// newCapacity为新的容量
void resize(int newCapacity) {
// 小数组,临时过度下
Entry[] oldTable = table;
// 扩容前的容量
int oldCapacity = oldTable.length;
// MAXIMUM_CAPACITY 为最大容量,2 的 30 次方 = 1<<30
if (oldCapacity == MAXIMUM_CAPACITY) {
// 容量调整为 Integer 的最大值 0x7fffffff(十六进制)=2 的 31 次方-1
threshold = Integer.MAX_VALUE;
return;
}
// 初始化一个新的数组(大容量)
Entry[] newTable = new Entry[newCapacity];
// 把小数组的元素转移到大数组中
transfer(newTable, initHashSeedAsNeeded(newCapacity));
// 引用新的大数组
table = newTable;
// 重新计算阈值
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
该方法接收一个新的容量newCapacity,然后将HashMap的容量扩大到newCapcacity。
首先,方法获取当前HashMap的旧数组oldTable和oldCapacity。如果旧容量已经达到HashMap支持的最大容量MAXIMUM_CAPACITY(2的30次方),就将新的阈值threshold调整为Integer.MAX_VALUE(2的31次方-1),这是因为HashMap的值不能超过MAXIMUM_CAPACITY。
被初始化为原容量oldCapacity的两倍。然后,如果newCapacity超过了HashMap的容量限制 MAXIMUM_CAPACITY(2^30),就将 newCapacity 设置为 MAXIMUM_CAPACITY。如果newCapacity 小于默认初始容量 DEFAULT_INITIAL_CAPACITY(16),就将 newCapacity 设置为DEFAULT_INITIAL_CAPACITY。这样可以避免新容量太小或太大导致哈希冲突过多或者浪费空间。
Java8的时候,newCapacity的计算方式发生了一些细微的变化。
int newCapacity = oldCapacity << 1;
if (newCapacity >= DEFAULT_INITIAL_CAPACITY && oldCapacity >=
DEFAULT_INITIAL_CAPACITY) {
if (newCapacity > MAXIMUM_CAPACITY)
newCapacity = MAXIMUM_CAPACITY;
} else {
if (newCapacity < DEFAULT_INITIAL_CAPACITY)
newCapacity = DEFAULT_INITIAL_CAPACITY;
}
JDK8的扩容源代码:
final Node[] resize() {
Node[] oldTab = table;//获取原来的数组table
int oldCap = (oldTab == null) ? 0 : oldTab.length;//获取数组长度oldcap
int oldThr = threshold;//获取阈值oldThr
int newCap, newThr = 0;
if (oldCap > 0) {//如果原来的数组table不为空
if (oldCap >= MAXIMUM_CAPACITY) {//超过最大值就不再扩充了,就只好随你碰撞区把
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&//没超过最大值,就扩充为原来的2倍
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);
}
//计算新的resize上限
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;//将新阈值赋值给成员变量threshold
@SuppressWarnings({"rawtypes","unchecked"})
Node[] newTab = (Node[])new Node[newCap];//创建新数组 newTab
table = newTab;//将新数组newTab赋值给成员变量table
if (oldTab != null) {//如果旧数组oldTab不为空
for (int j = 0; j < oldCap; ++j) {
Node 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)e).split(this, newTab, j, oldCap);
else { // preserve order
Node loHead = null, loTail = null;
Node hiHead = null, hiTail = null;
Node 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;
}