List，HashMap中都有modCount字段；今天分析一下modCount字段的作用。

 /**
     * The number of times this list has been <i>structurally modified</i>.
     * 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.
     *
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */

这是JDK中的描述，大致翻译一下。

该字段表示list结构上被修改的次数。结构上的修改指的是那些改变了list的长度大小或者使得遍历过程中产生不正确的结果的其它方式。

该字段被Iterator以及ListIterator的实现类所使用，如果该值被意外更改，Iterator或者ListIterator 将抛出ConcurrentModificationException异常，这是jdk在面对迭代遍历的时候为了避免不确定性而采取的快速失败原则。
子类对此字段的使用是可选的，如果子类希望支持快速失败，只需要覆盖该字段相关的所有方法即可。单线程调用不能添加删除Iterator正在遍历的对象，
否则将可能抛出ConcurrentModificationException异常，如果子类不希望支持快速失败，该字段可以直接忽略。