JDK1.8 HashMap中resize()方法解析
/* 初始化或者是将table大小加倍。如果为空，则按threshold分配空间，否则，加倍后，每个容器中的元素在新table中要么呆在原索引处，要么有一个2的次幂的位移
* @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 &&//容量加倍
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // 阈值加倍
}
else if (oldThr > 0) // 如果oldCap<=0，初始容量为阈值threshold
newCap = oldThr;
else { // 零初始化阈值表明使用默认值
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);//红黑树分裂
else { // 保持原有顺序
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);
//新表索引：hash & (newCap - 1)---》低x位为Index
//旧表索引：hash & (oldCap - 1)---》低x-1位为Index
//newCap = oldCap << 1
//举例说明：resize()之前为低x-1位为Index，resize()之后为低x位为Index
//则所有Entry中，hash值第x位为0的，不需要哈希到新位置，只需要呆在当前索引下的新位置j
//hash值第x位为1的，需要哈希到新位置，新位置为j+oldCap
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}