resize()扩容
若长度超过2^30(1073741824)则不进行扩容
否则新的长度扩大的为之前数组长度的两倍。
如果节点没有链表，用新数组计算节点位置，放入新数组中
如果节点是树split()方法
如果节点是链表(如果位置是0，直接放到新数组中位置不变，如果是其他位置，移动旧的数组个长度放入)1.7中放入链表会头尾变序
返回新数组
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) {//MAXIMUM_CAPACITY = 1073741824，如果旧的数组长度大于最大长度
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)//新的长度为旧的长度的两倍并且小于MAXIMUM_CAPACITY 并且旧的长度大于14
newThr = oldThr << 1; // double threshold 扩大两倍
}
else if (oldThr > 0) // initial capacity was placed in threshold 如果初始阀值大于0，把初始阀值赋值给初始容量
newCap = oldThr;
else { // zero initial threshold signifies using defaults 零初始阈值表示使用默认值。
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {//若新的阀值为0
float ft = (float)newCap * loadFactor;//新的容量*装载因子(默认0.75)
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);//新的容量<MAXIMUM_CAPACITY 并且 ft<MAXIMUM_CAPACITY 则新的阀值为ft，否则为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)//e的链表是空，直接把e放入新的数组中
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)//如果节点是树
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order 保留顺序
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) {//是否是第一位
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {//其他位置
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) {//移动原数组长度个位置
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
hash(Object key)
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
split()来切割树，如果树的节点小于等于6就进行切割然后取消树化，否则就把树放入相应位置中
final void split(HashMap<K,V> map, Node<K,V>[] tab, int index, int bit) {
TreeNode<K,V> b = this;
// Relink into lo and hi lists, preserving order
TreeNode<K,V> loHead = null, loTail = null;
TreeNode<K,V> hiHead = null, hiTail = null;
int lc = 0, hc = 0;
for (TreeNode<K,V> e = b, next; e != null; e = next) {
next = (TreeNode<K,V>)e.next;
e.next = null;
if ((e.hash & bit) == 0) {//如果是位置0，双链头是loHead双链尾是loTail
if ((e.prev = loTail) == null)
loHead = e;
else
loTail.next = e;
loTail = e;
++lc;
}
else {//如果不是位置0，双链头是hiHead双链尾是hiTail
if ((e.prev = hiTail) == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
++hc;
}
}
if (loHead != null) {
if (lc <= UNTREEIFY_THRESHOLD)//如果链的长度小于等于6，取消树化,把双链变成单链
tab[index] = loHead.untreeify(map);
else {
tab[index] = loHead;//放入0位置中
if (hiHead != null) // (else is already treeified)
loHead.treeify(tab);
}
}
if (hiHead != null) {
if (hc <= UNTREEIFY_THRESHOLD)//如果链的长度小于等于6，取消树化,把双链变成单链
tab[index + bit] = hiHead.untreeify(map);
else {
tab[index + bit] = hiHead;//放入index+旧长度位置中
if (loHead != null)
hiHead.treeify(tab);
}
}
}
untreeify()取消树化
final Node<K,V> untreeify(HashMap<K,V> map) {
Node<K,V> hd = null, tl = null;
for (Node<K,V> q = this; q != null; q = q.next) {
Node<K,V> p = map.replacementNode(q, null);
if (tl == null)
hd = p;
else
tl.next = p;
tl = p;
}
return hd;
}
get()
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) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))//若hash值相同，key也相同，返回当前节点
return first;
if ((e = first.next) != null) {//如果节点的next不为空，则可能是链表或者红黑树
if (first instanceof TreeNode)//如果节点是红黑树
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {//如果链表，循环获取链表中相同key和key的hash的节点并返回
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
getTreeNode中主要是find()
/**
* 如果要查找的hash小于p节点就查左边，
* 如果查找的hash大于p节点就去右边查，
* 左边空查右边，
* 右边空查左边
* 递归查找
*/
final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
TreeNode<K,V> p = this;
do {
int ph, dir; K pk;
TreeNode<K,V> pl = p.left, pr = p.right, q;
if ((ph = p.hash) > h)
p = pl;
else if (ph < h)
p = pr;
else if ((pk = p.key) == k || (k != null && k.equals(pk)))
return p;
else if (pl == null)
p = pr;
else if (pr == null)
p = pl;
else if ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
else if ((q = pr.find(h, k, kc)) != null)
return q;
else
p = pl;
} while (p != null);
return null;
}
put()
若数组长度为空进行扩容
若算出的位置为空，在当前位置新建新节点
若位置不为空，则确定是否value重复、确定是不是树，确定是不是链表
若key和hash相同，则用新value替换旧value，并把旧value返回
如果是树，添加新节点
若链表则去新增链节点，如果新增的节点大等8折转成树，若在链中key和hash相同，则用新value替换旧value，并把旧value返回
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)//若数组长度为空则进行扩容
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)//(n-1) & hash 算出来的值，小于n,相当于hash值 % n ，若算出的位置为空，新建新节点
tab[i] = newNode(hash, key, value, null);
else {//若位置不为空，则确定是否value重复、确定是不是树，确定是不是链表
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))//若key和hash相同，则用新value替换旧value，并把旧value返回
e = p;
else if (p instanceof TreeNode)//如果是树，添加新节点
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {//是链表则去新增链节点，如果新增的节点大等8折转成树，若在链中key和hash相同，则用新value替换旧value，并把旧value返回
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key 此时做新value替换旧value
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;//记录操作计数,并发计数，替换旧的value不算操作
if (++size > threshold)//若size大于阀值则进行扩容
resize();
afterNodeInsertion(evict);
return null;
}
putTreeVal()
final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,
int h, K k, V v) {
Class<?> kc = null;
boolean searched = false;
TreeNode<K,V> root = (parent != null) ? root() : this;
for (TreeNode<K,V> p = root;;) {
int dir, ph; K pk;
if ((ph = p.hash) > h)//传入的hash小于p的hash 则dir = -1；
dir = -1;
else if (ph < h)//传入的hash大雨p的hash 则dir = 1;
dir = 1;
else if ((pk = p.key) == k || (k != null && k.equals(pk)))//p的key和传入的key相同，用新的value替换旧的value并且把旧value返回
return p;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0) {
if (!searched) {
TreeNode<K,V> q, ch;
searched = true;
if (((ch = p.left) != null &&
(q = ch.find(h, k, kc)) != null) ||
((ch = p.right) != null &&
(q = ch.find(h, k, kc)) != null))
return q;
}
dir = tieBreakOrder(k, pk);
}
TreeNode<K,V> xp = p;
if ((p = (dir <= 0) ? p.left : p.right) == null) {//dir为负获取p的左子树，dir为正获取p的右子树，并且赋值给p，判断p是否为空
Node<K,V> xpn = xp.next;
TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);
if (dir <= 0)//小的放左边，大的放右边，小的放左边
xp.left = x;
else
xp.right = x;
xp.next = x;
x.parent = x.prev = xp;
if (xpn != null)
((TreeNode<K,V>)xpn).prev = x;
moveRootToFront(tab, balanceInsertion(root, x));//先平衡在移动root到链表首节点
return null;
}
//不为空，继续循环，直到找到为空的节点
}
}
balanceInsertion()
/**
* 平衡该树
* @param root
* @param x
* @param <K>
* @param <V>
* @return
*/
static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
TreeNode<K,V> x) {
x.red = true;//当前节点为red
for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {
if ((xp = x.parent) == null) {//当前节点的父节点为xp为空时，说明时根节点，把节点返回
x.red = false;
return x;
}
else if (!xp.red || (xpp = xp.parent) == null)//如果该节点不是红的或者xp的父节点xpp为空，返回该root，如果有两个节点时如此
return root;
if (xp == (xppl = xpp.left)) {//x节点的祖父节点的左子树为xppl，若xppl和xp相等
if ((xppr = xpp.right) != null && xppr.red) {//x节点的祖父节点的右节点为xppr，若xppr不为空，并且xppr为red
xppr.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {//x节点的祖父节点的右节点为xppr，若xppr为空或者xppr为黑
if (x == xp.right) {//若x的父节点的右子树为x
root = rotateLeft(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
if (xp != null) {//x的父节点不是空
xp.red = false;//x的父节点为黑
if (xpp != null) {//x的祖父节点不是空
xpp.red = true;//x的祖父节点为红
root = rotateRight(root, xpp);//右旋
}
}
}
}
else {
if (xppl != null && xppl.red) {
xppl.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {
if (x == xp.left) {
root = rotateRight(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
if (xp != null) {
xp.red = false;
if (xpp != null) {
xpp.red = true;
root = rotateLeft(root, xpp);//左旋
}
}
}
}
}
}
左旋和右旋
static <K,V> TreeNode<K,V> rotateLeft(TreeNode<K,V> root,
TreeNode<K,V> p) {
TreeNode<K,V> r, pp, rl;
if (p != null && (r = p.right) != null) {
if ((rl = p.right = r.left) != null)
rl.parent = p;
if ((pp = r.parent = p.parent) == null)
(root = r).red = false;
else if (pp.left == p)
pp.left = r;
else
pp.right = r;
r.left = p;
p.parent = r;
}
return root;
}
static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,
TreeNode<K,V> p) {//右旋
TreeNode<K,V> l, pp, lr;
if (p != null && (l = p.left) != null) {//p节点不是空，p节点的左子树为l不为空
if ((lr = p.left = l.right) != null)//p的左子树的右子树赋值给p的左子树为lr不为空
lr.parent = p;
if ((pp = l.parent = p.parent) == null)//p的父节点赋值给p的左子树的parent，赋值给pp，若pp为空
(root = l).red = false;//l变为根节点，并且red=false
else if (pp.right == p)
pp.right = l;
else
pp.left = l;
l.right = p;
p.parent = l;
}
return root;
}
moveRootToFront()
static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
int n;
if (root != null && tab != null && (n = tab.length) > 0) {
int index = (n - 1) & root.hash;
TreeNode<K,V> first = (TreeNode<K,V>)tab[index];//first指向数组中链表第一个节点
if (root != first) {//root是新转化成红黑树的root，first是之前tab中的tab【index】
Node<K,V> rn;
tab[index] = root;//把root放到数组中index的位置上
TreeNode<K,V> rp = root.prev;
if ((rn = root.next) != null)
((TreeNode<K,V>)rn).prev = rp;
if (rp != null)
rp.next = rn;
if (first != null)
first.prev = root;
root.next = first;
root.prev = null;
}
//使用断言来校验结构
assert checkInvariants(root);
}
}
checkInvariants()
static <K,V> boolean checkInvariants(TreeNode<K,V> t) {
TreeNode<K,V> tp = t.parent, tl = t.left, tr = t.right,
tb = t.prev, tn = (TreeNode<K,V>)t.next;
if (tb != null && tb.next != t)//t的前驱节点不为空 并且 t的前驱节点的后继节点不为t（应该为t）
return false;
if (tn != null && tn.prev != t)//t的后继节点不为空 并且 t的后继节点的前驱节点不为t（应该为t）
return false;
if (tp != null && t != tp.left && t != tp.right)//t的父节点不是空 并且 t不是tp的左子树 并且 t也不是tp的右子树
return false;
if (tl != null && (tl.parent != t || tl.hash > t.hash))//t的左子树不是空 并且 （t的左子树不是t 或者 t的左子树的hash值大于t的hash值）
return false;
if (tr != null && (tr.parent != t || tr.hash < t.hash))//t的右子树不是空 并且 （t的右子树不是t 或者 t的右子树的hash值小于t的hash值）
return false;
if (t.red && tl != null && tl.red && tr != null && tr.red)//t是红 并且 t的左子树不是空 并且 t的左子树是红 并且 t的右子树不为空 并且 t的右子树为红（就是根左右都红）
return false;
if (tl != null && !checkInvariants(tl))//tl不是空的 并且 校验递归校验tl为false
return false;
if (tr != null && !checkInvariants(tr))//tr不是空的 并且 校验递归校验tr为false
return false;
return true;
}
treeifyBin()进行树化
final void treeifyBin(Node<K,V>[] tab, int hash) {//变成双向的链表
int n, index; Node<K,V> e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
treeify()进行真正的树化
final void treeify(Node<K,V>[] tab) {
TreeNode<K,V> root = null;
//循环当前的树，让它成为红黑树
for (TreeNode<K,V> x = this, next; x != null; x = next) {
next = (TreeNode<K,V>)x.next;
x.left = x.right = null;
if (root == null) {
x.parent = null;
x.red = false;
root = x;
}
else {
K k = x.key;
int h = x.hash;
Class<?> kc = null;
for (TreeNode<K,V> p = root;;) {
int dir, ph;
K pk = p.key;
if ((ph = p.hash) > h)
dir = -1;
else if (ph < h)
dir = 1;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0)
dir = tieBreakOrder(k, pk);
TreeNode<K,V> xp = p;
//如果dir为负，放左边，dir为正，放右边（大的放左边，小的放右边）
if ((p = (dir <= 0) ? p.left : p.right) == null) {
x.parent = xp;
if (dir <= 0)
xp.left = x;
else
xp.right = x;
//去平衡二叉树
root = balanceInsertion(root, x);
break;
}
}
}
}
moveRootToFront(tab, root);
}
总结：
HashMap是一个由数组、链表、红黑树构成的数据结构。如果不设置初始大小，默认大小为16，如果设置初始大小，大小为2的倍数，则为该值，如果不是2的倍数，则向上取2的倍数（如设置100，取128）。默认的装载因子是0.75，每次扩容会判断当前所用空间是否是当前空间的0.75，大于0.75则扩容。
