextTheswitchstatementevaluatesanexpression thenattemptstomatchtheresulttooneofseveralpossiblecasesEachcasecontainsavalueandalistofstatementsTheflowofcontroltransferstostatementassociatedwiththefirstcasevaluethatmatches TheswitchStatement Thegeneralsyntaxofaswitchstatementis switch expression casevalue1 statement list1casevalue2 statement list2casevalue3 statement list3case Ifexpressionmatchesvalue2 controljumpstohere TheswitchStatement Oftenabreakstatementisusedasthelaststatementineachcase sstatementlistAbreakstatementcausescontroltotransfertotheendoftheswitchstatementIfabreakstatementisnotused theflowofcontrolwillcontinueintothenextcaseSometimesthismaybeappropriate butoftenwewanttoexecuteonlythestatementsassociatedwithonecase TheswitchStatement switch option case A aCount break case B bCount break case C cCount break Anexampleofaswitchstatement TheswitchStatement AswitchstatementcanhaveanoptionaldefaultcaseThedefaultcasehasnoassociatedvalueandsimplyusesthereservedworddefaultIfthedefaultcaseispresent controlwilltransfertoitifnoothercasevaluematchesIfthereisnodefaultcase andnoothervaluematches controlfallsthroughtothestatementaftertheswitch TheswitchStatement Thetypeofaswitchexpressionmustbeintegers 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 continue continueswitch 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 continueswitch 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 Outline TheswitchStatementTheConditionalOperatorThedoStatementTheforStatementDrawingwithLoopsandConditionalsDialogBoxes TheConditionalOperator TheconditionaloperatorevaluatestooneoftwoexpressionsbasedonabooleanconditionItssyntaxis condition expression1 expression2Iftheconditionistrue expression1isevaluated ifitisfalse expression2isevaluatedThevalueoftheentireconditionaloperatoristhevalueoftheselectedexpression TheConditionalOperator Theconditionaloperatorissimilartoanif elsestatement exceptthatitisanexpressionthatreturnsavalueForexample larger num1 num2 num1 num2 Ifnum1isgreaterthannum2 thennum1isassignedtolarger otherwise num2isassignedtolargerTheconditionaloperatoristernarybecauseitrequiresthreeoperands TheConditionalOperator Anotherexample Ifcountequals1 the Dime isprintedIfcountisanythingotherthan1 then Dimes isprinted System out println Yourchangeis count count 1 Dime Dimes QuickCheck Expressthefollowinglogicinasuccinctmannerusingtheconditionaloperator if val 10 System out println Itisnotgreaterthan10 elseSystem out println Itisgreaterthan10 QuickCheck Expressthefollowinglogicinasuccinctmannerusingtheconditionaloperator if val 10 System out println Itisnotgreaterthan10 elseSystem out println Itisgreaterthan10 System out println Itis val 10 not greaterthan10 Outline TheswitchStatementTheConditionalOperatorThedoStatementTheforStatementDrawingwithLoopsandConditionalsDialogBoxes ThedoStatement Adostatementhasthefollowingsyntax do statement list while condition Thestatement listisexecutedonceinitially andthentheconditionisevaluatedThestatementisexecutedrepeatedlyuntiltheconditionbecomesfalse LogicofadoLoop true false ThedoStatement Anexampleofadoloop ThebodyofadoloopexecutesatleastonceSeeReverseNumber java intcount 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 continue continueSystem 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 continueSystem 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 2896Thatnumberreversedis6982 Comparingwhileanddo Outline TheswitchStatementTheConditionalOperatorThedoStatementTheforStatementDrawingwithLoopsandConditionalsDialogBoxes TheforStatement Aforstatementhasthefollowingsyntax for initialization condition increment statement Logicofaforloop false TheforStatement Aforloopisfunctionallyequivalenttothefollowingwhileloopstructure initialization while condition statement increment TheforStatement Anexampleofaforloop for intcount 1 count 5 count System out println count TheinitializationsectioncanbeusedtodeclareavariableLikeawhileloop theconditionofaforloopistestedpriortoexecutingtheloopbodyTherefore thebodyofaforloopwillexecutezeroormoretimes TheforStatement Theincrementsectioncanperformanycalculation 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 continue continueSystem 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 continueSystem 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 714212835424956637077849198105112119126133140147154161168175182189196203210217224231238245252259266273280287294301308315322329336343350357364371378385392399 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 QuickCheck Writeacodefragmentthatrollsadie100timesandcountsthenumberoftimesa3comesup QuickCheck Writeacodefragmentthatrollsadie100timesandcountsthenumberoftimesa3comesup Diedie newDie intcount 0 for intnum 1 num 100 num if die roll 3 count Sytem out println count TheforStatement EachexpressionintheheaderofaforloopisoptionalIftheinitializationisleftout noinitializationisperformedIftheconditionisleftout itisalwaysconsideredtobetrue andthereforecreatesaninfiniteloopIftheincrementisleftout noincrementoperationisperformed For eachLoops AvariantoftheforloopsimplifiestherepetitiveprocessingofitemsinaniteratorForexample supposebookListisanArrayListobjectThefollowingloopwillprinteachbook for BookmyBook bookList System out println myBook Thisversionofaforloopisoftencalledafor eachloop For eachLoops Afor eachloopcanbeusedonanyobjectthatimplementstheIterableinterfaceIteliminatestheneedtoretrieveaniteratorandcallthehasNextandnextmethodsexplicitlyItalsowillbehelpfulwhenprocessingarrays whicharediscussedinChapter8 QuickCheck Writeafor eachloopthatprintsalloftheStudentobjectsinanArrayListobjectcalledroster QuickCheck Writeafor eachloopthatprintsalloftheStudentobjectsinanArrayListobjectcalledroster for Studentstudent roster System out println student Outline TheswitchStatementTheConditionalOperatorThedoStatementTheforStatementDrawingwithLoopsandConditionalsDialogBoxes DrawingTechniques ConditionalsandloopsenhanceourabilitytogenerateinterestinggraphicsSeeBullseye 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 continue continue 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 continue continue 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 continue continueif width THICKNESS checkfornarrowbox page setColor Color yellow page fillRect x y width height elseif height THICKNESS checkforshortbox page setColor Color green page fillRect x y width height else page setColor Color white page drawRect x y width height