今天同事写了几行类似这样的代码：
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public
static
void
main(String args[]) {
List<String> famous =
new
ArrayList<String>();
famous.add(
"liudehua"
);
famous.add(
"madehua"
);
famous.add(
"liushishi"
);
famous.add(
"tangwei"
);
for
(String s : famous) {
if
(s.equals(
"madehua"
)) {
famous.remove(s);
}
}
}
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运行出异常:
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.bes.Test.main(Test.java:15)
Java新手最容易犯的错误，对JAVA集合进行遍历删除时务必要用迭代器。切记。
其实对于如上for循环，运行过程中还是转换成了如下代码：
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for
(Iterator<String> it = famous.iterator();it.hasNext();){
String s = it.next();
if
(s.equals(
"madehua"
)){
famous.remove(s);
}
}
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仍然采用的是迭代器，但删除操作却用了错误的方法。如将famous.remove(s)改成it.remove()
则运行正常，结果也无误。
当然如果改成:
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for
(
int
i =
0
; i < famous.size(); i++) {
String s = famous.get(i);
if
(s.equals(
"madehua"
)) {
famous.remove(s);
}
}
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这种方法，也是可以完成功能，但一般也不这么写。
为什么用了迭代码器就不能采用famous.remove(s)操作? 这种因为ArrayList与Iterator混合使用时会导致各自的状态出现不一样，最终出现异常。
我们看一下ArrayList中的Iterator实现:
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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() {
if
(modCount != expectedModCount)
throw
new
ConcurrentModificationException();
}
}
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基本上ArrayList采用size属性来维护自已的状态，而Iterator采用cursor来来维护自已的状态。
当size出现变化时，cursor并不一定能够得到同步，除非这种变化是Iterator主动导致的。
从上面的代码可以看到当Iterator.remove方法导致ArrayList列表发生变化时，他会更新cursor来同步这一变化。但其他方式导致的ArrayList变化，Iterator是无法感知的。ArrayList自然也不会主动通知Iterator们，那将是一个繁重的工作。Iterator到底还是做了努力：为了防止状态不一致可能引发的无法设想的后果，Iterator会经常做checkForComodification检查，以防有变。如果有变，则以异常抛出，所以就出现了上面的异常。
本文转自 anranran 51CTO博客，原文链接:http://blog.51cto.com/guojuanjun/1348450
