Java Iterator接口实现代码解析

2023-07-26 10:12:04 64

Iterator接口

源代码

packagejava.util;

importjava.util.function.Consumer;

/**
*Aniteratoroveracollection.{@codeIterator}takestheplaceof
*{@linkEnumeration}intheJavaCollectionsFramework.Iterators
*differfromenumerationsintwoways:
*
*
*Iteratorsallowthecallertoremoveelementsfromthe
*underlyingcollectionduringtheiterationwithwell-defined
*semantics.
*
Methodnameshavebeenimproved.
*
*
*Thisinterfaceisamemberofthe
*
*JavaCollectionsFramework.
*
*@paramthetypeofelementsreturnedbythisiterator
*
*@authorJoshBloch
*@seeCollection
*@seeListIterator
*@seeIterable
*@since1.2
*/
publicinterfaceIterator{
/**
*Returns{@codetrue}iftheiterationhasmoreelements.
*(Inotherwords,returns{@codetrue}if{@link#next}would
*returnanelementratherthanthrowinganexception.)
*
*@return{@codetrue}iftheiterationhasmoreelements
*/
booleanhasNext();

/**
*Returnsthenextelementintheiteration.
*
*@returnthenextelementintheiteration
*@throwsNoSuchElementExceptioniftheiterationhasnomoreelements
*/
Enext();

/**
*Removesfromtheunderlyingcollectionthelastelementreturned
*bythisiterator(optionaloperation).Thismethodcanbecalled
*onlyoncepercallto{@link#next}.Thebehaviorofaniterator
*isunspecifiediftheunderlyingcollectionismodifiedwhilethe
*iterationisinprogressinanywayotherthanbycallingthis
*method.
*
*@implSpec
*Thedefaultimplementationthrowsaninstanceof
*{@linkUnsupportedOperationException}andperformsnootheraction.
*
*@throwsUnsupportedOperationExceptionifthe{@coderemove}
*operationisnotsupportedbythisiterator
*
*@throwsIllegalStateExceptionifthe{@codenext}methodhasnot
*yetbeencalled,orthe{@coderemove}methodhasalready
*beencalledafterthelastcalltothe{@codenext}
*method
*/
defaultvoidremove(){
thrownewUnsupportedOperationException("remove");
}

/**
*Performsthegivenactionforeachremainingelementuntilallelements
*havebeenprocessedortheactionthrowsanexception.Actionsare
*performedintheorderofiteration,ifthatorderisspecified.
*Exceptionsthrownbytheactionarerelayedtothecaller.
*
*@implSpec
*
Thedefaultimplementationbehavesasif:
*
{@code
*while(hasNext())
*action.accept(next());
*}
*
*@paramactionTheactiontobeperformedforeachelement
*@throwsNullPointerExceptionifthespecifiedactionisnull
*@since1.8
*/
defaultvoidforEachRemaining(Consumeraction){
Objects.requireNonNull(action);
while(hasNext())
action.accept(next());
}
}

阅读笔记

1.Iterator接口与Enumeration接口的关系/Iterator接口在Java集合库中的作用

Iterator接口是Java集合框架的一部分，被用于替代原有的Enumeration接口。（“Iterator”比“Enumeration”更简短、表意更清晰、功能更多，具体的信息下面Enumeration接口的注解中说的挺清楚，且Enumeration注解中也建议编程人员改用Iterator接口）

Java类库中，集合类的基本接口是Collection接口，而Collection接口实现了Iterable接口，Iterable接口中有一个iterator()方法用于获取Iterator对象。

packagejava.util; /** *AnobjectthatimplementstheEnumerationinterfacegeneratesa *seriesofelements,oneatatime.Successivecallstothe *nextElementmethodreturnsuccessiveelementsofthe *series. *

*Forexample,toprintallelementsofaVector<E>v: *

*for(Enumeration<E>e=v.elements();e.hasMoreElements();)
*System.out.println(e.nextElement());

*Methodsareprovidedtoenumeratethroughtheelementsofa *vector,thekeysofahashtable,andthevaluesinahashtable. *Enumerationsarealsousedtospecifytheinputstreamstoa *SequenceInputStream. *

*NOTE:ThefunctionalityofthisinterfaceisduplicatedbytheIterator *interface.Inaddition,Iteratoraddsanoptionalremoveoperation,and *hasshortermethodnames.Newimplementationsshouldconsiderusing *IteratorinpreferencetoEnumeration. * *@seejava.util.Iterator *@seejava.io.SequenceInputStream *@seejava.util.Enumeration#nextElement() *@seejava.util.Hashtable *@seejava.util.Hashtable#elements() *@seejava.util.Hashtable#keys() *@seejava.util.Vector *@seejava.util.Vector#elements() * *@authorLeeBoynton *@sinceJDK1.0 */ publicinterfaceEnumeration{ /** *Testsifthisenumerationcontainsmoreelements. * *@returntrueifandonlyifthisenumerationobject *containsatleastonemoreelementtoprovide; *falseotherwise. */ booleanhasMoreElements(); /** *Returnsthenextelementofthisenumerationifthisenumeration *objecthasatleastonemoreelementtoprovide. * *@returnthenextelementofthisenumeration. *@exceptionNoSuchElementExceptionifnomoreelementsexist. */ EnextElement(); }

2.hasNext()、next()、remove()方法的关系

hasNext()方法：判断是否还有元素可以进行迭代；

next()方法：迭代元素；

remove()方法：

/**
*Removefromtheunderlyingcollectionthelastelementreturnedbythisiterator
*（optionaloperation）.
*移除当前迭代器上一次从基础集合中迭代的元素（可选操作）
*
*Thismethodcanbecalledonlyoncepercalltonext().
*调用remove()方法前必须先调用next()方法，调用完一次remove()方法后想要再次调用remove()方法，
*必须先调用next()方法。
*
*Thebehaviorofaniteratorisunspecifiediftheunderlyingcollectionismodifyedwhile
*theiterationisinprogressisanywayotherthanbycallthismethod.
*如果在迭代进行过程中修改了基础集合，则迭代器的行为是不确定的。
*/
publicstaticvoidmain(String[]args){
CollectionstringCollection=newArrayList<>();
stringCollection.add("Hello");
stringCollection.add("World");
stringCollection.add("!");
IteratorstringIterator=stringCollection.iterator();

stringIterator.next();
stringIterator.remove();//OK
}
publicstaticvoidmain(String[]args){
......
stringIterator.next();
stringCollection.add("abc");//基本集合被改变
stringIterator.remove();//ERROR-java.util.ConcurrentModificationException
}
publicstaticvoidmain(String[]args){
......
stringIterator.next();
stringCollection.add("abc");//基本集合被改变
stringIterator.next();//ERROR-java.util.ConcurrentModificationException
}
publicstaticvoidmain(String[]args){
......
stringIterator.next();
stringCollection.add("abc");//基本集合改变
stringIterator=stringCollection.iterator();//重新获取迭代器
stringIterator.next();//OK
stringIterator.remove();//OK
}

三者关系：调用remove()方法前必须先调用next()方法，调用next()方法前最好先调用hasNext()方法。

3.具体实现类

AbstractList类中定义了一个实现了Iterator接口的内部类：

privateclassItrimplementsIterator{
/**
*Indexofelementtobereturnedbysubsequentcalltonext.
*/
intcursor=0;

/**
*Indexofelementreturnedbymostrecentcalltonextor
*previous.Resetto-1ifthiselementisdeletedbyacall
*toremove.
*/
intlastRet=-1;

/**
*ThemodCountvaluethattheiteratorbelievesthatthebacking
*Listshouldhave.Ifthisexpectationisviolated,theiterator
*hasdetectedconcurrentmodification.
*/
intexpectedModCount=modCount;

publicbooleanhasNext(){
returncursor!=size();
}

publicEnext(){
checkForComodification();
try{
inti=cursor;
Enext=get(i);
lastRet=i;//最近一次调用next()方法返回的元素的下标。
cursor=i+1;//下一次调用next()方法返回的元素的下标。
returnnext;
}catch(IndexOutOfBoundsExceptione){
checkForComodification();
thrownewNoSuchElementException();
}
}

publicvoidremove(){
if(lastRet<0)
thrownewIllegalStateException();//所以，调用remove()前必须先调用next()
checkForComodification();

try{
AbstractList.this.remove(lastRet);
if(lastRet
看完上面的代码，我对modCount、expectedModCount变量以及checkForComodification()方法的作用比较好奇，所以尝试着去搞清楚。
先来看modeCount变量，这个变量被声明在内部类的外部：
publicabstractclassAbstractListextendsAbstractCollectionimplementsList{
/**
*Thenumberoftimesthislisthasbeenstructurallymodified.
*Structuralmodificationsarethosethatchangethesizeofthe
*list,orotherwiseperturbitinsuchafashionthatiterationsin
*progressmayyieldincorrectresults.
*用于表示该列表发生结构性修改的次数。结构性修改是指*更改列表的大小*或*以其他
*方式干扰列表*，即正在进行的迭代可能会产生错误的结果。
*
*Thisfieldisusedbytheiteratorandlistiteratorimplementation
*returnedbythe{@codeiterator}and{@codelistIterator}methods.
*Ifthevalueofthisfieldchangesunexpectedly,theiterator(orlist
*iterator)willthrowa{@codeConcurrentModificationException}in
*responsetothe{@codenext},{@coderemove},{@codeprevious},
*{@codeset}or{@codeadd}operations.Thisprovides
*fail-fastbehavior,ratherthannon-deterministicbehaviorin
*thefaceofconcurrentmodificationduringiteration.
*设计者认为，与其因为基本集合被并发修改从而使迭代产生不确定行为，不如尽早给出错误。
*
*
Useofthisfieldbysubclassesisoptional.Ifasubclass
*wishestoprovidefail-fastiterators(andlistiterators),thenit
*merelyhastoincrementthisfieldinits{@codeadd(int,E)}and
*{@coderemove(int)}methods(andanyothermethodsthatitoverrides
*thatresultinstructuralmodificationstothelist).Asinglecallto
*{@codeadd(int,E)}or{@coderemove(int)}mustaddnomorethan
*onetothisfield,ortheiterators(andlistiterators)willthrow
*bogus{@codeConcurrentModificationExceptions}.Ifanimplementation
*doesnotwishtoprovidefail-fastiterators,thisfieldmaybe
*ignored.
*是否使用应需求决定。
*/
protectedtransientintmodCount=0;
}
看完上面的源码注解，已经大概能够知道modCount、expectedModCount以及checkForComodification()的作用了。
假如把基础集合当作一个银行账号，基础集合中的元素表示存款。那么modCount就相当于银行为每个账号做的消费记录，expectedModCount就相当于是账号持有人自己做的一份消费记录，一般银行和账号持有人自己做的消费记录都不会出错。
finalvoidcheckForComodification(){
if(modCount!=expectedModCount)
thrownewConcurrentModificationException();
}
一旦银行那边的消费记录和自己手里的那份消费记录对不上，肯定是账号被盗用了。
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Java Iterator接口实现代码解析

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