深入理解Java的迭代器Iterator

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package com.thinking4java;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class ListModifyDemo {
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
Iterator<Integer> iter = list.iterator();
while (iter.hasNext()) {
Integer next = iter.next();
System.out.println("next=" + next);
list.add(40);
}
}
}

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复制代码


复制代码

复制代码next=10
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.thinking4java.ListModifyDemo.main(ListModifyDemo.java:16)
next=10
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.thinking4java.ListModifyDemo.main(ListModifyDemo.java:16)

Iterator的工作机制。Iterator是工作在一个独立的线程中，并且拥有一个
mutex锁，就是说Iterator在工作的时候，是不允许被迭代的对象被改变的。Iterator被创建的时候，建立了一个内存索引表（单链表），这 个索引表指向原来的对象，当原来的对象数量改变的时候，这个索引表的内容没有同步改变，所以当索引指针往下移动的时候，便找不到要迭代的对象，于是产生错 误。List、Set等是动态的，可变对象数量的数据结构，但是Iterator则是单向不可变，只能顺序读取，不能逆序操作的数据结构，当 Iterator指向的原始数据发生变化时，Iterator自己就迷失了方向。
再回过头看下List迭代器的源码：
……
复制代码public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
int cursor = 0;
/**
* Index of element returned by most recent call to next or previous.
* Reset to -1 if this element is deleted by a call to remove.
*/
int lastRet = -1;
/**
* The modCount value that the iterator believes that the backing List
* should have. If this expectation is violated, the iterator has
* detected concurrent modification.
*/
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size();
}
public E next() {
checkForComodification();
try {
E next = get(cursor);
lastRet = cursor++;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
public void remove() {
if (lastRet == -1)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
int cursor = 0;
/**
* Index of element returned by most recent call to next or previous.
* Reset to -1 if this element is deleted by a call to remove.
*/
int lastRet = -1;
/**
* The modCount value that the iterator believes that the backing List
* should have. If this expectation is violated, the iterator has
* detected concurrent modification.
*/
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size();
}
public E next() {
checkForComodification();
try {
E next = get(cursor);
lastRet = cursor++;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
public void remove() {
if (lastRet == -1)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
复制代码//此处的modCount是Itr的外部类即：AbstractList的属性(protected transient int modCount = 0;)
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

//此处的modCount是Itr的外部类即：AbstractList的属性(protected transient int modCount = 0;)
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}