package com.example.demo.test;
import java.util.HashMap;
import java.util.Map;
/**
* <p>
* <code>HashMapTest</code>
* </p>
* Description: JDK 1.8
* @see java.util.HashMap 本质上是Node<K,V>[]
*
* @author Jalen
* @date 2019/8/26 14:03
*/
public class HashMapTest {
public static void main(String[] args) {
//createEmptyHashMap();
createNonEmptyHashMap();
}
/**
* @see java.util.HashMap#HashMap() loadFactor=DEFAULT_LOAD_FACTOR=0.75f
* @see java.util.HashMap#put
* {@link java.util.HashMap#hash(Object key)}
* {@link java.lang.String#hashCode}
* {@link java.util.HashMap#putVal}
* {@link java.util.HashMap#resize}
* static final int DEFAULT_INITIAL_CAPACITY = 1 << 4 as newCapacity
* (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY) = 0.75f * 16 = 12 as newThreshold
* Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap] 创建16长度的kv节点
* {@link java.util.HashMap#newNode}
* create new Node<K,V> 赋值到newTab[0]
* modCount++
* {@link java.util.HashMap#afterNodeInsertion}
*/
private static void createEmptyHashMap(){
Map<String, String> map = new HashMap<>(); //loadFactor=DEFAULT_LOAD_FACTOR=0.75f
map.put("a", "China");
System.out.println(map.toString());
}
/**
* @see java.util.HashMap#HashMap(int initialCapacity)
* this(initialCapacity, DEFAULT_LOAD_FACTOR);
* {@link java.util.HashMap#HashMap(int initialCapacity, float loadFactor)}
* check initialCapacity(不能小于0，如果大于MAXIMUM_CAPACITY则赋值为MAXIMUM_CAPACITY)，MAXIMUM_CAPACITY=1 << 30
* loadFactor设为DEFAULT_LOAD_FACTOR，即0.75f
* {@link java.util.HashMap#tableSizeFor(int cap)}
* 通过initialCapacity计算得到threshold为8，此时map的capacity并未确定，在第一次put value时才会定下来
* @see java.util.HashMap#put (第一次put值时会触发resize，重新计算threshold)
* {@link java.util.HashMap#hash(Object key)}
* {@link java.lang.String#hashCode}
* {@link java.util.HashMap#putVal}
* {@link java.util.HashMap#resize}
* (int)(DEFAULT_LOAD_FACTOR * initialCapacity) = 0.75f * 8 = 6 as newThreshold
* Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap] 创建8长度的kv节点
* (此时capacity为8，threshold重新计算为6)
* {@link java.util.HashMap#newNode}
* create new Node<K,V> 赋值到newTab[0]
* modCount++
* {@link java.util.HashMap#afterNodeInsertion}
* @see java.util.HashMap#put
* {@link java.util.HashMap#hash(Object key)}
* {@link java.util.HashMap#putVal}
* {@link java.util.HashMap#afterNodeAccess}
* @see java.util.HashMap#put
* {@link java.util.HashMap#hash(Object key)}
* {@link java.util.HashMap#putVal}
* {@link java.util.HashMap#newNode}
* {@link java.util.HashMap#afterNodeInsertion}
* ... ...
* @see java.util.HashMap#put
* {@link java.util.HashMap#hash(Object key)}
* {@link java.util.HashMap#putVal}
* {@link java.util.HashMap#newNode}
* {@link java.util.HashMap#resize}
* oldCap=8，oldThr=6；
* newCap=oldCap << 1，newThr=16*0.75=12
* Node<K, V> oldTab length=8(但实际上只有6个元素，2个空位), Node<K, V> newTab length=16(new Node[newCap])
* for循环将oldTab数据移动到newTab中，返回newTab
* {@link java.util.HashMap#afterNodeInsertion}
*/
private static void createNonEmptyHashMap(){
Map<String, String> map = new HashMap<>(5);
map.put("1", "China");
map.put("1", "America");
map.put("2", "England");
map.put("3", "France");
map.put("4", "Japan");
map.put("5", "Australia");
map.put("6", "Canada");
map.put("7", "Russia"); // 达到threshold，触发resize，oldCap=8，oldThr=6，newCap=oldCap << 1，newThr=16*0.75=12, 此时将oldTab数据移动到newTab里，返回newTab
// ... ... 当添加到第13个元素时还会触发resize扩容，此时oldCap << 1=32，newThr = 24
// ... ...
System.out.println(map.toString());
}
}
tableSizeFor 方法
/**
* 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;
}
resize 方法
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
