Function used by the key class does not spread values evenly over the hash range or has a large number of hash collisions , certain bucket chains may be much longer than others , and traversing a long hash chain and calling 函数不能将value均匀地分布在整个hash表范围内，或者存在大量的hash冲突，那么某些bucket链就会比其他的链长很多，而遍历一个长的hash链以及对该hash链上一定百分比的元素调用
This means that elements cannot be added to or removed from the middle or end of the hash chain - elements can only be added at the beginning , and removal involves cloning all or part of the chain and updating the list head pointer 这就意味着不能将元素添加到hash链的中间或末尾，或者从hash链的中间或末尾删除元素而只能从hash链的开头添加元素，并且删除操作包括克隆整个链或链的一部分并更新列表的头指针。
