[Java 8 HashMap 详解系列] 2.HashMap 中 Key 的 index 是怎样计算的?
[Java 8 HashMap 详解系列] 2.HashMap 中 Key 的 index 是怎样计算的?
一个会写诗的程序员
发布于 2020-03-24 16:45:43
发布于 2020-03-24 16:45:43
文章被收录于专栏：一个会写诗的程序员的博客
2.HashMap 中 Key 的 index 是怎样计算的?
HashMap中的 table 是怎样确定数组索引位置的?
对于HashMap内的所有实现来说，首先第一步是定位对键值对所在数组的索引下标位置，这是后续所有操作的基础.
如下代码是展示索引下标获取的基本逻辑:
/* ---------------- Static utilities -------------- */
/**
* Computes key.hashCode() and spreads (XORs) higher bits of hash
* to lower. Because the table uses power-of-two masking, sets of
* hashes that vary only in bits above the current mask will
* always collide. (Among known examples are sets of Float keys
* holding consecutive whole numbers in small tables.) So we
* apply a transform that spreads the impact of higher bits
* downward. There is a tradeoff between speed, utility, and
* quality of bit-spreading. Because many common sets of hashes
* are already reasonably distributed (so don't benefit from
* spreading), and because we use trees to handle large sets of
* collisions in bins, we just XOR some shifted bits in the
* cheapest possible way to reduce systematic lossage, as well as
* to incorporate impact of the highest bits that would otherwise
* never be used in index calculations because of table bounds.
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}然后, 数组元素的下标算法是:
public static final int index(int hash, int n) {
return (n - 1) & hash;
}代码拆解:
public static final int hash(Object key) {
if (key == null) {
return 0;
}
int h = key.hashCode();
int rh = h >>> 16;
out.println("===================== hash(" + key + ") ===========================");
out.println("key \t\t" + key);
out.println("h=key.hashCode() \t\t" + toBinary(h) + "\t\t" + h);
out.println("rh=h>>>16 \t\t" + toBinary(rh) + "\t\t" + rh);
return h ^ rh;
}
public static final int index(int hash, int n) {
out.println("hash=h ^ rh \t\t" + toBinary(hash) + "\t\t" + hash);
out.println("n = \t\t" + toBinary(n) + "\t\t" + (n));
out.println("n - 1= \t\t" + toBinary(n - 1) + "\t\t" + (n - 1));
int index = (n - 1) & hash;
out.println("index=(n - 1) & hash \t\t" + toBinary(index) + "\t\t" + index);
out.println();
return index;
}运行测试数据:
===================== hash(a) ===========================
key a
h=key.hashCode() 00000000000000000000000001100001 97
rh=h>>>16 00000000000000000000000000000000 0
hash(a)=97
hash=h ^ rh 00000000000000000000000001100001 97
n = 00000000000000000000000000000001 1
n - 1= 00000000000000000000000000000000 0
index=(n - 1) & hash 00000000000000000000000000000000 0
===================== hash(b) ===========================
key b
h=key.hashCode() 00000000000000000000000001100010 98
rh=h>>>16 00000000000000000000000000000000 0
hash(b)=98
hash=h ^ rh 00000000000000000000000001100010 98
n = 00000000000000000000000000000010 2
n - 1= 00000000000000000000000000000001 1
index=(n - 1) & hash 00000000000000000000000000000000 0
===================== hash(abc) ===========================
key abc
h=key.hashCode() 00000000000000010111100001100010 96354
rh=h>>>16 00000000000000000000000000000001 1
hash(abc)=96355
hash=h ^ rh 00000000000000010111100001100011 96355
n = 00000000000000000000000000000100 4
n - 1= 00000000000000000000000000000011 3
index=(n - 1) & hash 00000000000000000000000000000011 3
===================== hash(abcde) ===========================
key abcde
h=key.hashCode() 00000101100001001111010001100011 92599395
rh=h>>>16 00000000000000000000010110000100 1412
hash(abcde)=92598759
hash=h ^ rh 00000101100001001111000111100111 92598759
n = 00000000000000000000000000001000 8
n - 1= 00000000000000000000000000000111 7
index=(n - 1) & hash 00000000000000000000000000000111 7
===================== hash(abcdefg) ===========================
key abcdefg
h=key.hashCode() 10111000000110010111010001100100 -1206291356
rh=h>>>16 00000000000000001011100000011001 47129
hash(abcdefg)=-1206268803
hash=h ^ rh 10111000000110011100110001111101 -1206268803
n = 00000000000000000000000000001000 8
n - 1= 00000000000000000000000000000111 7
index=(n - 1) & hash 00000000000000000000000000000101 5算法图解:
源码分析:
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) { // index = (n - 1) & hash
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}HashMap 中的 tableSizeFor() 方法详解
输入: int cap; 返回: n = tableSizeFor(cap) 其中, n % 2 ==0, and cap <=n.
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
// 先 cap 减 1
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; // 再加 1
}我们来运行测试一下:
===============tableSizeFor(2)==================
cap = 2
n = cap - 1 00000000000000000000000000000001
n = n | (n >>> 1) 00000000000000000000000000000001
n = n | (n >>> 2) 00000000000000000000000000000001
n = n | (n >>> 4) 00000000000000000000000000000001
n = n | (n >>> 8) 00000000000000000000000000000001
n = n | (n >>> 16) 00000000000000000000000000000001
tableSizeFor = 2
2
===============tableSizeFor(5)==================
cap = 5
n = cap - 1 00000000000000000000000000000100
n = n | (n >>> 1) 00000000000000000000000000000110
n = n | (n >>> 2) 00000000000000000000000000000111
n = n | (n >>> 4) 00000000000000000000000000000111
n = n | (n >>> 8) 00000000000000000000000000000111
n = n | (n >>> 16) 00000000000000000000000000000111
tableSizeFor = 8
8
===============tableSizeFor(10)==================
cap = 10
n = cap - 1 00000000000000000000000000001001
n = n | (n >>> 1) 00000000000000000000000000001101
n = n | (n >>> 2) 00000000000000000000000000001111
n = n | (n >>> 4) 00000000000000000000000000001111
n = n | (n >>> 8) 00000000000000000000000000001111
n = n | (n >>> 16) 00000000000000000000000000001111
tableSizeFor = 16
16
===============tableSizeFor(25)==================
cap = 25
n = cap - 1 00000000000000000000000000011000
n = n | (n >>> 1) 00000000000000000000000000011100
n = n | (n >>> 2) 00000000000000000000000000011111
n = n | (n >>> 4) 00000000000000000000000000011111
n = n | (n >>> 8) 00000000000000000000000000011111
n = n | (n >>> 16) 00000000000000000000000000011111
tableSizeFor = 32
32
===============tableSizeFor(58)==================
cap = 58
n = cap - 1 00000000000000000000000000111001
n = n | (n >>> 1) 00000000000000000000000000111101
n = n | (n >>> 2) 00000000000000000000000000111111
n = n | (n >>> 4) 00000000000000000000000000111111
n = n | (n >>> 8) 00000000000000000000000000111111
n = n | (n >>> 16) 00000000000000000000000000111111
tableSizeFor = 64
64
===============tableSizeFor(100)==================
cap = 100
n = cap - 1 00000000000000000000000001100011
n = n | (n >>> 1) 00000000000000000000000001110011
n = n | (n >>> 2) 00000000000000000000000001111111
n = n | (n >>> 4) 00000000000000000000000001111111
n = n | (n >>> 8) 00000000000000000000000001111111
n = n | (n >>> 16) 00000000000000000000000001111111
tableSizeFor = 128
128
===============tableSizeFor(896)==================
cap = 896
n = cap - 1 00000000000000000000001101111111
n = n | (n >>> 1) 00000000000000000000001111111111
n = n | (n >>> 2) 00000000000000000000001111111111
n = n | (n >>> 4) 00000000000000000000001111111111
n = n | (n >>> 8) 00000000000000000000001111111111
n = n | (n >>> 16) 00000000000000000000001111111111
tableSizeFor = 1024
1024
===============tableSizeFor(1073741000)==================
cap = 1073741000
n = cap - 1 00111111111111111111110011000111
n = n | (n >>> 1) 00111111111111111111111011100111
n = n | (n >>> 2) 00111111111111111111111111111111
n = n | (n >>> 4) 00111111111111111111111111111111
n = n | (n >>> 8) 00111111111111111111111111111111
n = n | (n >>> 16) 00111111111111111111111111111111
tableSizeFor = 1073741824
1073741824参考资料
https://blog.csdn.net/fan2012huan/article/details/51097331 https://www.jianshu.com/p/cbe3f22793be https://www.jianshu.com/p/dbbecc36200d
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