今天同事写了几行类似这样的代码：
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);}}}
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：
for(Iterator<String> it = famous.iterator();it.hasNext();){String s = it.next();if(s.equals(“madehua”)){famous.remove(s);}}
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:
for (int i = 0; i < famous.size(); i++) {String s = famous.get(i);if (s.equals(“madehua”)) {famous.remove(s);}}
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:
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(); } }
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。
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