标签：pre ref xtend float ini and extends max treenode
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {}
tableSizeFor(int)
根据给定的容量大小，返回指定的int类型的数，这个数是2的次幂。看源码：
/** * Returns a power of two size for the given target capacity. */ static final int tableSizeFor(int cap) { int n = cap - 1; n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16; return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1; }
姿势：
public void run(){ int n = 16; System.out.println(n>>>1);//除以2的1次方 System.out.println(n<<1);//乘以2的1次方 System.out.println(n>>>2);//除以2的2次方 } /* 结果 */ 8 32 4
let a = 5; // 00000000000000000000000000000101 a |= 3; // 00000000000000000000000000000011 console.log(a); // 00000000000000000000000000000111
姿势总结：
源码中对给定的数进行除以2的1、2、4、8、16次幂的同时进行或等运算，这样就将给定数的二进制第一个1开始之后的所有位都补齐1。
栗子：
/* 32 的 二进制 100000 补齐1 之后是 111111 返回63 31 的 二进制 011111 补齐1 之后是 011111 返回31 */
给定数的对应2的次幂返回值。
姿势：
transient /* 实现Serializable接口，使用transient关键字的属性不会被序列化 该变量内容在序列化后无法获得访问。*/
/** * The default initial capacity - MUST be a power of two. * 初始化的容量16，一定是2的倍数 */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. * 最大容量等于2的30次方 */ static final int MAXIMUM_CAPACITY = 1 << 30;
/** * The load factor used when none specified in constructor. * 默认加载因子是0.75 */ 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. * nodes长度大于8 bin 就会转为红黑树 */ 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. * nodes长度等于6 bin 时变为链表 */ static final int UNTREEIFY_THRESHOLD = 6;
一直使用链表：当key离散性不好时，这个key的value会很长查询时间慢。在8之前时间复杂度都是差不多的。所以一直使用链表、而且8出现的概率在理想状态下非常低。个别bins中有红黑树对于这个key的查询效率有明显提升，相对整体hashmap的空间影响不大。
一直使用红黑树：红黑树的空间结构是链表的两倍，当nodes很少key很多时就会浪费空间。
/* 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 */
标签：pre ref xtend float ini and extends max treenode
原文地址：https://www.cnblogs.com/wyzbky/p/14242257.html
