为什么HashMap链表长度超过8会转成红黑树结构

底层数据结构： JDK1.8 以后的 HashMap 在解决哈希冲突时有了较大的变化，当链表长度大于阈值（默认为8）时，将链表转化为红黑树，以减少搜索时间。
为什么HashMap链表长度超过8会转成红黑树结构？

红黑树的插入、删除和遍历的最坏时间复杂度都是log(n)，

log(6)=2.6

链表 8/2=4

红黑树 log(8)=3

2.由频率表可以看出，桶的长度超过8的概率非常非常小。 阈值为8。

Because TreeNodes are about twice the size of regular nodes, we use them only when bins contain enough nodes to warrant use (see TREEIFY_THRESHOLD). And when they become too small (due to removal or resizing) they are converted back to plain bins. In usages with well-distributed user hashCodes, tree bins are rarely used. Ideally, under random hashCodes, the frequency of nodes in bins follows a Poisson distribution (http://en.wikipedia.org/wiki/Poisson_distribution) with a parameter of about 0.5 on average for the default resizing threshold of 0.75, although with a large variance because of resizing granula