Java課件第二章補充-數(shù)組

Chapter2Array2.1DeclaringandCreatingArrays2.4CopyingArrays2.2ArrayInitialize2.3Two-demensionArrays2.5PassingArraytomethods2021/7/131Chapter2Array2.1DeclariIntroducingArraysArrayisadatastructurethatrepresentsacollectionofthesametypesofdata.2021/7/132IntroducingArraysArrayisadDeclaringArrayVariablesdatatype[]arrayRefVar;

Example:double[]myList;datatypearrayRefVar[];//Thisstyleiscorrect,butnotpreferredExample:doublemyList[];inta[]；=int[]a；int[]a，b；≠inta[]，b；2021/7/133DeclaringArrayVariablesdatatCreatingArraysFormat:arrayRefVar=newdatatype[arraySize];Example:myList=newdouble[10];DefaultValue:Whenanarrayiscreated,itselementsareassignedthedefaultvalueof

0forthenumericprimitivedatatypes'\u0000'forchartypesfalseforbooleantypes2021/7/134CreatingArraysFormat:DefaultCreatingArrayse.g.:myList.lengthreturns10TheLengthofanArray:Onceanarrayiscreated,itssizeisfixed.Itcannotbechanged.Youcanfinditssizeusing

arrayRefVar.length2021/7/135CreatingArrayse.g.:myList.leusenewtocreat:c=newchar[5]；

\u0000

\u0000

\u0000

\u0000

\u0000

50x1234(chararray，5chardata)cCreatingArrays2021/7/136usenewtocreat:\u0000\u00CreatingArray：referencedatatypeArray:Point[]p=newPoint[3]；null

nullnull30x1245(PointArray，3Pointdata)pp[0]p[1]p[2]CreatingArrays2021/7/137CreatingArray：referencedataweshouldusenewtoallocatememoryforeveryelements.0,0p[0]0x1245p0,0null30x24640x2466p[1]p[0]=newPoint（）；p[1]=newPoint（）；CreatingArray：CreatingArrays2021/7/138weshouldusenewtoallocateDeclaringandCreatinginOneStepdatatype[]arrayRefVar=newdatatype[arraySize];e.g.:double[]myList=newdouble[10];datatypearrayRefVar[]=newdatatype[arraySize];e.g.:doublemyList[]=newdouble[10];2021/7/139DeclaringandCreatinginOne00030x4567ainta[]=newint[3]；a[0]a[1]a[2]a.lengthExample：DeclaringandCreatinginOneStep2021/7/131000030x4567ainta[]=newArrayInitializeDeclaring,creating,initializinginonestep:e.g.:int[]a={1，2，3，4，5}；String[]s={“abc”，“def”}；String[]s={newString(“abc”),newString(“def”)};Thisshorthandsyntaxmustbeinonestatement.2021/7/1311ArrayInitializeDeclaring,credouble[]myList={1.9,2.9,3.4,3.5};Thisshorthandnotationisequivalenttothefollowingstatements:double[]myList=newdouble[4];myList[0]=1.9;myList[1]=2.9;myList[2]=3.4;myList[3]=3.5;

ArrayInitialize2021/7/1312double[]myList={1.9,2.9,3CAUTIONUsingtheshorthandnotation,youhavetodeclare,create,andinitializethearrayallinonestatement.Splittingitwouldcauseasyntaxerror.Forexample,thefollowingiswrong:double[]myList;myList={1.9,2.9,3.4,3.5};2021/7/1313CAUTIONUsingtheshorthandnotArrayInitialize：

Case:

ArrayClassObj.java

Case:

ArrayInit.javaArraysInitializeExample2021/7/1314ArrayInitialize：Case:Arimportjava.util.*;publicclassArrayClassObj{ staticRandomrand=newRandom(); staticintpRand(intmod){ returnMath.abs(rand.nextInt())%mod; } publicstaticvoidmain(String[]args){ Integer[]a=newInteger[pRand(20)]; prt("lengthofa="+a.length); for(inti=0;i<a.length;i++){ a[i]=newInteger(pRand(500)); prt("a["+i+"]="+a[i]); } } staticvoidprt(Strings){ System.out.println(s); }}ArrayClassObj.java2021/7/1315importjava.util.*;ArrayClassOpublicclassArrayInit{ publicstaticvoidmain(String[]args){ Integer[]a={ newInteger(1), newInteger(2), newInteger(3), };

//Java1.1only: Integer[]b=newInteger[]{ newInteger(1), newInteger(2), newInteger(3), }; }}2021/7/1316publicclassArrayInit{2021/7Two-dimensionalArraysDeclarearrayrefvar:dataType[][]refVar;e.g.:CreatearrayandassignitsreferencetovariablerefVar=newdataType[10][10];CombinedeclarationandcreationinonestatementdataType[][]refVar=newdataType[10][10];AlternativesyntaxdataTyperefVar[][]=newdataType[10][10];inta[][]；int[]a[]；int[][]a；2021/7/1317Two-dimensionalArraysDeclareTwo-dimensionalArrayIllustration2021/7/1318Two-dimensionalArrayIllustraTwo-dimensionalArraysLengthofTwo-dimensionalArrays:e.g.:int[][]x=newint[3][4];2021/7/1319Two-dimensionalArraysLengthoCaution:Accessinganarrayoutofboundsisacommonprogrammingerror,whichthrowsaruntimeArrayIndexOutOfBoundsException.Toavoidit,makesurethatyoudonotuseanindexbeyondarrayRefVar.length-1.Two-dimensionalArrays2021/7/1320Caution:Accessinganarrayouint[][]array={{1,2,3},{4,5,6},{7,8,9},{10,11,12}};array.lengtharray[0].lengtharray[1].lengtharray[2].lengtharray[3].lengtharray[4].lengthArrayIndexOutOfBoundsExceptionTwo-dimensionalArraysExample：2021/7/1321int[][]array={array.lengthaCreating：【e.g.】int[][]a=newint[3][]；null

nullnull30x4978aa[0]a[1]a[2]a.lengthTwo-dimensionalArrays2021/7/1322Creating：【e.g.】int[][]a=nnull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[0][0]a[0][1]a[0][2]a[0].length0

0032021/7/1323nullnullnull30x4978aa[0]a[1]anull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[1]=newint[2]；a[0][0]a[0][1]a[0][2]a[0].length0

003002a[1][0]a[1][1]a[1].length2021/7/1324nullnullnull30x4978aa[0]a[1]anull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[1]=newint[2]；a[2]=newint[4]；a[0][0]a[0][1]a[0][2]a[0].length0

003002a[1][0]a[1][1]a[1].lengtha[2][0]a[2][1]a[2][2]a[2][3]00004a[2].length2021/7/1325nullnullnull30x4978aa[0]a[1]aDeclaring,Creating,andInitializing

UsingShorthandNotationsYoucanalsouseanarrayinitializertodeclare,createandinitializeatwo-dimensionalarray.Forexample,int[][]array=newint[4][3];array[0][0]=1;array[0][1]=2;array[0][2]=3;array[1][0]=4;array[1][1]=5;array[1][2]=6;array[2][0]=7;array[2][1]=8;array[2][2]=9;array[3][0]=10;array[3][1]=11;array[3][2]=12;

int[][]array={{1,2,3},{4,5,6},{7,8,9},{10,11,12}};Sameas2021/7/1326Declaring,Creating,andInitiEnhancedforLoopJDK1.5introducedanewforloopthatenablesyoutotraversethecompletearraysequentiallywithoutusinganindexvariable.

Ingeneral,thesyntaxis

Youstillhavetouseanindexvariableifyouwishtotraversethearrayinadifferentorderorchangetheelementsinthearray.for(elementTypevalue:arrayRefVar){//Processthevalue}

Arraytest.java2021/7/1327EnhancedforLoopJDK1.5introString[][]s=newString[10][];

a)Thislineofcodeisillegal.b)sisatwo-dimensionalarraycontaining10rowsand10columnsc)Eachelementinsissetto""d)Eachelementinsisuninitializedandmustbeinitializedbeforeitisreferenced.【Exercise】2021/7/1328String[][]s=newString[10CopyingArraysOften,inaprogram,youneedtoduplicateanarrayorapartofanarray.Insuchcasesyoucouldattempttousetheassignmentstatement(=),asfollows:

list2=list1;2021/7/1329CopyingArraysOften,inaprogCopyingArraysUsingaloop:int[]sourceArray={2,3,1,5,10};int[]targetArray=newint[sourceArray.length];for(inti=0;i<sourceArrays.length;i++)targetArray[i]=sourceArray[i];2021/7/1330CopyingArraysUsingaloop:202ThearraycopyUtilityarraycopy(sourceArray,src_pos,targetArray,tar_pos,length);Example:System.arraycopy(sourceArray,0,targetArray,0,sourceArray.length);2021/7/1331ThearraycopyUtilityarraycopyCopyingArray:System.arraycopy(object,int,object,int,int)inta[]={1,2,3};intb[]={4,5,6,7,8,9};

System.arraycopy(a,0,b,0,a.length);

b[]={1,2,3,7,8,9};Arraycopy.javaThearraycopyUtility2021/7/1332CopyingArray:Arraycopy.javaThPassingArraystoMethodspublicstaticvoidprintArray(int[]array){for(inti=0;i<array.length;i++){System.out.print(array[i]+"");}}Invokethemethodint[]list={3,1,2,6,4,2};printArray(list);InvokethemethodprintArray(newint[]{3,1,2,6,4,2});Anonymousarray2021/7/1333PassingArraystoMethodspubliAnonymousArrayThestatementprintArray(newint[]{3,1,2,6,4,2});createsanarrayusingthefollowingsyntax:

newdataType[]{literal0,...,literalk};Thereisnoexplicitreferencevariableforthearray.Sucharrayiscalledananonymousarray.

Case:

VarArgs.java2021/7/1334AnonymousArrayThestatementCclassA{inti;}publicclassVarArgs{ staticvoidf(Object[]x){ for(inti=0;i<x.length;i++) System.out.println(x[i]); } publicstaticvoidmain(String[]args){ f(newObject[]{ newInteger(47),newVarArgs(), newFloat(3.14),newDouble(11.11)}); f(newObject[]{"one","two","three"}); f(newObject[]{newA(),newA(),newA()}); }}VarArgs.java2021/7/1335classA{inti;}VarArgs.javaPassByValueJavausespassbyvaluetopassparameterstoamethod.Thereareimportantdifferencesbetweenpassingavalueofvariablesofprimitivedatatypesandpassingarrays.Foraparameterofaprimitivetypevalue,theactualvalueispassed.Changingthevalueofthelocalparameterinsidethemethoddoesnotaffectthevalueofthevariableoutsidethemethod.Foraparameterofanarraytype,thevalueoftheparametercontainsareferencetoanarray;thisreferenceispassedtothemethod.Anychangestothearraythatoccurinsidethemethodbodywillaffecttheoriginalarraythatwaspassedastheargument.2021/7/1336PassByValueJavausespassbypublicclassPassvalue{publicstaticvoidmain(String[]args){intx=1;//xrepresentsanintvalueint[]y=newint[10];//yrepresentsanarrayofintvalues

m(x,y);//Invokemwithargumentsxandy

System.out.println("xis"+x);System.out.println("y[0]is"+y[0]);}

publicstaticvoidm(intnumber,int[]numbers){number=1001;//Assignanewvaluetonumbernumbers[0]=5555;//Assignanewvaluetonumbers[0]}}SimpleExamplePassvalue.java2021/7/1337publicclassPassvalue{Simple問題？問題？Chapter2Array2.1DeclaringandCreatingArrays2.4CopyingArrays2.2ArrayInitialize2.3Two-demensionArrays2.5PassingArraytomethods2021/7/1339Chapter2Array2.1DeclariIntroducingArraysArrayisadatastructurethatrepresentsacollectionofthesametypesofdata.2021/7/1340IntroducingArraysArrayisadDeclaringArrayVariablesdatatype[]arrayRefVar;

Example:double[]myList;datatypearrayRefVar[];//Thisstyleiscorrect,butnotpreferredExample:doublemyList[];inta[]；=int[]a；int[]a，b；≠inta[]，b；2021/7/1341DeclaringArrayVariablesdatatCreatingArraysFormat:arrayRefVar=newdatatype[arraySize];Example:myList=newdouble[10];DefaultValue:Whenanarrayiscreated,itselementsareassignedthedefaultvalueof

0forthenumericprimitivedatatypes'\u0000'forchartypesfalseforbooleantypes2021/7/1342CreatingArraysFormat:DefaultCreatingArrayse.g.:myList.lengthreturns10TheLengthofanArray:Onceanarrayiscreated,itssizeisfixed.Itcannotbechanged.Youcanfinditssizeusing

arrayRefVar.length2021/7/1343CreatingArrayse.g.:myList.leusenewtocreat:c=newchar[5]；

\u0000

\u0000

\u0000

\u0000

\u0000

50x1234(chararray，5chardata)cCreatingArrays2021/7/1344usenewtocreat:\u0000\u00CreatingArray：referencedatatypeArray:Point[]p=newPoint[3]；null

nullnull30x1245(PointArray，3Pointdata)pp[0]p[1]p[2]CreatingArrays2021/7/1345CreatingArray：referencedataweshouldusenewtoallocatememoryforeveryelements.0,0p[0]0x1245p0,0null30x24640x2466p[1]p[0]=newPoint（）；p[1]=newPoint（）；CreatingArray：CreatingArrays2021/7/1346weshouldusenewtoallocateDeclaringandCreatinginOneStepdatatype[]arrayRefVar=newdatatype[arraySize];e.g.:double[]myList=newdouble[10];datatypearrayRefVar[]=newdatatype[arraySize];e.g.:doublemyList[]=newdouble[10];2021/7/1347DeclaringandCreatinginOne00030x4567ainta[]=newint[3]；a[0]a[1]a[2]a.lengthExample：DeclaringandCreatinginOneStep2021/7/134800030x4567ainta[]=newArrayInitializeDeclaring,creating,initializinginonestep:e.g.:int[]a={1，2，3，4，5}；String[]s={“abc”，“def”}；String[]s={newString(“abc”),newString(“def”)};Thisshorthandsyntaxmustbeinonestatement.2021/7/1349ArrayInitializeDeclaring,credouble[]myList={1.9,2.9,3.4,3.5};Thisshorthandnotationisequivalenttothefollowingstatements:double[]myList=newdouble[4];myList[0]=1.9;myList[1]=2.9;myList[2]=3.4;myList[3]=3.5;

ArrayInitialize2021/7/1350double[]myList={1.9,2.9,3CAUTIONUsingtheshorthandnotation,youhavetodeclare,create,andinitializethearrayallinonestatement.Splittingitwouldcauseasyntaxerror.Forexample,thefollowingiswrong:double[]myList;myList={1.9,2.9,3.4,3.5};2021/7/1351CAUTIONUsingtheshorthandnotArrayInitialize：

Case:

ArrayClassObj.java

Case:

ArrayInit.javaArraysInitializeExample2021/7/1352ArrayInitialize：Case:Arimportjava.util.*;publicclassArrayClassObj{ staticRandomrand=newRandom(); staticintpRand(intmod){ returnMath.abs(rand.nextInt())%mod; } publicstaticvoidmain(String[]args){ Integer[]a=newInteger[pRand(20)]; prt("lengthofa="+a.length); for(inti=0;i<a.length;i++){ a[i]=newInteger(pRand(500)); prt("a["+i+"]="+a[i]); } } staticvoidprt(Strings){ System.out.println(s); }}ArrayClassObj.java2021/7/1353importjava.util.*;ArrayClassOpublicclassArrayInit{ publicstaticvoidmain(String[]args){ Integer[]a={ newInteger(1), newInteger(2), newInteger(3), };

//Java1.1only: Integer[]b=newInteger[]{ newInteger(1), newInteger(2), newInteger(3), }; }}2021/7/1354publicclassArrayInit{2021/7Two-dimensionalArraysDeclarearrayrefvar:dataType[][]refVar;e.g.:CreatearrayandassignitsreferencetovariablerefVar=newdataType[10][10];CombinedeclarationandcreationinonestatementdataType[][]refVar=newdataType[10][10];AlternativesyntaxdataTyperefVar[][]=newdataType[10][10];inta[][]；int[]a[]；int[][]a；2021/7/1355Two-dimensionalArraysDeclareTwo-dimensionalArrayIllustration2021/7/1356Two-dimensionalArrayIllustraTwo-dimensionalArraysLengthofTwo-dimensionalArrays:e.g.:int[][]x=newint[3][4];2021/7/1357Two-dimensionalArraysLengthoCaution:Accessinganarrayoutofboundsisacommonprogrammingerror,whichthrowsaruntimeArrayIndexOutOfBoundsException.Toavoidit,makesurethatyoudonotuseanindexbeyondarrayRefVar.length-1.Two-dimensionalArrays2021/7/1358Caution:Accessinganarrayouint[][]array={{1,2,3},{4,5,6},{7,8,9},{10,11,12}};array.lengtharray[0].lengtharray[1].lengtharray[2].lengtharray[3].lengtharray[4].lengthArrayIndexOutOfBoundsExceptionTwo-dimensionalArraysExample：2021/7/1359int[][]array={array.lengthaCreating：【e.g.】int[][]a=newint[3][]；null

nullnull30x4978aa[0]a[1]a[2]a.lengthTwo-dimensionalArrays2021/7/1360Creating：【e.g.】int[][]a=nnull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[0][0]a[0][1]a[0][2]a[0].length0

0032021/7/1361nullnullnull30x4978aa[0]a[1]anull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[1]=newint[2]；a[0][0]a[0][1]a[0][2]a[0].length0

003002a[1][0]a[1][1]a[1].length2021/7/1362nullnullnull30x4978aa[0]a[1]anull

nullnull30x4978aa[0]a[1]a[2]a.lengtha=newint[3][]；a[0]=newint[3]；a[1]=newint[2]；a[2]=newint[4]；a[0][0]a[0][1]a[0][2]a[0].length0

003002a[1][0]a[1][1]a[1].lengtha[2][0]a[2][1]a[2][2]a[2][3]00004a[2].length2021/7/1363nullnullnull30x4978aa[0]a[1]aDeclaring,Creating,andInitializing

UsingShorthandNotationsYoucanalsouseanarrayinitializertodeclare,createandinitializeatwo-dimensionalarray.Forexample,int[][]array=newint[4][3];array[0][0]=1;array[0][1]=2;array[0][2]=3;array[1][0]=4;array[1][1]=5;array[1][2]=6;array[2][0]=7;array[2][1]=8;array[2][2]=9;array[3][0]=10;array[3][1]=11;array[3][2]=12;

int[][]array={{1,2,3},{4,5,6},{7,8,9},{10,11,12}};Sameas2021/7/1364Declaring,Creating,andInitiEnhancedforLoopJDK1.5introducedanewforloopthatenablesyoutotraversethecompletearraysequentiallywithoutusinganindexvariable.

Ingeneral,thesyntaxis

Youstillhavetouseanindexvariableifyouwishtotraversethearrayinadifferentorderorchangetheelementsinthearray.for(elementTypevalue:arrayRefVar){//Processthevalue}

Arraytest.java2021/7/1365EnhancedforLoopJDK1.5introString[][]s=newString[10][];

a)Thislineofcodeisillegal.b)sisatwo-dimensionalarraycontaining10rowsand10columnsc)Eachelementinsissetto""d)Eachelementinsisuninitializedandmustbeinitializedbeforeitisreferenced.【Exercise】2021/7/1366String[][]s=newString[10CopyingArraysOften,inaprogram,youneedtoduplicateanarrayorapartofanarray.Insuchcasesyoucouldattempttousetheassignmentstatement(=),asfollows:

list2=list1;2021/7/1367CopyingArraysOften,inaprogCopyingArraysUsingaloop:int[]sourceArray={2,3,1,5,10};int[]targetArray=newint[sourceArray.length];for(inti=0;i<sourceArrays.length;i++)targetArray[i]=sourceArray[i];2021/7/1368CopyingArraysUsingaloop:202ThearraycopyUtilityarraycopy(sourceArray,src_pos,targetArray,tar_pos,length);Example:System.arraycopy(sourceArray,0,targetArray,0,sourceArray.length);2021/7/1369ThearraycopyUtilityarraycopyCopyingArray:System.arraycopy(object,int,object,int,int)inta[]={1,2,3};intb[]={4,5,6,7,8,9};

System.arraycopy(a,0,b,0,a.length);

b[]={1,2,3,7,8,9};Arraycopy.javaThearraycopyUtility2021/7/1370CopyingArray:Arraycopy.javaThPassingArraystoMethodspublicstaticvoidprintArray(int[]array){for(inti=0;i<array.length;i++){System.out.print(array[i]+"