java语言程序设计基础篇(第八版)第九章课件

Chapter9StringsandTextI/O1MotivationsOftenyouencountertheproblemsthatinvolvestringprocessingandandoutput.Supposeyouneedtowriteaprogramtoreplacealloccurrencesofawordwithanewwordinafile.Howdoyousolvethisproblem?Thischapterintroducesstringsandtextfiles,whichwillenableyoutosolvethisproblem.2ObjectivesTousetheStringclasstoprocessfixedstrings(§9.2).TousetheCharacterclasstoprocessasinglecharacter(§9.3).TousetheStringBuilder/StringBufferclasstoprocessflexiblestrings(§9.4).TodistinguishamongtheString,StringBuilder,andStringBufferclasses(§9.2-9.4).Tolearnhowtopassargumentstothemainmethodfromthecommandline(§9.5).TodiscoverandtodeleteandrenamefilesusingtheFileclass(§9.6).TowritedatatoathePrintWriterclass(§9.7.1).ToreaddatafromatheScannerclass(§9.7.2).(GUI)Toopenfilesusingadialogbox(§9.8).3TheStringClassConstructingaString:Stringmessage="WelcometoJava“;Stringmessage=newString("WelcometoJava“);Strings=newString();ObtainingStringlengthandRetrievingIndividualCharactersinastringStringConcatenation(concat)Substrings(substring(index),substring(start,end))Comparisons(equals,compareTo)StringConversionsFindingaCharacteroraSubstringinaStringConversionsbetweenStringsandArraysConvertingCharactersandNumericValuestoStrings4ConstructingStringsStringnewString=newString(stringLiteral);

Stringmessage=newString("WelcometoJava");Sincestringsareusedfrequently,Javaprovidesashorthandinitializerforcreatingastring:Stringmessage="WelcometoJava";5StringsAreImmutableAStringobjectisimmutable;itscontentscannotbechanged.Doesthefollowingcodechangethecontentsofthestring?Strings="Java";s="HTML";6TraceCode

Strings="Java";s="HTML";animation7TraceCode

Strings="Java";s="HTML";animation8InternedStringsSincestringsareimmutableandarefrequentlyused,toimproveefficiencyandsavememory,theJVMusesauniqueinstanceforstringliteralswiththesamecharactersequence.Suchaninstanceiscalledinterned.Forexample,thefollowingstatements:9Examplesdisplay

s1==sisfalses1==s3istrueAnewobjectiscreatedifyouusethenewoperator.Ifyouusethestringinitializer,nonewobjectiscreatediftheinternedobjectisalreadycreated.10TraceCodeanimation11TraceCode12TraceCode13StringComparisons14StringComparisonsequals Strings1=newString("Welcome“); Strings2="welcome";

if(s1.equals(s2)){//s1ands2havethesamecontents

}

if(s1==s2){//s1ands2havethesamereference

}15StringComparisons,cont.compareTo(Objectobject) Strings1=newString("Welcome“); Strings2="welcome";

if(pareTo(s2)>0){//s1isgreaterthans2}elseif(pareTo(s2)==0){//s1ands2havethesamecontents}else//s1islessthans216StringLength,Characters,andCombiningStrings17FindingStringLengthFindingstringlengthusingthelength()method:message="Welcome";message.length()(returns7)18RetrievingIndividualCharactersinaStringDonotusemessage[0]Usemessage.charAt(index)Indexstartsfrom019StringConcatenationStrings3=s1.concat(s2);Strings3=s1+s2;s1+s2+s3+s4+s5sameas(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);20ExtractingSubstrings21ExtractingSubstringsYoucanextractasinglecharacterfromastringusingthecharAtmethod.YoucanalsoextractasubstringfromastringusingthesubstringmethodintheStringclass.Strings1="WelcometoJava";Strings2=s1.substring(0,11)+"HTML";22Converting,Replacing,andSplittingStrings23Examples"Welcome".toLowerCase()returnsanewstring,welcome."Welcome".toUpperCase()returnsanewstring,WELCOME."Welcome".trim()returnsanewstring,Welcome."Welcome".replace('e','A')returnsanewstring,WAlcomA."Welcome".replaceFirst("e","AB")returnsanewstring,WABlcome."Welcome".replace("e","AB")returnsanewstring,WABlcomAB."Welcome".replace("el","AB")returnsanewstring,WABlcome.24SplittingaStringString[]tokens="Java#HTML#Perl".split("#",0);for(inti=0;i<tokens.length;i++)System.out.print(tokens[i]+"");JavaHTMLPerldisplays25Matching,ReplacingandSplittingbyPatterns

Youcanmatch,replace,orsplitastringbyspecifyingapattern.Thisisanextremelyusefulandpowerfulfeature,commonlyknownasregularexpression.Regularexpressioniscomplextobeginningstudents.Forthisreason,twosimplepatternsareusedinthissection.PleaserefertoSupplementIII.F,“RegularExpressions,”forfurtherstudies.

"Java".matches("Java");"Java".equals("Java");"Javaisfun".matches("Java.*");"Javaiscool".matches("Java.*");26Matching,ReplacingandSplittingbyPatterns

ThereplaceAll,replaceFirst,andsplitmethodscanbeusedwitharegularexpression.Forexample,thefollowingstatementreturnsanewstringthatreplaces$,+,or#in"a+b$#c"bythestringNNN.Strings="a+b$#c".replaceAll("[$+#]","NNN");System.out.println(s);Heretheregularexpression[$+#]specifiesapatternthatmatches$,+,or#.So,theoutputisaNNNbNNNNNNc.

27Matching,ReplacingandSplittingbyPatterns

Thefollowingstatementsplitsthestringintoanarrayofstringsdelimitedbysomepunctuationmarks.String[]tokens="Java,C?C#,C++".split("[.,:;?]");

for(inti=0;i<tokens.length;i++)System.out.println(tokens[i]);28FindingaCharacteroraSubstringinaString29FindingaCharacteroraSubstringinaString"WelcometoJava".indexOf('W')

returns0."WelcometoJava".indexOf('x')

returns-1."WelcometoJava".indexOf('o',5)

returns9."WelcometoJava".indexOf("come")

returns3."WelcometoJava".indexOf("Java",5)

returns11."WelcometoJava".indexOf("java",5)

returns-1."WelcometoJava".lastIndexOf('a')

returns14.

30ConvertCharacterandNumberstoStringsTheStringclassprovidesseveralstaticvalueOfmethodsforconvertingacharacter,anarrayofcharacters,andnumericvaluestostrings.ThesemethodshavethesamenamevalueOfwithdifferentargumenttypeschar,char[],double,long,int,andfloat.Forexample,toconvertadoublevaluetoastring,useString.valueOf(5.44).Thereturnvalueisstringconsistsofcharacters‘5’,‘.’,‘4’,and‘4’.31Problem:FindingPalindromesObjective:Checkingwhetherastringisapalindrome:astringthatreadsthesameforwardandbackward.CheckPalindromeRun32TheCharacterClass33ExamplesCharactercharObject=newCharacter('b');charOpareTo(newCharacter('a'))returns1charOpareTo(newCharacter('b'))returns0charOpareTo(newCharacter('c'))returns-1charOpareTo(newCharacter('d')returns–2charObject.equals(newCharacter('b'))returnstruecharObject.equals(newCharacter('d'))returnsfalse34Problem:CountingEachLetterinaStringThisexamplegivesaprogramthatcountsthenumberofoccurrenceofeachletterinastring.Assumethelettersarenotcase-sensitive.

CountEachLetterRun35StringBuilderandStringBufferTheStringBuilder/StringBufferclassisanalternativetotheStringclass.Ingeneral,aStringBuilder/StringBuffercanbeusedwhereverastringisused.StringBuilder/StringBufferismoreflexiblethanString.Youcanadd,insert,orappendnewcontentsintoastringbuffer,whereasthevalueofaStringobjectisfixedoncethestringiscreated.36StringBuilderConstructors37ModifyingStringsintheBuilder38ExamplesstringBuilder.append("Java");stringBuilder.insert(11,"HTMLand");stringBuilder.delete(8,11)changesthebuildertoWelcomeJava.stringBuilder.deleteCharAt(8)changesthebuildertoWelcomeoJava.stringBuilder.reverse()changesthebuildertoavaJotemocleW.stringBuilder.replace(11,15,"HTML")changesthebuildertoWelcometoHTML.stringBuilder.setCharAt(0,'w')setsthebuildertowelcometoJava.39ThetoString,capacity,length,setLength,andcharAtMethods40Problem:CheckingPalindromesIgnoringNon-alphanumericCharactersThisexamplegivesaprogramthatcountsthenumberofoccurrenceofeachletterinastring.Assumethelettersarenotcase-sensitive.PalindromeIgnoreNonAlphanumericRun41MainMethodIsJustaRegularMethodYoucancallaregularmethodbypassingactualparameters.Canyoupassargumentstomain?Ofcourse,yes.Forexample,themainmethodinclassBisinvokedbyamethodinA,asshownbelow:42Command-LineParametersclassTestMain{ publicstaticvoidmain(String[]args){...}}javaTestMainarg0arg1arg2...argn43Processing

Command-LineParametersInthemainmethod,gettheargumentsfromargs[0],args[1],...,args[n],whichcorrespondstoarg0,arg1,...,argninthecommandline.44Problem:CalculatorObjective:Writeaprogramthatwillperformbinaryoperationsonintegers.Theprogramreceivesthreeparameters:anoperatorandtwointegers.

CalculatorjavaCalculator2+3javaCalculator2-3RunjavaCalculator2/3javaCalculator2“*”345RegularExpressionsAregularexpression(abbreviatedregex)isastringthatdescribesapatternformatchingasetofstrings.Regularexpressionisapowerfultoolforstringmanipulations.Youcanuseregularexpressionsformatching,replacing,andsplittingstrings.CompanionWebsite46MatchingStrings"Java".matches("Java");"Java".equals("Java");"Javaisfun".matches("Java.*")"Javaiscool".matches("Java.*")"Javaispowerful".matches("Java.*")CompanionWebsite47RegularExpressionSyntaxCompanionWebsite48ReplacingandSplittingStringsCompanionWebsite49ExamplesStrings="JavaJavaJava".replaceAll("v\\w","wi");Strings="JavaJavaJava".replaceFirst("v\\w","wi");String[]s="Java1HTML2Perl".split("\\d");CompanionWebsite50TheTheFileclassisintendedtoprovideanabstractionthatdealswithmostofthemachine-dependentcomplexitiesoffilesandpathnamesinamachine-independentfashion.Theisastring.TheFileclassisawrapperclassfortheanditsdirectorypath.51Obtainingandmanipulatingfile52Problem:ExploreTestRunObjective:Writeaprogramthatdemonstrateshowtocreatefilesinap