扩容
源码：
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;}
复制代码
Node<K,V>[] oldTab = table;扩容前的数组
int oldCap = (oldTab == null) ? 0 : oldTab.length;扩容前的数组的大小和阈值
int newCap, newThr = 0;预定义新数组的大小和阈值
if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; }判断是否超过最大值，超多则不再扩容
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold扩大容量为当前容量的两倍，但不能超过 MAXIMUM_CAPACITY
else if (oldThr > 0) newCap = oldThr;
判断当前数组是否没有数据，满足条件则使用初始化的值
newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); 如果初始化的值为 0，则使用默认的初始化容量
if (newThr == 0) { }对新的容量是否等于 0 进行判断
table = newTab;开始扩容，将新的容量赋值给 table
if (oldTab != null) {原数据不为空，将原数据复制到新 table 中
for (int j = 0; j < oldCap; ++j) {根据容量循环数组，复制非空元素到新 table
if (e.next == null) newTab[e.hash & (newCap - 1)] = e;如果链表只有一个，则进行直接赋值
if (loTail != null) { loTail.next = null; newTab[j] = loHead; }将原索引放到哈希桶中
f (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hiHead;将原索引 + oldCap 放到哈希桶中
总结
从以上源码可以看出，在扩容通过高位运算（e.hash & oldCap）来确定元素是否需要移动。
HashMap 插入元素使用尾插法，扩容时使用了首尾指针保证了链表元素顺序不会倒置
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