1.  modCount

  /**

     * The number of times this Hashtable has been structurally modified      * Structural modifications are those that change the number of entries in      * the Hashtable or otherwise modify its internal structure (e.g.,      * rehash).  This field is used to make iterators on Collection-views of      * the Hashtable fail-fast.  (See ConcurrentModificationException).      */     private transient int modCount = 0;

记录被改变的次数.主要用于iterators 的快速失败判断.

快速失败:

The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.

2.