Java notes.docx

Java notes.docx

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Javanotes

《CoreJavaI9th》

1.Thestandardnamingconventionisthatclassnamesarenounsthatstartwithanuppercaseletter.Ifanameconsistsofmultiplewords,useaninitialuppercaseletterineachofthewords.（Thisuseofuppercaselettersinthemiddleofawordissometimescalled“camelcase”or,self-referentially,“CamelCase”.）

2.Youneedtomakethefilenameforthesourcecodethesameasthenameofthepublicclass,withtheextension.javaappended.Thus,youmuststorethiscodeinafilecalledFirstSample.java.（Again,caseisimportant—don’tusefirstsample.java.）Ifyouhavenamedthefilecorrectlyandnotmadeanytypos（错别字）inthesourcecode,thenwhenyoucompilethissourcecode,youendupwithafilecontainingthebytecodes（字节码）forthisclass.TheJavacompilerautomaticallynamesthebytecodefileFirstSample.classandstoresitinthesamedirectoryasthesourcefile.Finally,launchtheprogrambyissuingthefollowingcommand:

javaFirstSample（Remembertoleaveoffthe.classextension.）Whentheprogramexecutes,itsimplydisplaysthestringWewillnotuse'Hello,World!

'ontheconsole.WhenyouusejavaClassNametorunacompiledprogram,theJavavirtualmachinealwaysstartsexecutionwiththecodeinthemainmethodintheclassyouindicate.（youmusthaveamainmethodinthesourcefileforyourclassforyourcodetoexecute）。

themainmethodmustbedeclaredpublic.

3.asinC/C++,thevoidkeywordindicatesthatthismethoddoesnotreturnavalue.UnlikeC/C++,themainmethoddoesnotreturnan“exitcode”totheoperatingsystem.Ifthemainmethodexitsnormally,theJavaprogramhastheexitcode0,indicatingsuccessfulcompletion.Toterminate（使终止）theprogramwithadifferentexitcode,usetheSystem.exitmethod.

4.Javausesthegeneralsyntax：

object（对象）.method（parameters（参数））asitsequivalentofafunctioncall.

5./**/commentsdonotnest（嵌套）inJava.Thatis,youmaynotbeabletodeactivatecodesimplybysurroundingitwith/*and*/becausethecodeyouwanttodeactivatemightitselfcontaina*/delimiter.

6.Byte:

1字节（-128_127）short:

2字节（-32768_32767）int:

4字节（-2147483648_2147483647）long:

8字节（-9223372036854775808_9223372036854775807）

7.UnderJava,therangesoftheintegertypesdonotdependonthemachineonwhichyouwillberunningtheJavacode.Thisalleviates（减轻）amajorpainfortheprogrammerwhowantstomovesoftwarefromoneplatformtoanother,orevenbetweenoperatingsystemsonthesameplatform.Javadoesnothaveanyunsignedtypes

8.Longintegernumbershaveasuffix（后缀）L（forexample,4000000000L）.Hexadecimal（十六进制）numbershaveaprefix（前缀）0x（forexample,0xCAFE）.Octal（八进制）numbershaveaprefix0.Forexample,010is8.Naturally,thiscanbeconfusing,sowerecommendagainsttheuseofoctalconstants.StartingwithJava7,youcanwritenumbersinbinary,withaprefix0b.Forexample,0b1001is9.AlsostartingwithJava7,youcanaddunderscorestonumberliterals,suchas1_000_000（or0b1111_0100_0010_0100_0000）todenote（标志）onemillion.Theunderscoresareforhumaneyesonly.TheJavacompilersimplyremovesthem.Float:

4字节（+_3.40282347E+38F6-7位有效数字）double:

8字节（15位有效数字）

9.Thenamedoublereferstothefactthatthesenumbershavetwicetheprecision（精度）ofthefloattype.（Somepeoplecallthesedouble-precisionnumbers.）Here,thetypetochooseinmostapplicationsisdouble.NumbersoftypefloathaveasuffixF（forexample,3.14F）.Floating-pointnumberswithoutanFsuffix（suchas3.14）arealwaysconsideredtobeoftypedouble.YoucanoptionallysupplytheDsuffix（forexample,3.14D）.Youcanspecify（指定,规定）floating-pointliteralsinhexadecimal（十六进制的）.Forexample,0.125=2–3canbewrittenas0x1.0p-3.Inhexadecimalnotation,youuseap,notane,todenotetheexponent.（Aneisahexadecimaldigit.）Note（注意）thatthemantissa（浮点数）iswritteninhexadecimalandtheexponent（指数）indecimal（十进制）.Thebaseoftheexponentis2,not10.

10.Allfloating-pointcomputationsfollowtheIEEE754specification.Inparticular,therearethreespecialfloating-pointvaluestodenoteoverflowsanderrors:

•Positiveinfinity（正无穷大）•Negativeinfinity（负无穷大）•NaN（notanumber）Forexample,theresultofdividingapositivenumberby0ispositiveinfinity.Computing0/0orthesquarerootofanegativenumberyieldsNaN.

11.Theconstants（常量）Double.POSITIVE_INFINITY,Double.NEGATIVE_INFINITY,andDouble.NaN（aswellascorrespondingFloatconstants）representthesespecialvalues,buttheyarerarelyusedinpractice.Inparticular,youcannottestif（x==Double.NaN）//isnevertruetocheckwhetheraparticularresultequalsDouble.NaN.All“notanumber”valuesareconsidereddistinct.However,youcanusetheDouble.isNaNmethod:

if（Double.isNaN（x））//checkwhetherxis"notanumber".Floating-pointnumbersarenotsuitableforfinancialcalculationsinwhichroundofferrorscannotbetolerated.Forexample,thecommandSystem.out.println（2.0-1.1）prints0.8999999999999999,not0.9asyouwouldexpect.Suchroundofferrorsarecausedbythefactthatfloating-pointnumbersarerepresentedinthebinarynumbersystem.Thereisnoprecisebinaryrepresentationofthefraction1/10,justasthereisnoaccuraterepresentationofthefraction1/3inthedecimalsystem.Ifyouneedprecisenumericalcomputationswithoutroundofferrors,usetheBigDecimalclass,whichisintroducedlaterinthischapter.

12.Thechartypeisusedtodescribeindividualcharacters.Mostcommonly,thesewillbecharacterconstants.Forexample,'A'isacharacterconstantwithvalue65.Itisdifferentfrom"A",astringcontainingasinglecharacter.Unicodecodeunitscanbeexpressedashexadecimalvaluesthatrunfrom\u0000to\uFFFF.Forexample,\u2122isthetrademarksymbol（™）and\u03C0istheGreekletterpi（π）.Besidesthe\uescapesequences（转义字符）thatindicatetheencoding（编码）ofUnicodecodeunits,thereareseveralescapesequencesforspecialcharacters,asshowninTable3.3.Youcanusetheseescapesequencesinsidequotedcharacterconstantsandstrings,suchas'\u2122'or"Hello\n".The\uescapesequence（butnoneoftheotherescapesequences）canevenbeusedoutsidequotedcharacterconstantsandstrings.Forexample,publicstaticvoidmain（String\u005B\u005Dargs）

Important:

isperfectlylegal—\u005Band\u005Daretheencodingsfor[and].

13.Youcannotconvertbetweenintegersandbooleanvalues.YoualsocannotuseaJavareservedwordforavariablename.Youcanhavemultipledeclarationsonasingleline:

inti,j;//bothareintegers.Asyousaw,namesarecasesensitive,forexample,hiredayandhireDayaretwoseparatenames.Ingeneral,youshouldnothavetwonamesthatonlydifferintheirlettercase.

14.Afteryoudeclareavariable（变量）,youmustexplicitly（明确的）initialize（初始化）itbymeansofanassignment（赋值）statement—youcanneverusethevalueofanuninitializedvariable.

15.Thekeywordfinalindicatesthatyoucanassigntothevariableonce,andthenitsvalueissetonceandforall.Itiscustomarytonameconstantsinalluppercase.ItisprobablymorecommoninJavatocreateaconstantsoit’savailabletomultiplemethodsinsideasingleclass.Theseareusuallycalledclassconstants.Setupaclassconstantwiththekeywordsstaticfinal.Hereisanexampleofusingaclassconstant:

publicclassConstants2

{

publicstaticfinaldoubleCM_PER_INCH=2.54;

publicstaticvoidmain（String[]args）

{

^^^^^^;

}

}

Notethatthedefinitionoftheclassconstantappearsoutsidethemainmethod.Thus,theconstantcanalsobeusedinothermethodsofthesameclass.Furthermore,iftheconstantisdeclared,asinourexample,public,methodsofotherclassescanalsouseit—inourexample,asConstants2.CM_PER_INCH.

16.Thereisaconvenientshortcutforusingbinary（二元运算符）arithmeticoperatorsinanassignment.Forexample,x+=4;isequivalenttox=x+4;（Ingeneral,placetheoperator（运算符）totheleftofthe=sign,suchas*=or%=.）

inta=2*++m;//nowais16,mis8

intb=2*n++;//nowbis14,nis8

17.Totestforequalityyouuseadoubleequalsign,==.Forexample,thevalueof3==7isfalse.Usea!

=forinequality.Forexample,thevalueof3!

=7istrue.Finally,youhavetheusual<（lessthan）,>（greaterthan）,<=（lessthanorequal）,and>=（greaterthanorequal）operators.Javauses&&forthelogical“and”operatorand||forthelogical“or”operator.Asyoucaneasilyrememberfromthe!

=operator,theexclamationpoint!

isthelogicalnegationoperator.The&&and||operatorsareevaluatedin“shortcircuit”fashion:

Thesecondargumentisnotevaluatedifthefirstargumentalreadydeterminesthevalue.Ifyoucombinetwoexpressionswiththe&&operator,expression1&&expression2andthetruthvalueofthefirstexpressionhasbeendeterminedtobefalse,thenitisimpossiblefortheresulttobetrue.Thus,thevalueforthesecondexpressionisnotcalculated.Thisbehaviorcanbeexploited（利用）toavoiderrors.Forexample,intheexpressionx!

=0&&1/x>x+y//nodivisionby0thesecondpartisneverevaluatedifxequalszero.Thus,1/xisnotcomputedifxiszero,andnodivide-by-zeroerrorcanoccur.Similarly,thevalueofexpression1||expression2isautomaticallytrueifthefirstexpressionistrue,withoutevaluatingthesecondexpression.Whenappliedtobooleanvalues,the&and|operatorsyieldabooleanvalue.Theseoperatorsaresimilartothe&&and||operators,exceptthatthe&and|operatorsarenotevaluatedin“shortcircuit”fashion.Thatis,bothargumentsareevaluatedbeforetheresultiscomputed.Finally,Javasupportstheternary（三元运算符）?

:

operatorthatisoccasionallyuseful.Theexpressioncondition?

expression1:

expression2evalu