/** * The default initial capacity - MUST be a power of two. * 解释：为了节省空间和让元素均匀分布，所以初始化容量，需要为2的乘方。 */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; /** * The load factor used when none specified in constructor. * 解释：默认的加载因子 */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * The bin count threshold for using a tree rather than list for a * bin. Bins are converted to trees when adding an element to a * bin with at least this many nodes. The value must be greater * than 2 and should be at least 8 to mesh with assumptions in * tree removal about conversion back to plain bins upon * shrinkage. * 解释：当一个桶中的元素个数达到8个时候就要，数据存储的数据结构就由链表变为了红黑树 */ static final int TREEIFY_THRESHOLD = 8; /** * The bin count threshold for untreeifying a (split) bin during a * resize operation. Should be less than TREEIFY_THRESHOLD, and at * most 6 to mesh with shrinkage detection under removal. * 解释：数据结构由红黑树转化为链表 */ static final int UNTREEIFY_THRESHOLD = 6;
Q&A：
1. 为什么变成红黑树是8，而转化成链表是6？
有人从源码分析，有人从查找时间复杂度分析。部分源码如下：
* 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
*/
2. 容量为什么是2的乘方呢？https://blog.csdn.net/sd_csdn_scy/article/details/57083619