Java集合框架Iterator具有fast-fail的特性，在创建Iterator以后，如果其他线程对该Collection做了修改或者当前线程调用非Iterator接口对集合做了修改那么就会抛出ConcurrentModificationException 异常，
。以ArrayList为例，其中有一个modCount成员变量来记录对该list修改次数。下面是jdk源码具体描述：
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.
jdk5.0中的for each特性实际也是通过迭代器实现(javap看一下字节码就清楚了)。fast-fail的实现也是有些代价的，比如上面的modCount.在使用迭代器遍历或操作Collection时每次都要checkForComodification，因此如果批量对集合进行某些操作，尽量调用批处理操作的接口如：removeAll或者addAll().
代码的正确性不要依赖 ConcurrentModificationException这个异常：下面也是jdk doc的描述
Note that fail-fast behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.
