hashmap 为什么设置桶个数大于8转为红黑树

hashmap 为什么设置桶个数大于8转为红黑树

时间和空间的平衡

为什么要转换

要弄明白这个问题，我们首先要明白为什么要转换，这个问题比较简单，因为Map中桶的元素初始化是链表保存的，其查找性能是O(n)，而树结构能将查找性能提升到O(log(n))。当链表长度很小的时候，即使遍历，速度也非常快，但是当链表长度不断变长，肯定会对查询性能有一定的影响，所以才需要转成树。
源码中有一段话:

* 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 granularity. Ignoring variance, the expected
* occurrences of list size k are (exp(-0.5) * pow(0.5, k) /
* factorial(k)). The first values are:
*
* 0:    0.60653066
* 1:    0.30326533
* 2:    0.07581633
* 3:    0.01263606
* 4:    0.00157952
* 5:    0.00015795
* 6:    0.00001316
* 7:    0.00000094
* 8:    0.00000006
* more: less than 1 in ten million

因为bin的大小几乎不能为8个,如果为8个说明hash取的不好,分配的过于不均匀