[Java] 用 Comparator 实现排序

本文内容纲要：

最近正好用到Comparator，发现能对不同类型的对象进行排序(当然排序依据还是基本类型)，也不用自己实现排序算法，用起来很方便，所以简单记录一下。

本文地址：http://www.cnblogs.com/rossoneri/p/4549529.html

InterfaceComparator

对任意类型集合对象进行整体排序，排序时将此接口的实现传递给Collections.sort方法或者Arrays.sort方法排序。

实现intcompare(To1,To2);方法，返回正数，零，负数各代表大于，等于，小于。具体看代码。

简单例子：

publicclassTest{ privatefinalclassCompareNameimplementsComparator<Milan>{ booleanis_Ascend; publicCompareName(booleanb){ //TODOAuto-generatedconstructorstub is_Ascend=b; } @Override publicintcompare(Milano1,Milano2){ //TODOAuto-generatedmethodstub if(is_Ascend) returno1.p_Name.compareTo(o2.p_Name); else returno2.p_Name.compareTo(o1.p_Name); } } privatefinalclassCompareIdimplementsComparator<Milan>{ booleanis_Ascend; publicCompareId(booleanb){ //TODOAuto-generatedconstructorstub is_Ascend=b; } @Override publicintcompare(Milano1,Milano2){ //TODOAuto-generatedmethodstub inta,b; if(is_Ascend){ a=o1.p_Id; b=o2.p_Id; }else{ a=o2.p_Id; b=o1.p_Id; } if(a>b) return1; elseif(a==b) return0; else return-1; } } publicstaticvoidmain(String[]args){ Testt=newTest(); Milanp1=newMilan(1,"Dida"); Milanp2=newMilan(2,"Cafu"); Milanp3=newMilan(3,"Maldini"); MilanP4=newMilan(6,"Baresi"); Milanp5=newMilan(9,"Inzaghi"); MilanP6=newMilan(10,"Costa"); List<Milan>mList=newArrayList<Milan>(); mList.add(p1); mList.add(P6); mList.add(P4); mList.add(p2); mList.add(p5); mList.add(p3); System.out.println("初始顺序"); System.out.println("姓名 | 号码"); for(Milanp:mList){ System.out.println(p.p_Name+" | "+p.p_Id); } System.out.println(); System.out.println("对号码降序"); System.out.println("姓名 | 号码"); Collections.sort(mList,t.newCompareId(false)); for(Milanp:mList){ System.out.println(p.p_Name+" | "+p.p_Id); } System.out.println(); System.out.println("对姓名升序"); System.out.println("姓名 | 号码"); Collections.sort(mList,t.newCompareName(true)); for(Milanp:mList){ System.out.println(p.p_Name+" | "+p.p_Id); } } }

输出结果：

初始顺序

姓名 | 号码

Dida | 1

Costa | 10

Baresi | 6

Cafu | 2

Inzaghi | 9

Maldini | 3

对号码降序

姓名 | 号码

Costa | 10

Inzaghi | 9

Baresi | 6

Maldini | 3

Cafu | 2

Dida | 1

对姓名升序

姓名 | 号码

Baresi | 6

Cafu | 2

Costa | 10

Dida | 1

Inzaghi | 9

Maldini | 3

sort方法第一个参数是一个list对象，排序结束后的结果就继续保存在这个list中，可以直接使用。

附上文档，不用再去查了

publicinterfaceComparator<T>

Acomparisonfunction,whichimposesatotalorderingonsomecollectionofobjects.Comparatorscanbepassedtoasortmethod(suchasCollections.sortorArrays.sort)toallowprecisecontroloverthesortorder.Comparatorscanalsobeusedtocontroltheorderofcertaindatastructures(suchassortedsetsorsortedmaps),ortoprovideanorderingforcollectionsofobjectsthatdonthaveanaturalordering. TheorderingimposedbyacomparatorconasetofelementsSissaidtobeconsistentwithequalsifandonlyifc.compare(e1,e2)==0hasthesamebooleanvaluease1.equals(e2)foreverye1ande2inS. Cautionshouldbeexercisedwhenusingacomparatorcapableofimposinganorderinginconsistentwithequalstoorderasortedset(orsortedmap).Supposeasortedset(orsortedmap)withanexplicitcomparatorcisusedwithelements(orkeys)drawnfromasetS.IftheorderingimposedbyconSisinconsistentwithequals,thesortedset(orsortedmap)willbehave"strangely."Inparticularthesortedset(orsortedmap)willviolatethegeneralcontractforset(ormap),whichisdefinedintermsofequals. Forexample,supposeoneaddstwoelementsaandbsuchthat(a.equals(b)&&c.compare(a,b)!=0)toanemptyTreeSetwithcomparatorc.Thesecondaddoperationwillreturntrue(andthesizeofthetreesetwillincrease)becauseaandbarenotequivalentfromthetreesetsperspective,eventhoughthisiscontrarytothespecificationoftheSet.addmethod. Note:Itisgenerallyagoodideaforcomparatorstoalsoimplementjava.io.Serializable,astheymaybeusedasorderingmethodsinserializabledatastructures(likeTreeSet,TreeMap).Inorderforthedatastructuretoserializesuccessfully,thecomparator(ifprovided)mustimplementSerializable. Forthemathematicallyinclined,therelationthatdefinestheimposedorderingthatagivencomparatorcimposesonagivensetofobjectsSis: {(x,y)suchthatc.compare(x,y)<=0}. Thequotientforthistotalorderis: {(x,y)suchthatc.compare(x,y)==0}. ItfollowsimmediatelyfromthecontractforcomparethatthequotientisanequivalencerelationonS,andthattheimposedorderingisatotalorderonS.WhenwesaythattheorderingimposedbyconSisconsistentwithequals,wemeanthatthequotientfortheorderingistheequivalencerelationdefinedbytheobjectsequals(Object)method(s): {(x,y)suchthatx.equals(y)}. UnlikeComparable,acomparatormayoptionallypermitcomparisonofnullarguments,whilemaintainingtherequirementsforanequivalencerelation.

intcompare(To1,To2)

Comparesitstwoargumentsfororder.Returnsanegativeinteger,zero,orapositiveintegerasthefirstargumentislessthan,equalto,orgreaterthanthesecond. Intheforegoingdescription,thenotationsgn(expression)designatesthemathematicalsignumfunction,whichisdefinedtoreturnoneof-1,0,or1accordingtowhetherthevalueofexpressionisnegative,zeroorpositive. Theimplementormustensurethatsgn(compare(x,y))==-sgn(compare(y,x))forallxandy.(Thisimpliesthatcompare(x,y)mustthrowanexceptionifandonlyifcompare(y,x)throwsanexception.) Theimplementormustalsoensurethattherelationistransitive:((compare(x,y)>0)&&(compare(y,z)>0))impliescompare(x,z)>0. Finally,theimplementormustensurethatcompare(x,y)==0impliesthatsgn(compare(x,z))==sgn(compare(y,z))forallz. Itisgenerallythecase,butnotstrictlyrequiredthat(compare(x,y)==0)==(x.equals(y)).Generallyspeaking,anycomparatorthatviolatesthisconditionshouldclearlyindicatethisfact.Therecommendedlanguageis"Note:thiscomparatorimposesorderingsthatareinconsistentwithequals." Parameters: o1-thefirstobjecttobecompared. o2-thesecondobjecttobecompared. Returns: anegativeinteger,zero,orapositiveintegerasthefirstargumentislessthan,equalto,orgreaterthanthesecond.

booleanequals(Objectobj)

Indicateswhethersomeotherobjectis"equalto"thiscomparator.ThismethodmustobeythegeneralcontractofObject.equals(Object).Additionally,thismethodcanreturntrueonlyifthespecifiedobjectisalsoacomparatoranditimposesthesameorderingasthiscomparator.Thus,comp1.equals(comp2)impliesthatsgn(comp1.compare(o1,o2))==sgn(comp2.compare(o1,o2))foreveryobjectreferenceo1ando2. NotethatitisalwayssafenottooverrideObject.equals(Object).However,overridingthismethodmay,insomecases,improveperformancebyallowingprogramstodeterminethattwodistinctcomparatorsimposethesameorder. Overrides: equalsinclassObject Parameters: obj-thereferenceobjectwithwhichtocompare. Returns: trueonlyifthespecifiedobjectisalsoacomparatoranditimposesthesameorderingasthiscomparator.

publicstatic<T>voidsort(List<T>list,Comparator<?superT>c)

Sortsthespecifiedlistaccordingtotheorderinducedbythespecifiedcomparator.Allelementsinthelistmustbemutuallycomparableusingthespecifiedcomparator(thatis,c.compare(e1,e2)mustnotthrowaClassCastExceptionforanyelementse1ande2inthelist). Thissortisguaranteedtobestable:equalelementswillnotbereorderedasaresultofthesort. Thesortingalgorithmisamodifiedmergesort(inwhichthemergeisomittedifthehighestelementinthelowsublistislessthanthelowestelementinthehighsublist).Thisalgorithmoffersguaranteednlog(n)performance.Thespecifiedlistmustbemodifiable,butneednotberesizable.Thisimplementationdumpsthespecifiedlistintoanarray,sortsthearray,anditeratesoverthelistresettingeachelementfromthecorrespondingpositioninthearray.Thisavoidsthen2log(n)performancethatwouldresultfromattemptingtosortalinkedlistinplace. Parameters: list-thelisttobesorted. c-thecomparatortodeterminetheorderofthelist.Anullvalueindicatesthattheelementsnaturalorderingshouldbeused.

本文内容总结：

原文链接：https://www.cnblogs.com/rossoneri/p/4549529.html