HashMap 源代码

1、简单介绍HashMap

HashMap 的实例有两个参数影响其性能：初始容量和加载因子。容量是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。

加载因子：是哈希表在其容量自动增加之前可以达到多满的一种尺度。当哈希表中的条目数超出了加载因子与当前容量的乘积时，

则要对该哈希表进行 rehash 操作（即重建内部数据结构），从而哈希表将具有大约两倍的桶数。

默认加载因子 (默认为0.75f)： 在时间和空间成本上寻求一种折衷。加载因子过高虽然减少了空间开销，但同时也增加了查询成本
（在大多数 HashMap 类的操作中，包括 get 和 put 操作，都反映了这一点）。
在设置初始容量时应该考虑到映射中所需的条目数及其加载因子，以便最大限度地减少 rehash 操作次数。
如果初始容量大于最大条目数除以加载因子，则不会发生 rehash 操作。

2、初始化容量、加载因子及最大容量

    /**     * The default initial capacity - MUST be a power of two.     */    //默认的初始化容量，为16    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    /**     * The maximum capacity, used if a higher value is implicitly specified     * by either of the constructors with arguments.     * MUST be a power of two <= 1<<30.     */    //最大容量为 2^30    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * The load factor used when none specified in constructor.     */    //默认加载因子为 0.75f    static final float DEFAULT_LOAD_FACTOR = 0.75f;

3、初始化机制

初始化的容量一定为2的n次方，比如：cap为65个，则初始化容量为2^n，n满足 2^(n-1) < cap <=2^n

代码如下

    /**     * Returns a power of two size for the given target capacity.     */    //初始化机制    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;    }

4、扩容机制

源代码中有个阀值 threshold = cap * loadFactor，当插入键值对后，当前容量超过threshold，则扩容， 扩容为原来的2倍

newCap = oldCap << 1

5、底层实现

JDK8中新增的特性，以前都是用桶/链表来实现HashMap的，现在添加一个键值对，当添加的桶的个数到达TREEIFY_THRESHOLD(8)，则检测HashMap中桶数，如果到达MIN_TREEIFY_CAPACITY(64)，则底层改为桶/红黑树来实现

代码如下

    /**     * The bin count threshold for using a tree rather than list for a     * bin.  Bins are converted to trees when adding an element to a     * bin with at least this many nodes. The value must be greater     * than 2 and should be at least 8 to mesh with assumptions in     * tree removal about conversion back to plain bins upon     * shrinkage.     */    //当桶中的键值对数量到达8个，且桶数量大于等于64，则将底层实现从链表转为红黑树    // 如果桶中的键值对到达该阀值，则检测桶数量    static final int TREEIFY_THRESHOLD = 8;    /**     * The smallest table capacity for which bins may be treeified.     * (Otherwise the table is resized if too many nodes in a bin.)     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts     * between resizing and treeification thresholds.     */    //当所有键值对到达64个的时候，则将链表转为红黑树    static final int MIN_TREEIFY_CAPACITY = 64;    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st        //如果同数量大于阀值，则调用treeifyBin方法，当桶数量大于64则需要将底层实现改为红黑树，        // 如果桶数量不到64则重构一下链表        treeifyBin(tab, hash);    /**     * Replaces all linked nodes in bin at index for given hash unless     * table is too small, in which case resizes instead.     */    //当桶的数量太多的时候，底层则改为红黑树实现    final void treeifyBin(Node<K,V>[] tab, int hash) {        int n, index; Node<K,V> e;        //当桶的数量有 MIN_TREEIFY_CAPACITY （64）个时才将链表改为红黑树        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)            //节点太少则resize()            resize();        else if ((e = tab[index = (n - 1) & hash]) != null) {            //将链接的结点转为二叉树结构            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);        }    }

6、红黑树源代码

里面的插入、删除，情况几的标记是结合维基百科中的红黑树文章中的插入、删除分的几种情况，该文链接：维基百科中的红黑树

 /* ------------------------------------------------------------ */    // Tree bins    /**     * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn     * extends Node) so can be used as extension of either regular or     * linked node.     */    /*    性质1. 节点是红色或黑色。    性质2. 根是黑色。    性质3. 所有叶子都是黑色（叶子是NIL节点）。    性质4. 每个红色节点必须有两个黑色的子节点。(从每个叶子到根的所有路径上不能有两个连续的红色节点。)    性质5. 从任一节点到其每个叶子的所有简单路径都包含相同数目的黑色节点。     */    //红黑树实现类    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {        //节点的父亲        TreeNode<K,V> parent;  // red-black tree links        //节点的左孩子        TreeNode<K,V> left;        //节点的右孩子        TreeNode<K,V> right;        //节点的前一个节点        TreeNode<K,V> prev;    // needed to unlink next upon deletion        //true表示红节点，false表示黑节点        boolean red;        TreeNode(int hash, K key, V val, Node<K,V> next) {            super(hash, key, val, next);        }        /**         * Returns root of tree containing this node.         */        //获取红黑树的根        final TreeNode<K,V> root() {            for (TreeNode<K,V> r = this, p;;) {                if ((p = r.parent) == null)                    return r;                r = p;            }        }        /**         * Ensures that the given root is the first node of its bin.         */        //确保root是桶中的第一个元素        //将root移到中中的第一个        static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {            int n;            if (root != null && tab != null && (n = tab.length) > 0) {                //获取下标值                int index = (n - 1) & root.hash;                TreeNode<K,V> first = (TreeNode<K,V>)tab[index];                if (root != first) {                    //root不是桶中第一个元素                    Node<K,V> rn;                    //将桶中的第一个元素设置为root                    tab[index] = root;                    TreeNode<K,V> rp = root.prev;                    //将结点root删掉，rn.prev = rp; rp.next = rn;                    if ((rn = root.next) != null)                        ((TreeNode<K,V>)rn).prev = rp;                    if (rp != null)                        rp.next = rn;                    //将first插入到root后面                    if (first != null)                        first.prev = root;                    root.next = first;                    root.prev = null;                }                assert checkInvariants(root);            }        }        /**         * Finds the node starting at root p with the given hash and key.         * The kc argument caches comparableClassFor(key) upon first use         * comparing keys.         */        //查找hash为h，key为k的节点        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)                    //h小于节点的hash值，则找左节点                    p = pl;                else if (ph < h)                    //h大于结点的hash值，则找右节点                    p = pr;                else if ((pk = p.key) == k || (k != null && k.equals(pk)))                    //找到则返回                    return p;                else if (pl == null)                    //左节点为空，则找右节点                    p = pr;                else if (pr == null)                    //右节点为空，则找左节点                    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)                    //通过右节点查找                    return q;                else                    p = pl;            } while (p != null);            return null;        }        /**         * Calls find for root node.         */        //获取树节点，通过根节点查找        final TreeNode<K,V> getTreeNode(int h, Object k) {            return ((parent != null) ? root() : this).find(h, k, null);        }        /**         * Tie-breaking utility for ordering insertions when equal         * hashCodes and non-comparable. We don't require a total         * order, just a consistent insertion rule to maintain         * equivalence across rebalancings. Tie-breaking further than         * necessary simplifies testing a bit.         */        //比较2个对象的大小        static int tieBreakOrder(Object a, Object b) {            int d;            if (a == null || b == null ||                (d = a.getClass().getName().                 compareTo(b.getClass().getName())) == 0)                d = (System.identityHashCode(a) <= System.identityHashCode(b) ?                     -1 : 1);            return d;        }        /**         * Forms tree of the nodes linked from this node.         * @return root of tree         */        //将链表转为二叉树        final void treeify(Node<K,V>[] tab) {            TreeNode<K,V> root = null;            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) {                    //根节点设置为黑色                    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;                        if ((p = (dir <= 0) ? p.left : p.right) == null) {                            x.parent = xp;                            if (dir <= 0)                                xp.left = x;                            else                                xp.right = x;                            root = balanceInsertion(root, x);                            break;                        }                    }                }            }            moveRootToFront(tab, root);        }        /**         * Returns a list of non-TreeNodes replacing those linked from         * this node.         */        //将二叉树转为链表        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;        }        /**         * Tree version of putVal.         */        //添加一个键值对        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)                    dir = -1;                else if (ph < h)                    dir = 1;                else if ((pk = p.key) == k || (pk != null && k.equals(pk)))                    //键值对为root，则返回                    return p;                else if ((kc == null &&                          (kc = comparableClassFor(k)) == null) ||                         (dir = compareComparables(kc, k, pk)) == 0) {                    if (!searched) {                        //只运行第一次，检测是否以存在该键值对                        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) {                    Node<K,V> xpn = xp.next;                    //插入节点                    TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);                    if (dir <= 0)                        xp.left = x;                    else                        xp.right = x;                    xp.next = x;                    x.parent = x.prev = xp;                    if (xpn != null)                        ((TreeNode<K,V>)xpn).prev = x;                    //检测平衡                    moveRootToFront(tab, balanceInsertion(root, x));                    return null;                }            }        }        /**         * Removes the given node, that must be present before this call.         * This is messier than typical red-black deletion code because we         * cannot swap the contents of an interior node with a leaf         * successor that is pinned by "next" pointers that are accessible         * independently during traversal. So instead we swap the tree         * linkages. If the current tree appears to have too few nodes,         * the bin is converted back to a plain bin. (The test triggers         * somewhere between 2 and 6 nodes, depending on tree structure).         */        final void removeTreeNode(HashMap<K,V> map, Node<K,V>[] tab,                                  boolean movable) {            int n;            if (tab == null || (n = tab.length) == 0)                return;            int index = (n - 1) & hash;            TreeNode<K,V> first = (TreeNode<K,V>)tab[index], root = first, rl;            TreeNode<K,V> succ = (TreeNode<K,V>)next, pred = prev;            if (pred == null)                tab[index] = first = succ;            else                pred.next = succ;            if (succ != null)                succ.prev = pred;            if (first == null)                return;            if (root.parent != null)                root = root.root();            if (root == null || root.right == null ||                (rl = root.left) == null || rl.left == null) {                //太少就转为链表                tab[index] = first.untreeify(map);  // too small                return;            }            TreeNode<K,V> p = this, pl = left, pr = right, replacement;            if (pl != null && pr != null) {                TreeNode<K,V> s = pr, sl;                while ((sl = s.left) != null) // find successor                    s = sl;                boolean c = s.red; s.red = p.red; p.red = c; // swap colors                TreeNode<K,V> sr = s.right;                TreeNode<K,V> pp = p.parent;                if (s == pr) { // p was s's direct parent                    p.parent = s;                    s.right = p;                }                else {                    TreeNode<K,V> sp = s.parent;                    if ((p.parent = sp) != null) {                        if (s == sp.left)                            sp.left = p;                        else                            sp.right = p;                    }                    if ((s.right = pr) != null)                        pr.parent = s;                }                p.left = null;                if ((p.right = sr) != null)                    sr.parent = p;                if ((s.left = pl) != null)                    pl.parent = s;                if ((s.parent = pp) == null)                    root = s;                else if (p == pp.left)                    pp.left = s;                else                    pp.right = s;                if (sr != null)                    replacement = sr;                else                    replacement = p;            }            else if (pl != null)                replacement = pl;            else if (pr != null)                replacement = pr;            else                replacement = p;            if (replacement != p) {                TreeNode<K,V> pp = replacement.parent = p.parent;                if (pp == null)                    root = replacement;                else if (p == pp.left)                    pp.left = replacement;                else                    pp.right = replacement;                p.left = p.right = p.parent = null;            }            TreeNode<K,V> r = p.red ? root : balanceDeletion(root, replacement);            if (replacement == p) {  // detach                TreeNode<K,V> pp = p.parent;                p.parent = null;                if (pp != null) {                    if (p == pp.left)                        pp.left = null;                    else if (p == pp.right)                        pp.right = null;                }            }            if (movable)                moveRootToFront(tab, r);        }        /**         * Splits nodes in a tree bin into lower and upper tree bins,         * or untreeifies if now too small. Called only from resize;         * see above discussion about split bits and indices.         *         * @param map the map         * @param tab the table for recording bin heads         * @param index the index of the table being split         * @param bit the bit of hash to split on         */        //将结点太多的桶分割        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) {                    if ((e.prev = loTail) == null)                        loHead = e;                    else                        loTail.next = e;                    loTail = e;                    ++lc;                }                else {                    if ((e.prev = hiTail) == null)                        hiHead = e;                    else                        hiTail.next = e;                    hiTail = e;                    ++hc;                }            }            if (loHead != null) {                if (lc <= UNTREEIFY_THRESHOLD)                    //太小则转为链表                    tab[index] = loHead.untreeify(map);                else {                    tab[index] = loHead;                    if (hiHead != null) // (else is already treeified)                        loHead.treeify(tab);                }            }            if (hiHead != null) {                if (hc <= UNTREEIFY_THRESHOLD)                    tab[index + bit] = hiHead.untreeify(map);                else {                    tab[index + bit] = hiHead;                    if (loHead != null)                        hiHead.treeify(tab);                }            }        }        /* ------------------------------------------------------------ */        // Red-black tree methods, all adapted from CLR        //左旋转        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;        }        //右旋转        static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,                                               TreeNode<K,V> p) {            TreeNode<K,V> l, pp, lr;            if (p != null && (l = p.left) != null) {                if ((lr = p.left = l.right) != null)                    lr.parent = p;                if ((pp = l.parent = p.parent) == null)                    (root = l).red = false;                else if (pp.right == p)                    pp.right = l;                else                    pp.left = l;                l.right = p;                p.parent = l;            }            return root;        }        //保证插入后平衡        static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,                                                    TreeNode<K,V> x) {            x.red = true;            for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {                if ((xp = x.parent) == null) {                    //插入 情形1：插入的是根节点，将颜色设为黑色，其他不用处理                    x.red = false;                    return x;                }                else if (!xp.red || (xpp = xp.parent) == null)                    //插入 情形2：插入的节点的父节点是黑色，不用处理                    return root;                if (xp == (xppl = xpp.left)) {                    //对称的，该部分是插入到左边的树                    if ((xppr = xpp.right) != null && xppr.red) {                        //插入 情形3：将父节点和叔节点设为黑色，将祖父设为红色                        xppr.red = false;                        xp.red = false;                        xpp.red = true;                        x = xpp;                    }                    else {                        if (x == xp.right) {                            //插入 情形4：需要左旋转一次                            root = rotateLeft(root, x = xp);                            xpp = (xp = x.parent) == null ? null : xp.parent;                        }                        if (xp != null) {                            //插入 情形5：需要右旋一次                            xp.red = false;                            if (xpp != null) {                                xpp.red = true;                                root = rotateRight(root, xpp);                            }                        }                    }                }                else {                    //对称的，该部分是插入到右边的树                    if (xppl != null && xppl.red) {                        //插入 情形3                        xppl.red = false;                        xp.red = false;                        xpp.red = true;                        x = xpp;                    }                    else {                        if (x == xp.left) {                            //插入 情形4                            root = rotateRight(root, x = xp);                            xpp = (xp = x.parent) == null ? null : xp.parent;                        }                        if (xp != null) {                            //插入 情形5                            xp.red = false;                            if (xpp != null) {                                xpp.red = true;                                root = rotateLeft(root, xpp);                            }                        }                    }                }            }        }        //删除后调整平衡        static <K,V> TreeNode<K,V> balanceDeletion(TreeNode<K,V> root,                                                   TreeNode<K,V> x) {            for (TreeNode<K,V> xp, xpl, xpr;;)  {                if (x == null || x == root)                    //删除 情况1：根节点，不需要调整                    return root;                else if ((xp = x.parent) == null) {                    //删除的是根节点，将跟节点设为黑色                    x.red = false;                    return x;                }                else if (x.red) {                    x.red = false;                    return root;                }                else if ((xpl = xp.left) == x) {                    //在左边树                    if ((xpr = xp.right) != null && xpr.red) {                        //删除 情况2                        xpr.red = false;                        xp.red = true;                        root = rotateLeft(root, xp);                        xpr = (xp = x.parent) == null ? null : xp.right;                    }                    if (xpr == null)                        x = xp;                    else {                        TreeNode<K,V> sl = xpr.left, sr = xpr.right;                        if ((sr == null || !sr.red) &&                            (sl == null || !sl.red)) {                            //删除 情况3                            xpr.red = true;                            x = xp;                        }                        else {                            if (sr == null || !sr.red) {                                //删除 情况5                                if (sl != null)                                    sl.red = false;                                xpr.red = true;                                root = rotateRight(root, xpr);                                xpr = (xp = x.parent) == null ?                                    null : xp.right;                            }                            if (xpr != null) {                                xpr.red = (xp == null) ? false : xp.red;                                if ((sr = xpr.right) != null)                                    sr.red = false;                            }                            if (xp != null) {                                //删除 情况6                                xp.red = false;                                root = rotateLeft(root, xp);                            }                            x = root;                        }                    }                }                else { // symmetric                    //跟上面是对称的                    if (xpl != null && xpl.red) {                        xpl.red = false;                        xp.red = true;                        root = rotateRight(root, xp);                        xpl = (xp = x.parent) == null ? null : xp.left;                    }                    if (xpl == null)                        x = xp;                    else {                        TreeNode<K,V> sl = xpl.left, sr = xpl.right;                        if ((sl == null || !sl.red) &&                            (sr == null || !sr.red)) {                            xpl.red = true;                            x = xp;                        }                        else {                            if (sl == null || !sl.red) {                                if (sr != null)                                    sr.red = false;                                xpl.red = true;                                root = rotateLeft(root, xpl);                                xpl = (xp = x.parent) == null ?                                    null : xp.left;                            }                            if (xpl != null) {                                xpl.red = (xp == null) ? false : xp.red;                                if ((sl = xpl.left) != null)                                    sl.red = false;                            }                            if (xp != null) {                                xp.red = false;                                root = rotateRight(root, xp);                            }                            x = root;                        }                    }                }            }        }        /**         * Recursive invariant check         */        //检测是否符合红黑树        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)                return false;            if (tn != null && tn.prev != t)                return false;            if (tp != null && t != tp.left && t != tp.right)                return false;            if (tl != null && (tl.parent != t || tl.hash > t.hash))                return false;            if (tr != null && (tr.parent != t || tr.hash < t.hash))                return false;            if (t.red && tl != null && tl.red && tr != null && tr.red)                return false;            if (tl != null && !checkInvariants(tl))                return false;            if (tr != null && !checkInvariants(tr))                return false;            return true;        }    }

7、源代码

/* * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */package java.util;import java.io.IOException;import java.io.InvalidObjectException;import java.io.Serializable;import java.lang.reflect.ParameterizedType;import java.lang.reflect.Type;import java.util.function.BiConsumer;import java.util.function.BiFunction;import java.util.function.Consumer;import java.util.function.Function;/** * Hash table based implementation of the <tt>Map</tt> interface.  This * implementation provides all of the optional map operations, and permits * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt> * class is roughly equivalent to <tt>Hashtable</tt>, except that it is * unsynchronized and permits nulls.)  This class makes no guarantees as to * the order of the map; in particular, it does not guarantee that the order * will remain constant over time. * * <p>This implementation provides constant-time performance for the basic * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function * disperses the elements properly among the buckets.  Iteration over * collection views requires time proportional to the "capacity" of the * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number * of key-value mappings).  Thus, it's very important not to set the initial * capacity too high (or the load factor too low) if iteration performance is * important. * * <p>An instance of <tt>HashMap</tt> has two parameters that affect its * performance: <i>initial capacity</i> and <i>load factor</i>.  The * <i>capacity</i> is the number of buckets in the hash table, and the initial * capacity is simply the capacity at the time the hash table is created.  The * <i>load factor</i> is a measure of how full the hash table is allowed to * get before its capacity is automatically increased.  When the number of * entries in the hash table exceeds the product of the load factor and the * current capacity, the hash table is <i>rehashed</i> (that is, internal data * structures are rebuilt) so that the hash table has approximately twice the * number of buckets. * * <p>As a general rule, the default load factor (.75) offers a good * tradeoff between time and space costs.  Higher values decrease the * space overhead but increase the lookup cost (reflected in most of * the operations of the <tt>HashMap</tt> class, including * <tt>get</tt> and <tt>put</tt>).  The expected number of entries in * the map and its load factor should be taken into account when * setting its initial capacity, so as to minimize the number of * rehash operations.  If the initial capacity is greater than the * maximum number of entries divided by the load factor, no rehash * operations will ever occur. * * <p>If many mappings are to be stored in a <tt>HashMap</tt> * instance, creating it with a sufficiently large capacity will allow * the mappings to be stored more efficiently than letting it perform * automatic rehashing as needed to grow the table.  Note that using * many keys with the same {@code hashCode()} is a sure way to slow * down performance of any hash table. To ameliorate impact, when keys * are {@link Comparable}, this class may use comparison order among * keys to help break ties. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access a hash map concurrently, and at least one of * the threads modifies the map structurally, it <i>must</i> be * synchronized externally.  (A structural modification is any operation * that adds or deletes one or more mappings; merely changing the value * associated with a key that an instance already contains is not a * structural modification.)  This is typically accomplished by * synchronizing on some object that naturally encapsulates the map. * * If no such object exists, the map should be "wrapped" using the * {@link Collections#synchronizedMap Collections.synchronizedMap} * method.  This is best done at creation time, to prevent accidental * unsynchronized access to the map:<pre> *   Map m = Collections.synchronizedMap(new HashMap(...));</pre> * * <p>The iterators returned by all of this class's "collection view methods" * are <i>fail-fast</i>: if the map is structurally modified at any time after * the iterator is created, in any way except through the iterator's own * <tt>remove</tt> method, the iterator will throw a * {@link ConcurrentModificationException}.  Thus, in the face of concurrent * modification, the iterator fails quickly and cleanly, rather than risking * arbitrary, non-deterministic behavior at an undetermined time in the * future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification.  Fail-fast iterators * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values * * @author  Doug Lea * @author  Josh Bloch * @author  Arthur van Hoff * @author  Neal Gafter * @see     Object#hashCode() * @see     Collection * @see     Map * @see     TreeMap * @see     Hashtable * @since   1.2 *//*基于哈希表的 Map 接口的实现。此实现提供所有可选的映射操作，并允许使用 null 值和 null 键。（除了非同步和允许使用 null 之外，HashMap 类与 Hashtable 大致相同。）此类不保证映射的顺序，特别是它不保证该顺序恒久不变。此实现假定哈希函数将元素适当地分布在各桶之间，可为基本操作（get 和 put）提供稳定的性能。迭代 collection 视图所需的时间与 HashMap 实例的“容量”（桶的数量）及其大小（键-值映射关系数）成比例。所以，如果迭代性能很重要，则不要将初始容量设置得太高（或将加载因子设置得太低）。HashMap 的实例有两个参数影响其性能：初始容量 和加载因子。容量 是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。加载因子 是哈希表在其容量自动增加之前可以达到多满的一种尺度。当哈希表中的条目数超出了加载因子与当前容量的乘积时，则要对该哈希表进行 rehash 操作（即重建内部数据结构），从而哈希表将具有大约两倍的桶数。通常，默认加载因子 (.75) 在时间和空间成本上寻求一种折衷。加载因子过高虽然减少了空间开销，但同时也增加了查询成本（在大多数 HashMap 类的操作中，包括 get 和 put 操作，都反映了这一点）。在设置初始容量时应该考虑到映射中所需的条目数及其加载因子，以便最大限度地减少 rehash 操作次数。如果初始容量大于最大条目数除以加载因子，则不会发生 rehash 操作。如果很多映射关系要存储在 HashMap 实例中，则相对于按需执行自动的 rehash 操作以增大表的容量来说，使用足够大的初始容量创建它将使得映射关系能更有效地存储。注意，此实现不是同步的。如果多个线程同时访问一个哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须 保持外部同步。（结构上的修改是指添加或删除一个或多个映射关系的任何操作；仅改变与实例已经包含的键关联的值不是结构上的修改。）这一般通过对自然封装该映射的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedMap 方法来“包装”该映射。最好在创建时完成这一操作，以防止对映射进行意外的非同步访问，如下所示：   Map m = Collections.synchronizedMap(new HashMap(...));由所有此类的“collection 视图方法”所返回的迭代器都是快速失败的：在迭代器创建之后，如果从结构上对映射进行修改，除非通过迭代器本身的 remove 方法，其他任何时间任何方式的修改，迭代器都将抛出 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，而不冒在将来不确定的时间发生任意不确定行为的风险。注意，迭代器的快速失败行为不能得到保证，一般来说，存在非同步的并发修改时，不可能作出任何坚决的保证。快速失败迭代器尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的做法是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。 */public class HashMap<K,V> extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable {    private static final long serialVersionUID = 362498820763181265L;    /*     * Implementation notes.     *     * This map usually acts as a binned (bucketed) hash table, but     * when bins get too large, they are transformed into bins of     * TreeNodes, each structured similarly to those in     * java.util.TreeMap. Most methods try to use normal bins, but     * relay to TreeNode methods when applicable (simply by checking     * instanceof a node).  Bins of TreeNodes may be traversed and     * used like any others, but additionally support faster lookup     * when overpopulated. However, since the vast majority of bins in     * normal use are not overpopulated, checking for existence of     * tree bins may be delayed in the course of table methods.     *     * Tree bins (i.e., bins whose elements are all TreeNodes) are     * ordered primarily by hashCode, but in the case of ties, if two     * elements are of the same "class C implements Comparable<C>",     * type then their compareTo method is used for ordering. (We     * conservatively check generic types via reflection to validate     * this -- see method comparableClassFor).  The added complexity     * of tree bins is worthwhile in providing worst-case O(log n)     * operations when keys either have distinct hashes or are     * orderable, Thus, performance degrades gracefully under     * accidental or malicious usages in which hashCode() methods     * return values that are poorly distributed, as well as those in     * which many keys share a hashCode, so long as they are also     * Comparable. (If neither of these apply, we may waste about a     * factor of two in time and space compared to taking no     * precautions. But the only known cases stem from poor user     * programming practices that are already so slow that this makes     * little difference.)     *     * Because TreeNodes are about twice the size of regular nodes, we     * use them only when bins contain enough nodes to warrant use     * (see TREEIFY_THRESHOLD). And when they become too small (due to     * removal or resizing) they are converted back to plain bins.  In     * usages with well-distributed user hashCodes, tree bins are     * rarely used.  Ideally, under random hashCodes, the frequency of     * nodes in bins follows a Poisson distribution     * (http://en.wikipedia.org/wiki/Poisson_distribution) with a     * parameter of about 0.5 on average for the default resizing     * threshold of 0.75, although with a large variance because of     * resizing granularity. Ignoring variance, the expected     * occurrences of list size k are (exp(-0.5) * pow(0.5, k) /     * factorial(k)). The first values are:     *     * 0:    0.60653066     * 1:    0.30326533     * 2:    0.07581633     * 3:    0.01263606     * 4:    0.00157952     * 5:    0.00015795     * 6:    0.00001316     * 7:    0.00000094     * 8:    0.00000006     * more: less than 1 in ten million     *     * The root of a tree bin is normally its first node.  However,     * sometimes (currently only upon Iterator.remove), the root might     * be elsewhere, but can be recovered following parent links     * (method TreeNode.root()).     *     * All applicable internal methods accept a hash code as an     * argument (as normally supplied from a public method), allowing     * them to call each other without recomputing user hashCodes.     * Most internal methods also accept a "tab" argument, that is     * normally the current table, but may be a new or old one when     * resizing or converting.     *     * When bin lists are treeified, split, or untreeified, we keep     * them in the same relative access/traversal order (i.e., field     * Node.next) to better preserve locality, and to slightly     * simplify handling of splits and traversals that invoke     * iterator.remove. When using comparators on insertion, to keep a     * total ordering (or as close as is required here) across     * rebalancings, we compare classes and identityHashCodes as     * tie-breakers.     *     * The use and transitions among plain vs tree modes is     * complicated by the existence of subclass LinkedHashMap. See     * below for hook methods defined to be invoked upon insertion,     * removal and access that allow LinkedHashMap internals to     * otherwise remain independent of these mechanics. (This also     * requires that a map instance be passed to some utility methods     * that may create new nodes.)     *     * The concurrent-programming-like SSA-based coding style helps     * avoid aliasing errors amid all of the twisty pointer operations.     */    /**     * The default initial capacity - MUST be a power of two.     */    //默认的初始化容量，为16    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    /**     * The maximum capacity, used if a higher value is implicitly specified     * by either of the constructors with arguments.     * MUST be a power of two <= 1<<30.     */    //最大容量为 2^30    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * The load factor used when none specified in constructor.     */    //默认加载因子为 0.75f    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * The bin count threshold for using a tree rather than list for a     * bin.  Bins are converted to trees when adding an element to a     * bin with at least this many nodes. The value must be greater     * than 2 and should be at least 8 to mesh with assumptions in     * tree removal about conversion back to plain bins upon     * shrinkage.     */    //当桶中的键值对数量到达8个，且桶数量大于等于64，则将底层实现从链表转为红黑树    // 如果桶中的键值对到达该阀值，则检测桶数量    static final int TREEIFY_THRESHOLD = 8;    /**     * The bin count threshold for untreeifying a (split) bin during a     * resize operation. Should be less than TREEIFY_THRESHOLD, and at     * most 6 to mesh with shrinkage detection under removal.     */    static final int UNTREEIFY_THRESHOLD = 6;    /**     * The smallest table capacity for which bins may be treeified.     * (Otherwise the table is resized if too many nodes in a bin.)     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts     * between resizing and treeification thresholds.     */    //当桶数量到达64个的时候，则将链表转为红黑树    static final int MIN_TREEIFY_CAPACITY = 64;    /**     * Basic hash bin node, used for most entries.  (See below for     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)     */    //HashMap中节点的类    static class Node<K,V> implements Map.Entry<K,V> {        //结点的哈希值，不可变        final int hash;        //结点的key，不可变        final K key;        //节点的值，可变        V value;        //指向下一个节点        Node<K,V> next;        Node(int hash, K key, V value, Node<K,V> next) {            this.hash = hash;            this.key = key;            this.value = value;            this.next = next;        }        public final K getKey()        { return key; }        public final V getValue()      { return value; }        public final String toString() { return key + "=" + value; }        //将key和value的哈希值去异或        public final int hashCode() {            return Objects.hashCode(key) ^ Objects.hashCode(value);        }        //设置value        public final V setValue(V newValue) {            V oldValue = value;            value = newValue;            return oldValue;        }        //判断2个结点是否相同        public final boolean equals(Object o) {            if (o == this)                return true;            if (o instanceof Map.Entry) {                Map.Entry<?,?> e = (Map.Entry<?,?>)o;                if (Objects.equals(key, e.getKey()) &&                    Objects.equals(value, e.getValue()))                    return true;            }            return false;        }    }    /* ---------------- Static utilities -------------- */    /**     * Computes key.hashCode() and spreads (XORs) higher bits of hash     * to lower.  Because the table uses power-of-two masking, sets of     * hashes that vary only in bits above the current mask will     * always collide. (Among known examples are sets of Float keys     * holding consecutive whole numbers in small tables.)  So we     * apply a transform that spreads the impact of higher bits     * downward. There is a tradeoff between speed, utility, and     * quality of bit-spreading. Because many common sets of hashes     * are already reasonably distributed (so don't benefit from     * spreading), and because we use trees to handle large sets of     * collisions in bins, we just XOR some shifted bits in the     * cheapest possible way to reduce systematic lossage, as well as     * to incorporate impact of the highest bits that would otherwise     * never be used in index calculations because of table bounds.     */    static final int hash(Object key) {        int h;        //将key的哈希值h 与 h右移16位的值 去异或        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);    }    /**     * Returns x's Class if it is of the form "class C implements     * Comparable<C>", else null.     */    static Class<?> comparableClassFor(Object x) {        if (x instanceof Comparable) {            Class<?> c; Type[] ts, as; Type t; ParameterizedType p;            if ((c = x.getClass()) == String.class) // bypass checks                return c;            if ((ts = c.getGenericInterfaces()) != null) {                for (int i = 0; i < ts.length; ++i) {                    if (((t = ts[i]) instanceof ParameterizedType) &&                        ((p = (ParameterizedType)t).getRawType() ==                         Comparable.class) &&                        (as = p.getActualTypeArguments()) != null &&                        as.length == 1 && as[0] == c) // type arg is c                        return c;                }            }        }        return null;    }    /**     * Returns k.compareTo(x) if x matches kc (k's screened comparable     * class), else 0.     */    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable    static int compareComparables(Class<?> kc, Object k, Object x) {        return (x == null || x.getClass() != kc ? 0 :                ((Comparable)k).compareTo(x));    }    /**     * Returns a power of two size for the given target capacity.     */    //初始化机制    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;    }    /* ---------------- Fields -------------- */    /**     * The table, initialized on first use, and resized as     * necessary. When allocated, length is always a power of two.     * (We also tolerate length zero in some operations to allow     * bootstrapping mechanics that are currently not needed.)     */    //存放节点的缓冲，哈希表的桶    transient Node<K,V>[] table;    /**     * Holds cached entrySet(). Note that AbstractMap fields are used     * for keySet() and values().     */    //指向调用entrySet()返回的对象    transient Set<Map.Entry<K,V>> entrySet;    /**     * The number of key-value mappings contained in this map.     */    //键值对的容量    transient int size;    /**     * The number of times this HashMap has been structurally modified     * Structural modifications are those that change the number of mappings in     * the HashMap or otherwise modify its internal structure (e.g.,     * rehash).  This field is used to make iterators on Collection-views of     * the HashMap fail-fast.  (See ConcurrentModificationException).     */    //检测并发修改的标志参数    transient int modCount;    /**     * The next size value at which to resize (capacity * load factor).     *     * @serial     */    // (The javadoc description is true upon serialization.    // Additionally, if the table array has not been allocated, this    // field holds the initial array capacity, or zero signifying    // DEFAULT_INITIAL_CAPACITY.)    //记录容量 阀值    int threshold;    /**     * The load factor for the hash table.     *     * @serial     */    //加载因子    final float loadFactor;    /* ---------------- Public operations -------------- */    /**     * Constructs an empty <tt>HashMap</tt> with the specified initial     * capacity and load factor.     *     * @param  initialCapacity the initial capacity     * @param  loadFactor      the load factor     * @throws IllegalArgumentException if the initial capacity is negative     *         or the load factor is nonpositive     */    //构造方法    public HashMap(int initialCapacity, float loadFactor) {        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal initial capacity: " +                                               initialCapacity);        if (initialCapacity > MAXIMUM_CAPACITY)            initialCapacity = MAXIMUM_CAPACITY;        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new IllegalArgumentException("Illegal load factor: " +                                               loadFactor);        this.loadFactor = loadFactor;        this.threshold = tableSizeFor(initialCapacity);    }    /**     * Constructs an empty <tt>HashMap</tt> with the specified initial     * capacity and the default load factor (0.75).     *     * @param  initialCapacity the initial capacity.     * @throws IllegalArgumentException if the initial capacity is negative.     */    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }    /**     * Constructs an empty <tt>HashMap</tt> with the default initial capacity     * (16) and the default load factor (0.75).     */    public HashMap() {        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted    }    /**     * Constructs a new <tt>HashMap</tt> with the same mappings as the     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with     * default load factor (0.75) and an initial capacity sufficient to     * hold the mappings in the specified <tt>Map</tt>.     *     * @param   m the map whose mappings are to be placed in this map     * @throws  NullPointerException if the specified map is null     */    public HashMap(Map<? extends K, ? extends V> m) {        this.loadFactor = DEFAULT_LOAD_FACTOR;        //将m的键值对赋到本HashMap中        putMapEntries(m, false);    }    /**     * Implements Map.putAll and Map constructor     *     * @param m the map     * @param evict false when initially constructing this map, else     * true (relayed to method afterNodeInsertion).     */    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {        int s = m.size();        if (s > 0) {            if (table == null) { // pre-size                //计算需要的容量                float ft = ((float)s / loadFactor) + 1.0F;                int t = ((ft < (float)MAXIMUM_CAPACITY) ?                         (int)ft : MAXIMUM_CAPACITY);                if (t > threshold)                    threshold = tableSizeFor(t);            }            else if (s > threshold)                //需要重新分配                resize();            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {                K key = e.getKey();                V value = e.getValue();                //将每个键值对添加                putVal(hash(key), key, value, false, evict);            }        }    }    /**     * Returns the number of key-value mappings in this map.     *     * @return the number of key-value mappings in this map     */    //返回键值对的数量    public int size() {        return size;    }    /**     * Returns <tt>true</tt> if this map contains no key-value mappings.     *     * @return <tt>true</tt> if this map contains no key-value mappings     */    //判断是否为空    public boolean isEmpty() {        return size == 0;    }    /**     * Returns the value to which the specified key is mapped,     * or {@code null} if this map contains no mapping for the key.     *     * <p>More formally, if this map contains a mapping from a key     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :     * key.equals(k))}, then this method returns {@code v}; otherwise     * it returns {@code null}.  (There can be at most one such mapping.)     *     * <p>A return value of {@code null} does not <i>necessarily</i>     * indicate that the map contains no mapping for the key; it's also     * possible that the map explicitly maps the key to {@code null}.     * The {@link #containsKey containsKey} operation may be used to     * distinguish these two cases.     *     * @see #put(Object, Object)     */    //根据key获取相应的value    public V get(Object key) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? null : e.value;    }    /**     * Implements Map.get and related methods     *     * @param hash hash for key     * @param key the key     * @return the node, or null if none     */    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))))                //那个桶的第一个键值对，检测一下是否就是这个元素                return first;            if ((e = first.next) != null) {                if (first instanceof TreeNode)                    //如果是TreeNode，则调用getTreeNode                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);                //循环遍历每个键值对                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        return e;                } while ((e = e.next) != null);            }        }        return null;    }    /**     * Returns <tt>true</tt> if this map contains a mapping for the     * specified key.     *     * @param   key   The key whose presence in this map is to be tested     * @return <tt>true</tt> if this map contains a mapping for the specified     * key.     */    //判断map是否有key的键    public boolean containsKey(Object key) {        return getNode(hash(key), key) != null;    }    /**     * Associates the specified value with the specified key in this map.     * If the map previously contained a mapping for the key, the old     * value is replaced.     *     * @param key key with which the specified value is to be associated     * @param value value to be associated with the specified key     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     */    //添加键值对    public V put(K key, V value) {        return putVal(hash(key), key, value, false, true);    }    /**     * Implements Map.put and related methods     *     * @param hash hash for key     * @param key the key     * @param value the value to put     * @param onlyIfAbsent if true, don't change existing value     * @param evict if false, the table is in creation mode.     * @return previous value, or null if none     */    //添加一个键值对    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)            //如果table为空，则初始化            n = (tab = resize()).length;        if ((p = tab[i = (n - 1) & hash]) == null)            //第i个桶没有元素，直接添加到tab[i]            tab[i] = newNode(hash, key, value, null);        else {            //tab[i]有元素，则需要遍历结点后再添加            Node<K,V> e; K k;            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                //刚好桶顶是key相同的值                e = p;            else if (p instanceof TreeNode)                //是树节点，则调用putTreeVal                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            else {                for (int binCount = 0; ; ++binCount) {                    //遍历到要添加的那个键值对位置                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st                            //如果桶中结点数量（也即是链表的长度）大于阀值，则调用treeifyBin方法，当桶数量大于64则需要将底层实现改为红黑树，                            // 如果数量不到64则重构一下链表                            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                //存在有键为key的键值对，重新设值value                V oldValue = e.value;                if (!onlyIfAbsent || oldValue == null)                    e.value = value;                afterNodeAccess(e);                return oldValue;            }        }        //并发标志参数+1        ++modCount;        //添加后再检测是否到了阀值        if (++size > threshold)            resize();        afterNodeInsertion(evict);        return null;    }    /**     * 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) {                //如果容量大于最大容量，则阀值设置为Integer.MAX_VALUE                threshold = Integer.MAX_VALUE;                return oldTab;            }            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&                     oldCap >= DEFAULT_INITIAL_CAPACITY)                //阀值设置为原来2倍                newThr = oldThr << 1; // double threshold        }        else if (oldThr > 0) // initial capacity was placed in threshold            //阀值大于0，则将新的容量newCap设为阀值            newCap = oldThr;        else {               // zero initial threshold signifies using defaults            //初始化时为默认            newCap = DEFAULT_INITIAL_CAPACITY;            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);        }        if (newThr == 0) {            //新阀值为0，则需要计算新的阀值            float ft = (float)newCap * loadFactor;            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?                      (int)ft : Integer.MAX_VALUE);        }        //设置新的阀值        threshold = newThr;        //创建新的桶        @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) {                    //将旧的桶设为空，方便垃圾回收器回收                    oldTab[j] = null;                    //如果该桶只有一个元素，则直接赋到新的桶里面                    if (e.next == null)                        newTab[e.hash & (newCap - 1)] = e;                    else if (e instanceof TreeNode)                        //如果是红黑树，则调用红黑树自己的封装方法                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);                    else { // preserve order                        //桶中有多个元素，遍历将它们链接到相应的位置                        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) {                                //还是在原来的桶的键值对                                if (loTail == null)                                    loHead = e;                                else                                    loTail.next = e;                                loTail = e;                            }                            else {                                //不在原来桶的键值对                                if (hiTail == null)                                    hiHead = e;                                else                                    hiTail.next = e;                                hiTail = e;                            }                        } while ((e = next) != null);                        if (loTail != null) {                            //放在原来的桶                            loTail.next = null;                            newTab[j] = loHead;                        }                        if (hiTail != null) {                            //放在非原来的桶                            hiTail.next = null;                            newTab[j + oldCap] = hiHead;                        }                    }                }            }        }        return newTab;    }    /**     * Replaces all linked nodes in bin at index for given hash unless     * table is too small, in which case resizes instead.     */    //当桶的数量太多的时候，底层则改为红黑树实现    final void treeifyBin(Node<K,V>[] tab, int hash) {        int n, index; Node<K,V> e;        //当桶的数量有 MIN_TREEIFY_CAPACITY （64）个时才将链表改为红黑树        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)            //桶太少则resize()            resize();        else if ((e = tab[index = (n - 1) & hash]) != null) {            //将链接的结点转为二叉树结构            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)                //将链表结构转为二叉树                hd.treeify(tab);        }    }    /**     * Copies all of the mappings from the specified map to this map.     * These mappings will replace any mappings that this map had for     * any of the keys currently in the specified map.     *     * @param m mappings to be stored in this map     * @throws NullPointerException if the specified map is null     */    //将m的键值对添加到当前的map    public void putAll(Map<? extends K, ? extends V> m) {        putMapEntries(m, true);    }    /**     * Removes the mapping for the specified key from this map if present.     *     * @param  key key whose mapping is to be removed from the map     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     */    //移除掉键值对    public V remove(Object key) {        Node<K,V> e;        return (e = removeNode(hash(key), key, null, false, true)) == null ?            null : e.value;    }    /**     * Implements Map.remove and related methods     *     * @param hash hash for key     * @param key the key     * @param value the value to match if matchValue, else ignored     * @param matchValue if true only remove if value is equal     * @param movable if false do not move other nodes while removing     * @return the node, or null if none     */    final Node<K,V> removeNode(int hash, Object key, Object value,                               boolean matchValue, boolean movable) {        Node<K,V>[] tab; Node<K,V> p; int n, index;        if ((tab = table) != null && (n = tab.length) > 0 &&            (p = tab[index = (n - 1) & hash]) != null) {            //找到对应的桶            Node<K,V> node = null, e; K k; V v;            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                //等于桶第一个键值对                node = p;            else if ((e = p.next) != null) {                //需要遍历桶里面的键值对                if (p instanceof TreeNode)                    //属于红黑树结构                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);                else {                    //属于链表结构                    do {                        //遍历整个链表                        if (e.hash == hash &&                            ((k = e.key) == key ||                             (key != null && key.equals(k)))) {                            node = e;                            break;                        }                        p = e;                    } while ((e = e.next) != null);                }            }            if (node != null && (!matchValue || (v = node.value) == value ||                                 (value != null && value.equals(v)))) {                //找到了要remove的键值对                if (node instanceof TreeNode)                    //底层结构为红黑树                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);                else if (node == p)                    //要remove的键值对为桶第一个元素                    tab[index] = node.next;                else                    p.next = node.next;                ++modCount;                //容量减1                --size;                afterNodeRemoval(node);                return node;            }        }        return null;    }    /**     * Removes all of the mappings from this map.     * The map will be empty after this call returns.     */    //清除掉键值对    public void clear() {        Node<K,V>[] tab;        modCount++;        if ((tab = table) != null && size > 0) {            size = 0;            //将桶清为null,桶中的链表也就成为垃圾了，因为根据可达性分析，链表都已经是不可达了            for (int i = 0; i < tab.length; ++i)                tab[i] = null;        }    }    /**     * Returns <tt>true</tt> if this map maps one or more keys to the     * specified value.     *     * @param value value whose presence in this map is to be tested     * @return <tt>true</tt> if this map maps one or more keys to the     *         specified value     */    //判断是否包含值value    public boolean containsValue(Object value) {        Node<K,V>[] tab; V v;        if ((tab = table) != null && size > 0) {            for (int i = 0; i < tab.length; ++i) {                //遍历每个桶                for (Node<K,V> e = tab[i]; e != null; e = e.next) {                    //遍历桶中的每一个键值对                    if ((v = e.value) == value ||                        (value != null && value.equals(v)))                        return true;                }            }        }        return false;    }    /**     * Returns a {@link Set} view of the keys contained in this map.     * The set is backed by the map, so changes to the map are     * reflected in the set, and vice-versa.  If the map is modified     * while an iteration over the set is in progress (except through     * the iterator's own <tt>remove</tt> operation), the results of     * the iteration are undefined.  The set supports element removal,     * which removes the corresponding mapping from the map, via the     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>     * operations.     *     * @return a set view of the keys contained in this map     */    //返回所有键组成的Set    public Set<K> keySet() {        Set<K> ks;        return (ks = keySet) == null ? (keySet = new KeySet()) : ks;    }    final class KeySet extends AbstractSet<K> {        public final int size()                 { return size; }        public final void clear()               { HashMap.this.clear(); }        public final Iterator<K> iterator()     { return new KeyIterator(); }        public final boolean contains(Object o) { return containsKey(o); }        public final boolean remove(Object key) {            return removeNode(hash(key), key, null, false, true) != null;        }        public final Spliterator<K> spliterator() {            return new KeySpliterator<>(HashMap.this, 0, -1, 0, 0);        }        public final void forEach(Consumer<? super K> action) {            Node<K,V>[] tab;            if (action == null)                throw new NullPointerException();            if (size > 0 && (tab = table) != null) {                int mc = modCount;                for (int i = 0; i < tab.length; ++i) {                    for (Node<K,V> e = tab[i]; e != null; e = e.next)                        //遍历将每一个key都传都到action中                        action.accept(e.key);                }                if (modCount != mc)                    throw new ConcurrentModificationException();            }        }    }    /**     * Returns a {@link Collection} view of the values contained in this map.     * The collection is backed by the map, so changes to the map are     * reflected in the collection, and vice-versa.  If the map is     * modified while an iteration over the collection is in progress     * (except through the iterator's own <tt>remove</tt> operation),     * the results of the iteration are undefined.  The collection     * supports element removal, which removes the corresponding     * mapping from the map, via the <tt>Iterator.remove</tt>,     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not     * support the <tt>add</tt> or <tt>addAll</tt> operations.     *     * @return a view of the values contained in this map     */    //将所有value都存到集合里面    public Collection<V> values() {        Collection<V> vs;        return (vs = values) == null ? (values = new Values()) : vs;    }    final class Values extends AbstractCollection<V> {        public final int size()                 { return size; }        public final void clear()               { HashMap.this.clear(); }        public final Iterator<V> iterator()     { return new ValueIterator(); }        public final boolean contains(Object o) { return containsValue(o); }        public final Spliterator<V> spliterator() {            return new ValueSpliterator<>(HashMap.this, 0, -1, 0, 0);        }        public final void forEach(Consumer<? super V> action) {            Node<K,V>[] tab;            if (action == null)                throw new NullPointerException();            if (size > 0 && (tab = table) != null) {                int mc = modCount;                for (int i = 0; i < tab.length; ++i) {                    for (Node<K,V> e = tab[i]; e != null; e = e.next)                        //将所有值都传到action                        action.accept(e.value);                }                if (modCount != mc)                    throw new ConcurrentModificationException();            }        }    }    /**     * Returns a {@link Set} view of the mappings contained in this map.     * The set is backed by the map, so changes to the map are     * reflected in the set, and vice-versa.  If the map is modified     * while an iteration over the set is in progress (except through     * the iterator's own <tt>remove</tt> operation, or through the     * <tt>setValue</tt> operation on a map entry returned by the     * iterator) the results of the iteration are undefined.  The set     * supports element removal, which removes the corresponding     * mapping from the map, via the <tt>Iterator.remove</tt>,     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and     * <tt>clear</tt> operations.  It does not support the     * <tt>add</tt> or <tt>addAll</tt> operations.     *     * @return a set view of the mappings contained in this map     */    //将所有键值对转为Set    public Set<Map.Entry<K,V>> entrySet() {        Set<Map.Entry<K,V>> es;        return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;    }    final class EntrySet extends AbstractSet<Map.Entry<K,V>> {        public final int size()                 { return size; }        public final void clear()               { HashMap.this.clear(); }        public final Iterator<Map.Entry<K,V>> iterator() {            return new EntryIterator();        }        public final boolean contains(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry<?,?> e = (Map.Entry<?,?>) o;            Object key = e.getKey();            Node<K,V> candidate = getNode(hash(key), key);            return candidate != null && candidate.equals(e);        }        public final boolean remove(Object o) {            if (o instanceof Map.Entry) {                Map.Entry<?,?> e = (Map.Entry<?,?>) o;                Object key = e.getKey();                Object value = e.getValue();                return removeNode(hash(key), key, value, true, true) != null;            }            return false;        }        public final Spliterator<Map.Entry<K,V>> spliterator() {            return new EntrySpliterator<>(HashMap.this, 0, -1, 0, 0);        }        public final void forEach(Consumer<? super Map.Entry<K,V>> action) {            Node<K,V>[] tab;            if (action == null)                throw new NullPointerException();            if (size > 0 && (tab = table) != null) {                int mc = modCount;                for (int i = 0; i < tab.length; ++i) {                    for (Node<K,V> e = tab[i]; e != null; e = e.next)                        //遍历整个键值对，然后传到action                        action.accept(e);                }                if (modCount != mc)                    throw new ConcurrentModificationException();            }        }    }    // Overrides of JDK8 Map extension methods    //找键为key的值value    @Override    public V getOrDefault(Object key, V defaultValue) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? defaultValue : e.value;    }    //将键值对put到map中    @Override    public V putIfAbsent(K key, V value) {        return putVal(hash(key), key, value, true, true);    }    @Override    public boolean remove(Object key, Object value) {        return removeNode(hash(key), key, value, true, true) != null;    }    //替换值    @Override    public boolean replace(K key, V oldValue, V newValue) {        Node<K,V> e; V v;        if ((e = getNode(hash(key), key)) != null &&            ((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {            e.value = newValue;            afterNodeAccess(e);            return true;        }        return false;    }    //替换值    @Override    public V replace(K key, V value) {        Node<K,V> e;        if ((e = getNode(hash(key), key)) != null) {            V oldValue = e.value;            e.value = value;            afterNodeAccess(e);            return oldValue;        }        return null;    }    @Override    public V computeIfAbsent(K key,                             Function<? super K, ? extends V> mappingFunction) {        if (mappingFunction == null)            throw new NullPointerException();        int hash = hash(key);        Node<K,V>[] tab; Node<K,V> first; int n, i;        int binCount = 0;        TreeNode<K,V> t = null;        Node<K,V> old = null;        if (size > threshold || (tab = table) == null ||            (n = tab.length) == 0)            //需要扩容            n = (tab = resize()).length;        if ((first = tab[i = (n - 1) & hash]) != null) {            if (first instanceof TreeNode)                //底层是红黑树                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);            else {                //底层是链表                Node<K,V> e = first; K k;                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k)))) {                        old = e;                        break;                    }                    ++binCount;                } while ((e = e.next) != null);            }            V oldValue;            if (old != null && (oldValue = old.value) != null) {                afterNodeAccess(old);                return oldValue;            }        }        //如果没有该key的键值对        V v = mappingFunction.apply(key);        if (v == null) {            return null;        } else if (old != null) {            old.value = v;            afterNodeAccess(old);            return v;        }        else if (t != null)            //添加红黑树结点            t.putTreeVal(this, tab, hash, key, v);        else {            //添加链表结点            tab[i] = newNode(hash, key, v, first);            if (binCount >= TREEIFY_THRESHOLD - 1)                treeifyBin(tab, hash);        }        ++modCount;        ++size;        afterNodeInsertion(true);        return v;    }    public V computeIfPresent(K key,                              BiFunction<? super K, ? super V, ? extends V> remappingFunction) {        if (remappingFunction == null)            throw new NullPointerException();        Node<K,V> e; V oldValue;        int hash = hash(key);        if ((e = getNode(hash, key)) != null &&            (oldValue = e.value) != null) {            V v = remappingFunction.apply(key, oldValue);            if (v != null) {                e.value = v;                afterNodeAccess(e);                return v;            }            else                removeNode(hash, key, null, false, true);        }        return null;    }    @Override    public V compute(K key,                     BiFunction<? super K, ? super V, ? extends V> remappingFunction) {        if (remappingFunction == null)            throw new NullPointerException();        int hash = hash(key);        Node<K,V>[] tab; Node<K,V> first; int n, i;        int binCount = 0;        TreeNode<K,V> t = null;        Node<K,V> old = null;        if (size > threshold || (tab = table) == null ||            (n = tab.length) == 0)            //扩容            n = (tab = resize()).length;        if ((first = tab[i = (n - 1) & hash]) != null) {            if (first instanceof TreeNode)                //底层是红黑树                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);            else {                //底层是链表                Node<K,V> e = first; K k;                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k)))) {                        old = e;                        break;                    }                    ++binCount;                } while ((e = e.next) != null);            }        }        V oldValue = (old == null) ? null : old.value;        //返回计算结果        V v = remappingFunction.apply(key, oldValue);        if (old != null) {            if (v != null) {                //将计算结果设置为新值                old.value = v;                afterNodeAccess(old);            }            else                //新值为空则移除掉键值对                removeNode(hash, key, null, false, true);        }        else if (v != null) {            if (t != null)                //添加红黑树结点                t.putTreeVal(this, tab, hash, key, v);            else {                //添加链表结点                tab[i] = newNode(hash, key, v, first);                if (binCount >= TREEIFY_THRESHOLD - 1)                    //检测是否需要将链表转为红黑树                    treeifyBin(tab, hash);            }            ++modCount;            ++size;            afterNodeInsertion(true);        }        return v;    }    @Override    public V merge(K key, V value,                   BiFunction<? super V, ? super V, ? extends V> remappingFunction) {        if (value == null)            throw new NullPointerException();        if (remappingFunction == null)            throw new NullPointerException();        int hash = hash(key);        Node<K,V>[] tab; Node<K,V> first; int n, i;        int binCount = 0;        TreeNode<K,V> t = null;        Node<K,V> old = null;        if (size > threshold || (tab = table) == null ||            (n = tab.length) == 0)            //需要扩容            n = (tab = resize()).length;        if ((first = tab[i = (n - 1) & hash]) != null) {            //该桶有键值对            if (first instanceof TreeNode)                //红黑树                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);            else {                //链表结构                Node<K,V> e = first; K k;                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k)))) {                        old = e;                        break;                    }                    ++binCount;                } while ((e = e.next) != null);            }        }        if (old != null) {            //有该key的键值对            V v;            if (old.value != null)                //通过oldValue和value计算得到新value                v = remappingFunction.apply(old.value, value);            else                v = value;            if (v != null) {                //设置新value                old.value = v;                afterNodeAccess(old);            }            else                //新value为null,则remove键值对                removeNode(hash, key, null, false, true);            return v;        }        if (value != null) {            if (t != null)                //添加红黑树                t.putTreeVal(this, tab, hash, key, value);            else {                //添加链表                tab[i] = newNode(hash, key, value, first);                if (binCount >= TREEIFY_THRESHOLD - 1)                    //检测是否需要转换红黑树                    treeifyBin(tab, hash);            }            ++modCount;            ++size;            afterNodeInsertion(true);        }        return value;    }    @Override    public void forEach(BiConsumer<? super K, ? super V> action) {        Node<K,V>[] tab;        if (action == null)            throw new NullPointerException();        if (size > 0 && (tab = table) != null) {            int mc = modCount;            for (int i = 0; i < tab.length; ++i) {                for (Node<K,V> e = tab[i]; e != null; e = e.next)                    //将所有键值对传入action                    action.accept(e.key, e.value);            }            if (modCount != mc)                throw new ConcurrentModificationException();        }    }    @Override    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {        Node<K,V>[] tab;        if (function == null)            throw new NullPointerException();        if (size > 0 && (tab = table) != null) {            int mc = modCount;            for (int i = 0; i < tab.length; ++i) {                for (Node<K,V> e = tab[i]; e != null; e = e.next) {                    //通过计算后重新设值                    e.value = function.apply(e.key, e.value);                }            }            if (modCount != mc)                throw new ConcurrentModificationException();        }    }    /* ------------------------------------------------------------ */    // Cloning and serialization    /**     * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and     * values themselves are not cloned.     *     * @return a shallow copy of this map     */    //克隆    @SuppressWarnings("unchecked")    @Override    public Object clone() {        HashMap<K,V> result;        try {            result = (HashMap<K,V>)super.clone();        } catch (CloneNotSupportedException e) {            // this shouldn't happen, since we are Cloneable            throw new InternalError(e);        }        result.reinitialize();        result.putMapEntries(this, false);        return result;    }    // These methods are also used when serializing HashSets    final float loadFactor() { return loadFactor; }    final int capacity() {        return (table != null) ? table.length :            (threshold > 0) ? threshold :            DEFAULT_INITIAL_CAPACITY;    }    /**     * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,     * serialize it).     *     * @serialData The <i>capacity</i> of the HashMap (the length of the     *             bucket array) is emitted (int), followed by the     *             <i>size</i> (an int, the number of key-value     *             mappings), followed by the key (Object) and value (Object)     *             for each key-value mapping.  The key-value mappings are     *             emitted in no particular order.     */    //序列化 writeObject    private void writeObject(java.io.ObjectOutputStream s)        throws IOException {        int buckets = capacity();        // Write out the threshold, loadfactor, and any hidden stuff        s.defaultWriteObject();        //先写容量        s.writeInt(buckets);        //写大小        s.writeInt(size);        //写键值对        internalWriteEntries(s);    }    /**     * Reconstitute the {@code HashMap} instance from a stream (i.e.,     * deserialize it).     */    //序列化 readObject    private void readObject(java.io.ObjectInputStream s)        throws IOException, ClassNotFoundException {        // Read in the threshold (ignored), loadfactor, and any hidden stuff        s.defaultReadObject();        reinitialize();        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new InvalidObjectException("Illegal load factor: " +                                             loadFactor);        s.readInt();                // Read and ignore number of buckets        int mappings = s.readInt(); // Read number of mappings (size)        if (mappings < 0)            throw new InvalidObjectException("Illegal mappings count: " +                                             mappings);        else if (mappings > 0) { // (if zero, use defaults)            // Size the table using given load factor only if within            // range of 0.25...4.0            float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);            float fc = (float)mappings / lf + 1.0f;            int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?                       DEFAULT_INITIAL_CAPACITY :                       (fc >= MAXIMUM_CAPACITY) ?                       MAXIMUM_CAPACITY :                       tableSizeFor((int)fc));            float ft = (float)cap * lf;            threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?                         (int)ft : Integer.MAX_VALUE);            @SuppressWarnings({"rawtypes","unchecked"})                Node<K,V>[] tab = (Node<K,V>[])new Node[cap];            table = tab;            // Read the keys and values, and put the mappings in the HashMap            for (int i = 0; i < mappings; i++) {                @SuppressWarnings("unchecked")                    K key = (K) s.readObject();                @SuppressWarnings("unchecked")                    V value = (V) s.readObject();                putVal(hash(key), key, value, false, false);            }        }    }    /* ------------------------------------------------------------ */    // iterators    abstract class HashIterator {        Node<K,V> next;        // next entry to return        Node<K,V> current;     // current entry        int expectedModCount;  // for fast-fail        int index;             // current slot        HashIterator() {            expectedModCount = modCount;            Node<K,V>[] t = table;            current = next = null;            index = 0;            if (t != null && size > 0) { // advance to first entry                do {} while (index < t.length && (next = t[index++]) == null);            }        }        //是否有下一个键值对        public final boolean hasNext() {            return next != null;        }        //获取下一个键值对节点        final Node<K,V> nextNode() {            Node<K,V>[] t;            Node<K,V> e = next;            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            if (e == null)                throw new NoSuchElementException();            if ((next = (current = e).next) == null && (t = table) != null) {                do {} while (index < t.length && (next = t[index++]) == null);            }            return e;        }        public final void remove() {            Node<K,V> p = current;            if (p == null)                throw new IllegalStateException();            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            current = null;            K key = p.key;            removeNode(hash(key), key, null, false, false);            expectedModCount = modCount;        }    }    final class KeyIterator extends HashIterator        implements Iterator<K> {        //获取下一个key        public final K next() { return nextNode().key; }    }    final class ValueIterator extends HashIterator        implements Iterator<V> {        //获取下一个value        public final V next() { return nextNode().value; }    }    final class EntryIterator extends HashIterator        implements Iterator<Map.Entry<K,V>> {        //获取下一个键值对        public final Map.Entry<K,V> next() { return nextNode(); }    }    /* ------------------------------------------------------------ */    // spliterators    static class HashMapSpliterator<K,V> {        final HashMap<K,V> map;        //记录当前的节点        Node<K,V> current;          // current node        //当前节点的下标        int index;                  // current index, modified on advance/split        //堆大小        int fence;                  // one past last index        //估计大小        int est;                    // size estimate        int expectedModCount;       // for comodification checks        HashMapSpliterator(HashMap<K,V> m, int origin,                           int fence, int est,                           int expectedModCount) {            this.map = m;            this.index = origin;            this.fence = fence;            this.est = est;            this.expectedModCount = expectedModCount;        }        final int getFence() { // initialize fence and size on first use            int hi;            if ((hi = fence) < 0) {                HashMap<K,V> m = map;                est = m.size;                expectedModCount = m.modCount;                Node<K,V>[] tab = m.table;                hi = fence = (tab == null) ? 0 : tab.length;            }            return hi;        }        public final long estimateSize() {            getFence(); // force init            return (long) est;        }    }    static final class KeySpliterator<K,V>        extends HashMapSpliterator<K,V>        implements Spliterator<K> {        KeySpliterator(HashMap<K,V> m, int origin, int fence, int est,                       int expectedModCount) {            super(m, origin, fence, est, expectedModCount);        }        public KeySpliterator<K,V> trySplit() {            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;            return (lo >= mid || current != null) ? null :                new KeySpliterator<>(map, lo, index = mid, est >>>= 1,                                        expectedModCount);        }        public void forEachRemaining(Consumer<? super K> action) {            int i, hi, mc;            if (action == null)                throw new NullPointerException();            HashMap<K,V> m = map;            Node<K,V>[] tab = m.table;            if ((hi = fence) < 0) {                mc = expectedModCount = m.modCount;                hi = fence = (tab == null) ? 0 : tab.length;            }            else                mc = expectedModCount;            if (tab != null && tab.length >= hi &&                (i = index) >= 0 && (i < (index = hi) || current != null)) {                Node<K,V> p = current;                current = null;                do {                    if (p == null)                        p = tab[i++];                    else {                        action.accept(p.key);                        p = p.next;                    }                } while (p != null || i < hi);                if (m.modCount != mc)                    throw new ConcurrentModificationException();            }        }        public boolean tryAdvance(Consumer<? super K> action) {            int hi;            if (action == null)                throw new NullPointerException();            Node<K,V>[] tab = map.table;            if (tab != null && tab.length >= (hi = getFence()) && index >= 0) {                while (current != null || index < hi) {                    if (current == null)                        current = tab[index++];                    else {                        K k = current.key;                        current = current.next;                        action.accept(k);                        if (map.modCount != expectedModCount)                            throw new ConcurrentModificationException();                        return true;                    }                }            }            return false;        }        public int characteristics() {            return (fence < 0 || est == map.size ? Spliterator.SIZED : 0) |                Spliterator.DISTINCT;        }    }    static final class ValueSpliterator<K,V>        extends HashMapSpliterator<K,V>        implements Spliterator<V> {        ValueSpliterator(HashMap<K,V> m, int origin, int fence, int est,                         int expectedModCount) {            super(m, origin, fence, est, expectedModCount);        }        public ValueSpliterator<K,V> trySplit() {            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;            return (lo >= mid || current != null) ? null :                new ValueSpliterator<>(map, lo, index = mid, est >>>= 1,                                          expectedModCount);        }        public void forEachRemaining(Consumer<? super V> action) {            int i, hi, mc;            if (action == null)                throw new NullPointerException();            HashMap<K,V> m = map;            Node<K,V>[] tab = m.table;            if ((hi = fence) < 0) {                mc = expectedModCount = m.modCount;                hi = fence = (tab == null) ? 0 : tab.length;            }            else                mc = expectedModCount;            if (tab != null && tab.length >= hi &&                (i = index) >= 0 && (i < (index = hi) || current != null)) {                Node<K,V> p = current;                current = null;                do {                    if (p == null)                        p = tab[i++];                    else {                        action.accept(p.value);                        p = p.next;                    }                } while (p != null || i < hi);                if (m.modCount != mc)                    throw new ConcurrentModificationException();            }        }        public boolean tryAdvance(Consumer<? super V> action) {            int hi;            if (action == null)                throw new NullPointerException();            Node<K,V>[] tab = map.table;            if (tab != null && tab.length >= (hi = getFence()) && index >= 0) {                while (current != null || index < hi) {                    if (current == null)                        current = tab[index++];                    else {                        V v = current.value;                        current = current.next;                        action.accept(v);                        if (map.modCount != expectedModCount)                            throw new ConcurrentModificationException();                        return true;                    }                }            }            return false;        }        public int characteristics() {            return (fence < 0 || est == map.size ? Spliterator.SIZED : 0);        }    }    static final class EntrySpliterator<K,V>        extends HashMapSpliterator<K,V>        implements Spliterator<Map.Entry<K,V>> {        EntrySpliterator(HashMap<K,V> m, int origin, int fence, int est,                         int expectedModCount) {            super(m, origin, fence, est, expectedModCount);        }        public EntrySpliterator<K,V> trySplit() {            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;            return (lo >= mid || current != null) ? null :                new EntrySpliterator<>(map, lo, index = mid, est >>>= 1,                                          expectedModCount);        }        public void forEachRemaining(Consumer<? super Map.Entry<K,V>> action) {            int i, hi, mc;            if (action == null)                throw new NullPointerException();            HashMap<K,V> m = map;            Node<K,V>[] tab = m.table;            if ((hi = fence) < 0) {                mc = expectedModCount = m.modCount;                hi = fence = (tab == null) ? 0 : tab.length;            }            else                mc = expectedModCount;            if (tab != null && tab.length >= hi &&                (i = index) >= 0 && (i < (index = hi) || current != null)) {                Node<K,V> p = current;                current = null;                do {                    if (p == null)                        p = tab[i++];                    else {                        action.accept(p);                        p = p.next;                    }                } while (p != null || i < hi);                if (m.modCount != mc)                    throw new ConcurrentModificationException();            }        }        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {            int hi;            if (action == null)                throw new NullPointerException();            Node<K,V>[] tab = map.table;            if (tab != null && tab.length >= (hi = getFence()) && index >= 0) {                while (current != null || index < hi) {                    if (current == null)                        current = tab[index++];                    else {                        Node<K,V> e = current;                        current = current.next;                        action.accept(e);                        if (map.modCount != expectedModCount)                            throw new ConcurrentModificationException();                        return true;                    }                }            }            return false;        }        public int characteristics() {            return (fence < 0 || est == map.size ? Spliterator.SIZED : 0) |                Spliterator.DISTINCT;        }    }    /* ------------------------------------------------------------ */    // LinkedHashMap support    /*     * The following package-protected methods are designed to be     * overridden by LinkedHashMap, but not by any other subclass.     * Nearly all other internal methods are also package-protected     * but are declared final, so can be used by LinkedHashMap, view     * classes, and HashSet.     */    // Create a regular (non-tree) node    //创建一个链表结点    Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {        return new Node<>(hash, key, value, next);    }    // For conversion from TreeNodes to plain nodes    //替换一个链表节点    Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {        return new Node<>(p.hash, p.key, p.value, next);    }    // Create a tree bin node    //创建一个红黑树节点    TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {        return new TreeNode<>(hash, key, value, next);    }    // For treeifyBin    //替换一个红黑树节点    TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {        return new TreeNode<>(p.hash, p.key, p.value, next);    }    /**     * Reset to initial default state.  Called by clone and readObject.     */    void reinitialize() {        table = null;        entrySet = null;        keySet = null;        values = null;        modCount = 0;        threshold = 0;        size = 0;    }    // Callbacks to allow LinkedHashMap post-actions    void afterNodeAccess(Node<K,V> p) { }    void afterNodeInsertion(boolean evict) { }    void afterNodeRemoval(Node<K,V> p) { }    // Called only from writeObject, to ensure compatible ordering.    //通过序列表，初始化键值对    void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {        Node<K,V>[] tab;        if (size > 0 && (tab = table) != null) {            for (int i = 0; i < tab.length; ++i) {                for (Node<K,V> e = tab[i]; e != null; e = e.next) {                    s.writeObject(e.key);                    s.writeObject(e.value);                }            }        }    }    /* ------------------------------------------------------------ */    // Tree bins    /**     * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn     * extends Node) so can be used as extension of either regular or     * linked node.     */    /*    性质1. 节点是红色或黑色。    性质2. 根是黑色。    性质3. 所有叶子都是黑色（叶子是NIL节点）。    性质4. 每个红色节点必须有两个黑色的子节点。(从每个叶子到根的所有路径上不能有两个连续的红色节点。)    性质5. 从任一节点到其每个叶子的所有简单路径都包含相同数目的黑色节点。     */    //红黑树实现类    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {        //节点的父亲        TreeNode<K,V> parent;  // red-black tree links        //节点的左孩子        TreeNode<K,V> left;        //节点的右孩子        TreeNode<K,V> right;        //节点的前一个节点        TreeNode<K,V> prev;    // needed to unlink next upon deletion        //true表示红节点，false表示黑节点        boolean red;        TreeNode(int hash, K key, V val, Node<K,V> next) {            super(hash, key, val, next);        }        /**         * Returns root of tree containing this node.         */        //获取红黑树的根        final TreeNode<K,V> root() {            for (TreeNode<K,V> r = this, p;;) {                if ((p = r.parent) == null)                    return r;                r = p;            }        }        /**         * Ensures that the given root is the first node of its bin.         */        //确保root是桶中的第一个元素        //将root移到中中的第一个        static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {            int n;            if (root != null && tab != null && (n = tab.length) > 0) {                //获取下标值                int index = (n - 1) & root.hash;                TreeNode<K,V> first = (TreeNode<K,V>)tab[index];                if (root != first) {                    //root不是桶中第一个元素                    Node<K,V> rn;                    //将桶中的第一个元素设置为root                    tab[index] = root;                    TreeNode<K,V> rp = root.prev;                    //将结点root删掉，rn.prev = rp; rp.next = rn;                    if ((rn = root.next) != null)                        ((TreeNode<K,V>)rn).prev = rp;                    if (rp != null)                        rp.next = rn;                    //将first插入到root后面                    if (first != null)                        first.prev = root;                    root.next = first;                    root.prev = null;                }                assert checkInvariants(root);            }        }        /**         * Finds the node starting at root p with the given hash and key.         * The kc argument caches comparableClassFor(key) upon first use         * comparing keys.         */        //查找hash为h，key为k的节点        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)                    //h小于节点的hash值，则找左节点                    p = pl;                else if (ph < h)                    //h大于结点的hash值，则找右节点                    p = pr;                else if ((pk = p.key) == k || (k != null && k.equals(pk)))                    //找到则返回                    return p;                else if (pl == null)                    //左节点为空，则找右节点                    p = pr;                else if (pr == null)                    //右节点为空，则找左节点                    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)                    //通过右节点查找                    return q;                else                    p = pl;            } while (p != null);            return null;        }        /**         * Calls find for root node.         */        //获取树节点，通过根节点查找        final TreeNode<K,V> getTreeNode(int h, Object k) {            return ((parent != null) ? root() : this).find(h, k, null);        }        /**         * Tie-breaking utility for ordering insertions when equal         * hashCodes and non-comparable. We don't require a total         * order, just a consistent insertion rule to maintain         * equivalence across rebalancings. Tie-breaking further than         * necessary simplifies testing a bit.         */        //比较2个对象的大小        static int tieBreakOrder(Object a, Object b) {            int d;            if (a == null || b == null ||                (d = a.getClass().getName().                 compareTo(b.getClass().getName())) == 0)                d = (System.identityHashCode(a) <= System.identityHashCode(b) ?                     -1 : 1);            return d;        }        /**         * Forms tree of the nodes linked from this node.         * @return root of tree         */        //将链表转为二叉树        final void treeify(Node<K,V>[] tab) {            TreeNode<K,V> root = null;            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) {                    //根节点设置为黑色                    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;                        if ((p = (dir <= 0) ? p.left : p.right) == null) {                            x.parent = xp;                            if (dir <= 0)                                xp.left = x;                            else                                xp.right = x;                            root = balanceInsertion(root, x);                            break;                        }                    }                }            }            moveRootToFront(tab, root);        }        /**         * Returns a list of non-TreeNodes replacing those linked from         * this node.         */        //将二叉树转为链表        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;        }        /**         * Tree version of putVal.         */        //添加一个键值对        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)                    dir = -1;                else if (ph < h)                    dir = 1;                else if ((pk = p.key) == k || (pk != null && k.equals(pk)))                    //键值对为root，则返回                    return p;                else if ((kc == null &&                          (kc = comparableClassFor(k)) == null) ||                         (dir = compareComparables(kc, k, pk)) == 0) {                    if (!searched) {                        //只运行第一次，检测是否以存在该键值对                        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) {                    Node<K,V> xpn = xp.next;                    //插入节点                    TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);                    if (dir <= 0)                        xp.left = x;                    else                        xp.right = x;                    xp.next = x;                    x.parent = x.prev = xp;                    if (xpn != null)                        ((TreeNode<K,V>)xpn).prev = x;                    //检测平衡                    moveRootToFront(tab, balanceInsertion(root, x));                    return null;                }            }        }        /**         * Removes the given node, that must be present before this call.         * This is messier than typical red-black deletion code because we         * cannot swap the contents of an interior node with a leaf         * successor that is pinned by "next" pointers that are accessible         * independently during traversal. So instead we swap the tree         * linkages. If the current tree appears to have too few nodes,         * the bin is converted back to a plain bin. (The test triggers         * somewhere between 2 and 6 nodes, depending on tree structure).         */        final void removeTreeNode(HashMap<K,V> map, Node<K,V>[] tab,                                  boolean movable) {            int n;            if (tab == null || (n = tab.length) == 0)                return;            int index = (n - 1) & hash;            TreeNode<K,V> first = (TreeNode<K,V>)tab[index], root = first, rl;            TreeNode<K,V> succ = (TreeNode<K,V>)next, pred = prev;            if (pred == null)                tab[index] = first = succ;            else                pred.next = succ;            if (succ != null)                succ.prev = pred;            if (first == null)                return;            if (root.parent != null)                root = root.root();            if (root == null || root.right == null ||                (rl = root.left) == null || rl.left == null) {                //太少就转为链表                tab[index] = first.untreeify(map);  // too small                return;            }            TreeNode<K,V> p = this, pl = left, pr = right, replacement;            if (pl != null && pr != null) {                TreeNode<K,V> s = pr, sl;                while ((sl = s.left) != null) // find successor                    s = sl;                boolean c = s.red; s.red = p.red; p.red = c; // swap colors                TreeNode<K,V> sr = s.right;                TreeNode<K,V> pp = p.parent;                if (s == pr) { // p was s's direct parent                    p.parent = s;                    s.right = p;                }                else {                    TreeNode<K,V> sp = s.parent;                    if ((p.parent = sp) != null) {                        if (s == sp.left)                            sp.left = p;                        else                            sp.right = p;                    }                    if ((s.right = pr) != null)                        pr.parent = s;                }                p.left = null;                if ((p.right = sr) != null)                    sr.parent = p;                if ((s.left = pl) != null)                    pl.parent = s;                if ((s.parent = pp) == null)                    root = s;                else if (p == pp.left)                    pp.left = s;                else                    pp.right = s;                if (sr != null)                    replacement = sr;                else                    replacement = p;            }            else if (pl != null)                replacement = pl;            else if (pr != null)                replacement = pr;            else                replacement = p;            if (replacement != p) {                TreeNode<K,V> pp = replacement.parent = p.parent;                if (pp == null)                    root = replacement;                else if (p == pp.left)                    pp.left = replacement;                else                    pp.right = replacement;                p.left = p.right = p.parent = null;            }            TreeNode<K,V> r = p.red ? root : balanceDeletion(root, replacement);            if (replacement == p) {  // detach                TreeNode<K,V> pp = p.parent;                p.parent = null;                if (pp != null) {                    if (p == pp.left)                        pp.left = null;                    else if (p == pp.right)                        pp.right = null;                }            }            if (movable)                moveRootToFront(tab, r);        }        /**         * Splits nodes in a tree bin into lower and upper tree bins,         * or untreeifies if now too small. Called only from resize;         * see above discussion about split bits and indices.         *         * @param map the map         * @param tab the table for recording bin heads         * @param index the index of the table being split         * @param bit the bit of hash to split on         */        //将结点太多的桶分割        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) {                    if ((e.prev = loTail) == null)                        loHead = e;                    else                        loTail.next = e;                    loTail = e;                    ++lc;                }                else {                    if ((e.prev = hiTail) == null)                        hiHead = e;                    else                        hiTail.next = e;                    hiTail = e;                    ++hc;                }            }            if (loHead != null) {                if (lc <= UNTREEIFY_THRESHOLD)                    //太小则转为链表                    tab[index] = loHead.untreeify(map);                else {                    tab[index] = loHead;                    if (hiHead != null) // (else is already treeified)                        loHead.treeify(tab);                }            }            if (hiHead != null) {                if (hc <= UNTREEIFY_THRESHOLD)                    tab[index + bit] = hiHead.untreeify(map);                else {                    tab[index + bit] = hiHead;                    if (loHead != null)                        hiHead.treeify(tab);                }            }        }        /* ------------------------------------------------------------ */        // Red-black tree methods, all adapted from CLR        //左旋转        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;        }        //右旋转        static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,                                               TreeNode<K,V> p) {            TreeNode<K,V> l, pp, lr;            if (p != null && (l = p.left) != null) {                if ((lr = p.left = l.right) != null)                    lr.parent = p;                if ((pp = l.parent = p.parent) == null)                    (root = l).red = false;                else if (pp.right == p)                    pp.right = l;                else                    pp.left = l;                l.right = p;                p.parent = l;            }            return root;        }        //保证插入后平衡        static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,                                                    TreeNode<K,V> x) {            x.red = true;            for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {                if ((xp = x.parent) == null) {                    //插入 情形1：插入的是根节点，将颜色设为黑色，其他不用处理                    x.red = false;                    return x;                }                else if (!xp.red || (xpp = xp.parent) == null)                    //插入 情形2：插入的节点的父节点是黑色，不用处理                    return root;                if (xp == (xppl = xpp.left)) {                    //对称的，该部分是插入到左边的树                    if ((xppr = xpp.right) != null && xppr.red) {                        //插入 情形3：将父节点和叔节点设为黑色，将祖父设为红色                        xppr.red = false;                        xp.red = false;                        xpp.red = true;                        x = xpp;                    }                    else {                        if (x == xp.right) {                            //插入 情形4：需要左旋转一次                            root = rotateLeft(root, x = xp);                            xpp = (xp = x.parent) == null ? null : xp.parent;                        }                        if (xp != null) {                            //插入 情形5：需要右旋一次                            xp.red = false;                            if (xpp != null) {                                xpp.red = true;                                root = rotateRight(root, xpp);                            }                        }                    }                }                else {                    //对称的，该部分是插入到右边的树                    if (xppl != null && xppl.red) {                        //插入 情形3                        xppl.red = false;                        xp.red = false;                        xpp.red = true;                        x = xpp;                    }                    else {                        if (x == xp.left) {                            //插入 情形4                            root = rotateRight(root, x = xp);                            xpp = (xp = x.parent) == null ? null : xp.parent;                        }                        if (xp != null) {                            //插入 情形5                            xp.red = false;                            if (xpp != null) {                                xpp.red = true;                                root = rotateLeft(root, xpp);                            }                        }                    }                }            }        }        //删除后调整平衡        static <K,V> TreeNode<K,V> balanceDeletion(TreeNode<K,V> root,                                                   TreeNode<K,V> x) {            for (TreeNode<K,V> xp, xpl, xpr;;)  {                if (x == null || x == root)                    //删除 情况1：根节点，不需要调整                    return root;                else if ((xp = x.parent) == null) {                    //删除的是根节点，将跟节点设为黑色                    x.red = false;                    return x;                }                else if (x.red) {                    x.red = false;                    return root;                }                else if ((xpl = xp.left) == x) {                    //在左边树                    if ((xpr = xp.right) != null && xpr.red) {                        //删除 情况2                        xpr.red = false;                        xp.red = true;                        root = rotateLeft(root, xp);                        xpr = (xp = x.parent) == null ? null : xp.right;                    }                    if (xpr == null)                        x = xp;                    else {                        TreeNode<K,V> sl = xpr.left, sr = xpr.right;                        if ((sr == null || !sr.red) &&                            (sl == null || !sl.red)) {                            //删除 情况3                            xpr.red = true;                            x = xp;                        }                        else {                            if (sr == null || !sr.red) {                                //删除 情况5                                if (sl != null)                                    sl.red = false;                                xpr.red = true;                                root = rotateRight(root, xpr);                                xpr = (xp = x.parent) == null ?                                    null : xp.right;                            }                            if (xpr != null) {                                xpr.red = (xp == null) ? false : xp.red;                                if ((sr = xpr.right) != null)                                    sr.red = false;                            }                            if (xp != null) {                                //删除 情况6                                xp.red = false;                                root = rotateLeft(root, xp);                            }                            x = root;                        }                    }                }                else { // symmetric                    //跟上面是对称的                    if (xpl != null && xpl.red) {                        xpl.red = false;                        xp.red = true;                        root = rotateRight(root, xp);                        xpl = (xp = x.parent) == null ? null : xp.left;                    }                    if (xpl == null)                        x = xp;                    else {                        TreeNode<K,V> sl = xpl.left, sr = xpl.right;                        if ((sl == null || !sl.red) &&                            (sr == null || !sr.red)) {                            xpl.red = true;                            x = xp;                        }                        else {                            if (sl == null || !sl.red) {                                if (sr != null)                                    sr.red = false;                                xpl.red = true;                                root = rotateLeft(root, xpl);                                xpl = (xp = x.parent) == null ?                                    null : xp.left;                            }                            if (xpl != null) {                                xpl.red = (xp == null) ? false : xp.red;                                if ((sl = xpl.left) != null)                                    sl.red = false;                            }                            if (xp != null) {                                xp.red = false;                                root = rotateRight(root, xp);                            }                            x = root;                        }                    }                }            }        }        /**         * Recursive invariant check         */        //检测是否符合红黑树        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)                return false;            if (tn != null && tn.prev != t)                return false;            if (tp != null && t != tp.left && t != tp.right)                return false;            if (tl != null && (tl.parent != t || tl.hash > t.hash))                return false;            if (tr != null && (tr.parent != t || tr.hash < t.hash))                return false;            if (t.red && tl != null && tl.red && tr != null && tr.red)                return false;            if (tl != null && !checkInvariants(tl))                return false;            if (tr != null && !checkInvariants(tr))                return false;            return true;        }    }}