今天在测试代码的时候出现一个异常ConcurrentModificationException，该异常网上很多解决方案以及解释，但我还是再记录一遍吧。

代码抽象出来是这样的：

import java.util.ArrayList;

import java.util.List;

public class Test {

public static void main(String[] args) {

List list=new ArrayList();

list.add(1);

list.add(2);

list.add(3);

list.add(4);

list.add(5);

for (Integer i : list) {//这是迭代

if(i==3){

list.remove(new Integer(i));//引起异常的操作

}

}

}

}

该代码在运行期间就出现java.util.ConcurrentModificationException异常。

这个循环其实是对list进行迭代。

1.在迭代的时候怎么判断是否还有下一个(hasNext()方法怎么实现)：

public boolean hasNext() {

return cursor != size();

}

cursor:Index of element to be returned by subsequent call to next

size():是该list的size

所以只要两者不相等，就认为还有元素。

2.迭代的时候怎么取下一个(next()方法怎么实现)：

public E next() {

checkForComodification();

try {

E next = get(cursor);

lastRet = cursor++;

return next;

} catch (IndexOutOfBoundsException e) {

checkForComodification();

throw new NoSuchElementException();

}

}

final void checkForComodification() {

if (modCount != expectedModCount)

throw new ConcurrentModificationException();

}

modelCount:The number of times this list has been structurally modified.Structural modifications are those that change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.

expectedModCount:期望的modelCount

这2个变量是有迭代器自己来维护的。

上面这段程序出现异常是因为我们使用Collection里面的remove方法进行删除，ArrayList的remove方法实现：

public boolean remove(Object o) {

if (o == null) {

for (int index = 0; index < size; index++)

if (elementData[index] == null) {

fastRemove(index);

return true;

}

} else {

for (int index = 0; index < size; index++)

if (o.equals(elementData[index])) {

fastRemove(index);

return true;

}

}

return false;

}

private void fastRemove(int index) {

modCount++; //***

int numMoved = size - index - 1;

if (numMoved > 0)

System.arraycopy(elementData, index+1, elementData, index,

numMoved);

elementData[--size] = null; // Let gc do its work

}

modCount+1,导致modCount和expectedModCount不相等。

3.解决方法就是用迭代器自己的remove方法：

public void remove() {

if (lastRet == -1)

throw new IllegalStateException();

checkForComodification();

try {

AbstractList.this.remove(lastRet); //将modCount+1，实现如下

if (lastRet < cursor)

cursor--;

lastRet = -1;

expectedModCount = modCount; //维护

} catch (IndexOutOfBoundsException e) {

throw new ConcurrentModificationException();

}

}

public E remove(int index) {

RangeCheck(index);

modCount++; //***

E oldValue = (E) elementData[index];

int numMoved = size - index - 1;

if (numMoved > 0)

System.arraycopy(elementData, index+1, elementData, index,

numMoved);

elementData[--size] = null; // Let gc do its work

return oldValue;

}