public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
    // 序列号
    private static final long serialVersionUID = 362498820763181265L;
    // 默认的初始容量是16
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
    // 最大容量
    static final int MAXIMUM_CAPACITY = 1 << 30;
    // 默认的负载因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    // 当桶(bucket)上的结点数大于等于这个值时会转成红黑树
    static final int TREEIFY_THRESHOLD = 8;
    // 当桶(bucket)上的结点数小于等于这个值时树转链表
    static final int UNTREEIFY_THRESHOLD = 6;
    // 桶中结构转化为红黑树对应的table的最小容量
    static final int MIN_TREEIFY_CAPACITY = 64;
    // 存储元素的数组，总是2的幂次倍
    transient Node<k,v>[] table;
    // 存放具体元素的集
    transient Set<map.entry<k,v>> entrySet;
    // 存放元素的个数，注意这个不等于数组的长度。
    transient int size;
    // 每次扩容和更改map结构的计数器
    transient int modCount;
    // 阈值(容量*负载因子) 当实际大小超过阈值时，会进行扩容
    int threshold;
    // 负载因子
    final float loadFactor;
    ....
}

/**
 * 构造具有指定初始容量和负载因子的空HashMap。
 */
public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        //指定的容量大小不可以小于0,否则将抛出IllegalArgumentException异常
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    //判定指定的容量大小是否大于HashMap的容量极限
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        //指定的负载因子不可以小于0或为NaN，若判定成立则抛出IllegalArgumentException异常
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    // 初始容量暂时放到threshold，在resize中再赋值给newCap进行table初始化
    // 设置“HashMap阈值”，当HashMap中存储数据的数量达到threshold时，就需要将HashMap的容量加倍。
    //tableSizeFor用于查找到大于给定数值的最近2次幂值，比如给定18就是32。给定33就是64
    this.threshold = tableSizeFor(initialCapacity);
}

/**
 * 用指定的初始值构造一个空的HashMap容量和默认负载因子(0.75)。
 */
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/**
 * 使用默认初始容量(16)和默认负载因子(0.75)构造一个空的HashMap。
 */
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

/**
 * 构造一个与指定Map具有相同映射的新HashMap。
 * HashMap是用默认负载因子(0.75)创建的，初始容量足以在指定的Map中保存映射。
 */
public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

JDK1.7
    static int hash(int h) {
    	h ^= (h >>> 20) ^ (h >>> 12);
    	return h ^ (h >>> 7) ^ (h >>> 4);
	}

JDK1.8
	static final int hash(Object key) {
        int h;
        // ^ 异或   >>> 无符号右移，空位以0补齐
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

 /**
  * 将指定值与此映射中的指定键关联。如果映射以前包含键的映射，则替换旧值。
  */
 public V put(K key, V value) {
     return putVal(hash(key), key, value, false, true);
 }

 /**
  * 实现了Map.put和相关方法
  */
 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)
         // table未初始化或者长度为0，则进行扩容
         n = (tab = resize()).length;
     if ((p = tab[i = (n - 1) & hash]) == null)
// (n - 1) & hash 确定元素存放在哪个桶中
         // 桶为空，新生成节点放入桶中（此时，这个结点是放在数组中）
         tab[i] = newNode(hash, key, value, null);
     else {
         // 桶中已经存在元素，处理hash冲突
         Node<K,V> e; K k;
         if (p.hash == hash &&
             ((k = p.key) == key || (key != null && key.equals(k))))
             // 快速判断第一个节点table[i]的key是否与插入的key一样
             // 若相同则直接使用插入的p值替换掉旧的值e
             e = p;
         else if (p instanceof TreeNode)
             // 插入的节点是红黑树节点，放入树中
             e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
         else {
             // 插入的节点是链表节点
             // 在链表的最末端插入节点
             for (int binCount = 0; ; ++binCount) {
                 if ((e = p.next) == null) {
                     // 在尾部掺入新节点 
                     p.next = newNode(hash, key, value, null);
                     if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                         // 如果节点数量达到阈值（默认为8），执行treeifyBin方法
                         // 根据HashMap数组来决定是否转换为红黑树 
                         // 只有当数组长度大于或者等于64的情况下，才会执行红黑树操作，减少搜索次数
                         // 否则，只是对数组扩容
                         treeifyBin(tab, hash);
                     break;
                 }
                 if (e.hash == hash &&
                     ((k = e.key) == key || (key != null && key.equals(k))))
                     // 如果链表节点的key与插入元素的key相等，跳出循环
                     break;
                 // 用于遍历桶中的链表，与前面的 e = p.next组合，遍历链表
                 p = e;
             }
         }
         if (e != null) { // existing mapping for key
             // 在桶中找到key值、hash值与插入元素相等的节点 
             // 记录e的value
             V oldValue = e.value;
             if (!onlyIfAbsent || oldValue == null)
                 // onlyIfAbsent为false或者oldValue为null，用新值替换旧值
                 e.value = value;
             // 访问后回调 
             afterNodeAccess(e);
             // 返回旧值
             return oldValue;
         }
     }
     ++modCount; // 结构性修改
     if (++size > threshold)
         // 如果实际大小大于阈值则扩容
         resize();
     afterNodeInsertion(evict); // 插入后回调
     return null;
 }

/**
 * 返回指定键映射到的值，如果此映射不包含该键的映射，则返回null。
 * 
 * 更正式地说，如果这个映射包含一个从键k到值v的映射，
 * 使得(key==null ?K ==null: key.equals(K))，则此方法返回v;否则返回null。(最多只能有一个这样的映射。)
 *
 * 返回值为null并不一定表示映射不包含该键的映射;也有可能映射显式地将键映射为null。
 * containsKey操作可以用来区分这两种情况。
 */
public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

/**
 * 实现了Map.get相关方法
 */
final Node<K,V> getNode(int hash, Object key) {
    // table：当前map的数组  first：当前hash对应索引位置上的节点 e：遍历过程中临时存储的节点
    // n：table数组的长度
    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))))
            // 如果相等，直接返回
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                // 在树结构中get
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {
                // 在链表中get，遍历链表
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

//参数 newCapacity 为新数组的大小
   void resize(int newCapacity) {
       Entry[] oldTable = table;//引用扩容前的 Entry 数组
       int oldCapacity = oldTable.length;
       if (oldCapacity == MAXIMUM_CAPACITY) {//扩容前的数组大小如果已经达到最大(2^30)了
           threshold = Integer.MAX_VALUE;//修改阈值为int的最大值(2^31-1)，这样以后就不会扩容了
           return;
       }

       Entry[] newTable = new Entry[newCapacity];//初始化一个新的Entry数组
       transfer(newTable, initHashSeedAsNeeded(newCapacity));//将数组元素转移到新数组里面
       table = newTable;
       threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);//修改阈值
   }
   void transfer(Entry[] newTable, boolean rehash) {
       int newCapacity = newTable.length;
       for (Entry<K,V> e : table) {//遍历数组
           while(null != e) {
               Entry<K,V> next = e.next;
               if (rehash) {
                   e.hash = null == e.key ? 0 : hash(e.key);
               }
               int i = indexFor(e.hash, newCapacity);//重新计算每个元素在数组中的索引位置
               e.next = newTable[i];//标记下一个元素，添加是链表头添加
               newTable[i] = e;//将元素放在链上
               e = next;//访问下一个 Entry 链上的元素
           }
       }
   }

Hi = (H(key) + d1) % m
其中i=1,2,3...，s， H(key)为哈希函数，m为哈希表长，di为增量序列

Hi = RHi(key) i=1,2,3,...

void resize(int newCapacity) {   //传入新的容量
    Entry[] oldTable = table;    //引用扩容前的Entry数组
    int oldCapacity = oldTable.length;
    if (oldCapacity == MAXIMUM_CAPACITY) {  //扩容前的数组大小如果已经达到最大(2^30)了
        threshold = Integer.MAX_VALUE; //修改阈值为int的最大值(2^31-1)，这样以后就不会扩容了
        return;
    }
    Entry[] newTable = new Entry[newCapacity];  //初始化一个新的Entry数组
    transfer(newTable);                         //！！将数据转移到新的Entry数组里
    table = newTable;                           //HashMap的table属性引用新的Entry数组
    threshold = (int) (newCapacity * loadFactor);//修改阈值
}
void transfer(Entry[] newTable) {
    Entry[] src = table;                   //src引用了旧的Entry数组
    int newCapacity = newTable.length;
    for (int j = 0; j < src.length; j++) { //遍历旧的Entry数组
        Entry<K, V> e = src[j];             //取得旧Entry数组的每个元素
        if (e != null) {
            src[j] = null;//释放旧Entry数组的对象引用（for循环后，旧的Entry数组不再引用任何对象）
            do {
                Entry<K, V> next = e.next;
                int i = indexFor(e.hash, newCapacity); //！！重新计算每个元素在数组中的位置
                e.next = newTable[i]; //标记[1]
                newTable[i] = e;      //将元素放在数组上
                e = next;             //访问下一个Entry链上的元素
            } while (e != null);
        }
    }
}

do {
	Entry<K, V> next = e.next;  // 线程1执行到此处被调度挂起
	int i = indexFor(e.hash, newCapacity);
	e.next = newTable[i];
	newTable[i] = e;
	e = next;
} while(e != null);

package java.util;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
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;
import sun.misc.SharedSecrets;

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    private static final long serialVersionUID = 362498820763181265L;

    /*
     * 实现说明
     *
     * 这个映射通常作为一个binned(桶)哈希表，但是当桶变得太大时，
     * 它们被转换成treenode的桶，每个节点的结构与java.util.TreeMap中的相似。
     * 大多数方法尝试使用正常的桶，但在适用的情况下中继到TreeNode方法(只需通过检查节点的instanceof)。
     * TreeNodes的桶可以像其他桶一样遍历和使用，但在填充过多时还支持更快的查找。
     * 然而，由于绝大多数正常使用的容器都没有过度填充，因此检查树容器是否存在可能会在表方法的过程中被延迟。
     *
     * 树桶(即，其元素都是treenode的桶)主要由hashCode排序，但在关联的情况下，
     * 如果两个元素属于相同的“类C实现Comparable<C>”，则键入它们的compareTo方法用于排序。
     * (我们通过反射保守地检查泛型类型来验证这一点——参见方法comparableClassFor)。
     * 当键具有不同的哈希值或可排序时，树桶增加的复杂性在提供最坏情况O(log n)操作方面是值得的，
     * 因此，在意外或恶意使用hashCode()方法返回分布不佳的值以及许多键共享hashCode的情况下，
     * 只要它们也是可比较的，性能就会优雅地下降。
     * (如果这两项都不适用，与不采取预防措施相比，我们可能会浪费大约两倍的时间和空间。
     * 但唯一已知的案例源于糟糕的用户编程实践，这些实践已经非常缓慢，以至于这几乎没有什么区别。)
     *
     * 因为treenode的大小大约是常规节点的两倍，所以我们只在桶包含足够的节点以保证使用时才使用它们
     * (参见TREEIFY_THRESHOLD)。当它们变得太小(由于移除或调整大小)时，它们被转换回普通的桶。
     * 在用户hashCodes分布良好的用法中，很少使用树桶。
     * 理想情况下，在随机hashCodes下，箱中节点的频率遵循
     * 泊松分布(http://en.wikipedia.org/wiki/Poisson_distribution)，
     * 默认调整大小阈值为0.75，参数平均约为0.5，尽管由于调整大小粒度而存在很大差异。
     * 忽略方差，列表大小k的预期出现次数为(exp(-0.5) * pow(0.5, k) / factorial(k))。第一个值是:
     *
     * 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
     *
     * 树桶的根通常是它的第一个节点。然而，有时(目前仅在Iterator.remove上)，
     * 根可能在其他地方，但可以通过父链接(方法TreeNode.root())恢复。
     *
     * 所有适用的内部方法都接受哈希码作为参数(通常由公共方法提供)，允许它们相互调用而无需重新计算用户哈希码。
     * 大多数内部方法也接受“tab”参数，通常是当前表，但在调整大小或转换时可能是新表或旧表。
     *
     * 当bin列表被树化、拆分或非树化时，我们将它们保持相同的相对访问/遍历顺序(即字段Node.next)，
     * 以更好地保留局部性，并稍微简化调用iterator.remove的拆分和遍历处理。
     * 当在插入时使用比较器时，为了在重新平衡中保持总排序(或尽可能接近)，我们比较类和identityHashCodes作为决定因素。
     *
     * 由于子类LinkedHashMap的存在，普通模式与树模式之间的使用和转换变得复杂。
     * 请参阅下面定义的在插入、删除和访问时调用的钩子方法，这些方法允许LinkedHashMap
     * 内部保持独立于这些机制。(这还需要将映射实例传递给一些可能创建新节点的实用程序方法。)
     *
     * 类似于并发编程的基于ssa的编码风格有助于避免在所有扭曲指针操作中出现混叠错误。
     */

    /**
     * 默认初始容量-必须是2的幂。
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 如果隐式指定了更大的值，则使用最大容量。
     * 由任意一个带参数的构造函数调用。必须是2的幂<= 1<<30。
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 在构造函数中未指定时使用的负载因子（默认负载因子）。
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * 使用树而不是列表的bin计数阈值。当向至少有这么多节点的桶中添加元素时，桶将转换为树。
     * 该值必须大于2，并且应该至少为8，以符合在树木移除中关于收缩后转换回普通桶的假设。
     * （当桶上的节点数大于等于该值会由链表转换成红黑树）
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * 在调整大小操作期间取消树化(拆分)bin的bin计数阈值。
     * 应小于TREEIFY_THRESHOLD，且最多为6，以便在移除时进行收缩检测。
     *（当桶上的节点数小于等于该值会由红黑树转换成链表）
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * 可以对桶进行树化的最小表容量。(否则，如果一个bin中的节点太多，则会调整表的大小。)
     * 应该至少为4 * TREEIFY_THRESHOLD，以避免调整大小和树化阈值之间的冲突。
     *（桶中结构转换为红黑树对应的table的最小容量）
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

    /**
     * 基本哈希bin节点，用于大多数条目。(参见下面的TreeNode子类，以及LinkedHashMap中的Entry子类。)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash; // 哈希值，存放元素到hashMap中时用来与其他元素的hash值比较
        final K key; // 键
        V value; // 值
        Node<K,V> next; // 指向下一个节点 

        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; }

        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        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 -------------- */

    /**
     * 计算key.hashCode()并将哈希的高位数扩展到低位数。
     * 因为表使用了2的幂掩码，所以在当前掩码之上只变化几比特的哈希集总是会发生冲突。
     * (其中一个已知的例子是在小表格中保存连续整数的Float键集。)
     * 因此，我们应用一个变换，将高比特的影响向下扩散。比特传播的速度、效用和质量之间存在权衡。
     * 由于许多常见的哈希集已经合理分布(因此不会从扩展中受益)，并且由于我们使用树来处理箱中的大型碰撞集，
     * 因此我们只是以最便宜的方式对一些移位的位进行异或，以减少系统损失，并合并最高位的影响，
     * 否则由于表边界的原因，这些位在索引计算中永远不会使用。
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

    /**
     * 如果x的Class是" Class C implements Comparable<C>"的形式，则返回它的Class，否则返回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;
    }

    /**
     * 如果x匹配kc (k筛选的可比较类)，则返回k. compareto (x)，否则为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));
    }

    /**
     * 返回给定目标容量的2次幂大小。
     */
    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 -------------- */

    /**
     * 表，在第一次使用时初始化，并根据需要调整大小。
     * 在分配时，长度总是2的幂。(我们也允许某些操作的长度为零，以允许目前不需要的引导机制。)
     * （存储元素的数组，总是2的幂次方）
     */
    transient Node<K,V>[] table;

    /**
     * 保存缓存的entrySet()。注意，AbstractMap字段用于keySet()和values()。
     * （存放具体元素的集）
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * 此映射中包含的键值映射的数量。
     *（存放元素的个数，注意，这个不等于数组的长度）
     */
    transient int size;

    /**
     * 结构修改是指改变HashMap中的映射数量或以其他方式修改其内部结构(例如，重新散列)。
     * 该字段用于使HashMap的集合视图上的迭代器快速失败。(见ConcurrentModificationException)。
     *（每次扩容和更改map结构的计数器）
     */
    transient int modCount;

    /**
     * 要调整大小的下一个大小值(容量*负载因子)。
     * （阈值，当实际大小超过阈值时，会进行扩容）
     */
    // (序列化时javadoc描述为真。
    // 另外，如果表数组没有被分配
    // 字段保存初始数组容量，或者零表示
    // DEFAULT_INITIAL_CAPACITY)。
    int threshold;

    /**
     * 哈希表的负载因子。
     */
    final float loadFactor;

    /* ---------------- Public operations -------------- */

    /**
     * 构造具有指定初始容量和负载因子的空HashMap。
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            //指定的容量大小不可以小于0,否则将抛出IllegalArgumentException异常
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        //判定指定的容量大小是否大于HashMap的容量极限
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            //指定的负载因子不可以小于0或为NaN，若判定成立则抛出IllegalArgumentException异常
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        // 初始容量暂时放到threshold，在resize中再赋值给newCap进行table初始化
        // 设置“HashMap阈值”，当HashMap中存储数据的数量达到threshold时，就需要将HashMap的容量加倍。
        //tableSizeFor用于查找到大于给定数值的最近2次幂值，比如给定18就是32。给定33就是64
        this.threshold = tableSizeFor(initialCapacity);
    }

    /**
     * 用指定的初始值构造一个空的HashMap容量和默认负载因子(0.75)。
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * 使用默认初始容量(16)和默认负载因子(0.75)构造一个空的HashMap。
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    /**
     * 构造一个与指定Map具有相同映射的新HashMap。
     * HashMap是用默认负载因子(0.75)创建的，初始容量足以在指定的Map中保存映射。
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

    /**
     * 实现了Map.putAll和Map的构造
     */
    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        if (s > 0) {
            // 判断map是否已经初始化
            if (table == null) { // pre-size
                // 未初始化，s为m的实际元素个数
                // ft = s/loadFactor => s = ft*loadFactor
                // ft指的是要添加s个元素所需的最小容量
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                         (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)
                    // table未初始化，threshold实际上存放的是初始化容量
                    // 如果添加s个元素所需的最小容量大于初始化容量，则将最小容量扩容为最接近2的幂次方大小作为初始化
                    // 注意，这里不是初始化阈值
                    threshold = tableSizeFor(t);
            }
            else if (s > threshold)
                // 已经初始化，并且m元素个数大于阈值，进行扩容处理
                resize();
            // 将m中所有元素添加至HashMap中，如果table未初始化，putVal中会调用resize()初始化或者扩容
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);
            }
        }
    }

    /**
     * 返回此映射中键值映射的个数
     */
    public int size() {
        return size;
    }

    /**
     * 如果此映射不包含键值映射，则返回true
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 返回指定键映射到的值，如果此映射不包含该键的映射，则返回null。
     * 
     * 更正式地说，如果这个映射包含一个从键k到值v的映射，
     * 使得(key==null ?K ==null: key.equals(K))，则此方法返回v;否则返回null。(最多只能有一个这样的映射。)
     *
     * 返回值为null并不一定表示映射不包含该键的映射;也有可能映射显式地将键映射为null。
     * containsKey操作可以用来区分这两种情况。
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * 实现了Map.get相关方法
     */
    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))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

    /**
     * 如果此映射包含指定键的映射，则返回true
     */
    public boolean containsKey(Object key) {
        return getNode(hash(key), key) != null;
    }

    /**
     * 将指定值与此映射中的指定键关联。如果映射以前包含键的映射，则替换旧值。
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * 实现了Map.put和相关方法
     */
    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)
            // table未初始化或者长度为0，则进行扩容
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
			// (n - 1) & hash 确定元素存放在哪个桶中
            // 桶为空，新生成节点放入桶中（此时，这个结点是放在数组中）
            tab[i] = newNode(hash, key, value, null);
        else {
            // 桶中已经存在元素，处理hash冲突
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                // 快速判断第一个节点table[i]的key是否与插入的key一样
                // 若相同则直接使用插入的p值替换掉旧的值e
                e = p;
            else if (p instanceof TreeNode)
                // 插入的节点是红黑树节点，放入树中
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                // 插入的节点是链表节点
                // 在链表的最末端插入节点
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        // 在尾部掺入新节点 
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            // 如果节点数量达到阈值（默认为8），执行treeifyBin方法
                            // 根据HashMap数组来决定是否转换为红黑树 
                            // 只有当数组长度大于或者等于64的情况下，才会执行红黑树操作，减少搜索次数
                            // 否则，只是对数组扩容
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        // 如果链表节点的key与插入元素的key相等，跳出循环
                        break;
                    // 用于遍历桶中的链表，与前面的 e = p.next组合，遍历链表
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                // 在桶中找到key值、hash值与插入元素相等的节点 
                // 记录e的value
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    // onlyIfAbsent为false或者oldValue为null，用新值替换旧值
                    e.value = value;
                // 访问后回调 
                afterNodeAccess(e);
                // 返回旧值
                return oldValue;
            }
        }
        ++modCount; // 结构性修改
        if (++size > threshold)
            // 如果实际大小大于阈值则扩容
            resize();
        afterNodeInsertion(evict); // 插入后回调
        return null;
    }

    /**
     * 初始化或加倍表大小。如果为空，则按照字段阈值中持有的初始容量目标进行分配。
     * 否则，由于我们使用的是2的幂展开，因此每个桶中的元素必须保持在相同的索引上，或者在新表中以2的幂偏移量移动。
     */
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length; // 原数组如果为空，则长度赋值为0
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            // 如果原数组长度大于0，说明HashMap已经初始化过了，是一次正常的扩容
            if (oldCap >= MAXIMUM_CAPACITY) {
                // 判断旧的容量是否大于容量最大值(2^30)，如果是，则无法扩容，那么修改阈值为 Integer.MAX_VALUE
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                // // 原数组长度大于等于初始化长度16，并且原数组长度扩大1倍也小于 2^30次方，扩容为原来的2倍
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // 初始容量被置于阈值
            // 如果旧容量 = 0 且 旧阈值 > 0，说明是通过构造方法创建的HashMap
        	// 新容量直接等于旧阈值
            // 创建对象时初始化容量大小放在threshold中，此时只需要将其作为新的数组容量
            newCap = oldThr;
        else {               // 零初始阈值表示使用默认值
            // 无参构造函数创建的对象在这里计算容量和阈值
            newCap = DEFAULT_INITIAL_CAPACITY; // 数组长度初始化为16 
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); // 数组阈值初始化为12
        }
        if (newThr == 0) {
            // 创建时指定了初始化容量或者负载因子，在这里进行阈值初始化
            // 或者扩容前旧容量小于16，在这里计算新的resize上限
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr; // 更新
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            // 把每个bucket都移动到新的buckets中
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    // 当前桶位数据不为空，但是不能判断是链表还是红黑树结构
                    oldTab[j] = null;
                    if (e.next == null)
                        // 如果e结点不存在下一个结点，说明e是单个元素，则直接放入新数组的对应位置
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        // 将红黑树拆分成两棵子树，如果子树节点小于等于UNTREEIFY_THRESHOLD（默认为 6），
                        // 则将子树转换为链表，否则保持子树的树结构
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        // 否则e为链表，则对链表进行遍历
                        // 低位链表：存放在扩容之后的数组的下标位置，与当前数组下标位置一致
                        // loHead：低位链表头节点
                        // loTail低位链表尾节点
                        Node<K,V> loHead = null, loTail = null;
                        // 高位链表，存放扩容之后的数组的下标位置，=原索引+扩容之前数组容量
                        // hiHead:高位链表头节点
                        // hiTail:高位链表尾节点
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            // 通过 & 运算计算当前结点的hash值是高位为1还是0
                            if ((e.hash & oldCap) == 0) {
                                 // 高位为0，放入低位链表中
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                // 高位为1，放入高位链表中
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            // 原索引放到bucket里，将低位链表头结点指向原来的索引位置
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            // 原索引+oldCap放到bucket里，将高位链表头结点指向新的索引位置
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

    /**
     * 替换节点中给定哈希索引处的所有链接节点，除非表太小，在这种情况下调整大小。
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

    /**
     * 将指定映射中的所有映射复制到此映射。这些映射将替换此映射对指定映射中当前任何键的任何映射。
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        putMapEntries(m, true);
    }

    /**
     * 从此映射中删除指定键的映射(如果存在)。
     */
    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }

    /**
     * 实现Map.remove和相关方法
     */
    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) {
            Node<K,V> node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        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)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

    /**
     * 从该映射中删除所有映射。这个调用返回后，映射将为空
     */
    public void clear() {
        Node<K,V>[] tab;
        modCount++;
        if ((tab = table) != null && size > 0) {
            size = 0;
            for (int i = 0; i < tab.length; ++i)
                tab[i] = null;
        }
    }

    /**
     * 如果此映射将一个或多个键映射到指定值，则返回true
     */
    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) {
                for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                    if ((v = e.value) == value ||
                        (value != null && value.equals(v)))
                        return true;
                }
            }
        }
        return false;
    }

    /**
     * 返回此映射中包含的键的Set视图。集合由映射支持，因此对映射的更改反映在集合中，反之亦然。
     * 如果在对set进行迭代时修改map(通过迭代器自己的remove操作除外)，则迭代的结果是未定义的。
     * set支持移除元素，即通过Iterator从map中移除对应的映射。remove,Set.remove, removeAll, retainAll和clear操作。
     * 它不支持add或addAll操作。
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return 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)
                        action.accept(e.key);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * 返回此映射中包含的值的集合视图。集合由映射支持，因此对映射的更改将反映在集合中，反之亦然。
     * 如果在对集合进行迭代时修改map(通过迭代器自己的remove操作除外)，则迭代的结果是未定义的。
     * 该集合支持移除元素，即通过Iterator从map中移除对应的映射。Iterator.remove,Collection.remove,removeAll, retainAll和clear操作。
     * 它不支持add或addAll操作。
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return 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)
                        action.accept(e.value);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * 返回此映射中包含的映射的Set视图。集合由映射支持，因此对映射的更改反映在集合中，反之亦然。
     * 如果在对set进行迭代时修改map(通过迭代器自己的remove操作或通过对迭代器返回的map项执行setValue操作除外)，
     * 则迭代的结果是未定义的。set支持移除元素，即通过Iterator从map中移除对应的映射。Iterator.remove, Set.remove, removeAll, retainAll和clear操作。
     * 它不支持add或addAll操作。
     */
    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)
                        action.accept(e);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

    // Overrides of JDK8 Map extension methods

    @Override
    public V getOrDefault(Object key, V defaultValue) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? defaultValue : e.value;
    }

    @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;
    }

    @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;
    }

    @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)
            n = (tab = resize()).length;
        if ((first = tab[i = (n - 1) & hash]) != null) {
            if (first instanceof TreeNode)
                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);
            else {
                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;
            }
        }
        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)
            t.putTreeVal(this, tab, hash, key, v);
        else {
            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)
            n = (tab = resize()).length;
        if ((first = tab[i = (n - 1) & hash]) != null) {
            if (first instanceof TreeNode)
                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);
            else {
                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;
        V v = remappingFunction.apply(key, oldValue);
        if (old != null) {
            if (v != null) {
                old.value = v;
                afterNodeAccess(old);
            }
            else
                removeNode(hash, key, null, false, true);
        }
        else if (v != null) {
            if (t != null)
                t.putTreeVal(this, tab, hash, key, v);
            else {
                tab[i] = newNode(hash, key, v, first);
                if (binCount >= TREEIFY_THRESHOLD - 1)
                    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)
            n = (tab = resize()).length;
        if ((first = tab[i = (n - 1) & hash]) != null) {
            if (first instanceof TreeNode)
                old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key);
            else {
                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) {
            V v;
            if (old.value != null)
                v = remappingFunction.apply(old.value, value);
            else
                v = value;
            if (v != null) {
                old.value = v;
                afterNodeAccess(old);
            }
            else
                removeNode(hash, key, null, false, true);
            return v;
        }
        if (value != null) {
            if (t != null)
                t.putTreeVal(this, tab, hash, key, value);
            else {
                tab[i] = newNode(hash, key, value, first);
                if (binCount >= TREEIFY_THRESHOLD - 1)
                    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)
                    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) {
                    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
     */
    @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.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws IOException {
        int buckets = capacity();
        // Write out the threshold, loadfactor, and any hidden stuff
        s.defaultWriteObject();
        s.writeInt(buckets);
        s.writeInt(size);
        internalWriteEntries(s);
    }

    /**
     * Reconstitutes this map from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws IOException if an I/O error occurs
     */
    private void readObject(ObjectInputStream s)
        throws IOException, ClassNotFoundException {

        ObjectInputStream.GetField fields = s.readFields();

        // Read loadFactor (ignore threshold)
        float lf = fields.get("loadFactor", 0.75f);
        if (lf <= 0 || Float.isNaN(lf))
            throw new InvalidObjectException("Illegal load factor: " + lf);

        lf = Math.min(Math.max(0.25f, lf), 4.0f);
        HashMap.UnsafeHolder.putLoadFactor(this, lf);

        reinitialize();

        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) {
            // use defaults
        } else if (mappings > 0) {
            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);

            // Check Map.Entry[].class since it's the nearest public type to
            // what we're actually creating.
            SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap);
            @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);
            }
        }
    }

    // Support for resetting final field during deserializing
    private static final class UnsafeHolder {
        private UnsafeHolder() { throw new InternalError(); }
        private static final sun.misc.Unsafe unsafe
                = sun.misc.Unsafe.getUnsafe();
        private static final long LF_OFFSET;
        static {
            try {
                LF_OFFSET = unsafe.objectFieldOffset(HashMap.class.getDeclaredField("loadFactor"));
            } catch (NoSuchFieldException e) {
                throw new InternalError();
            }
        }
        static void putLoadFactor(HashMap<?, ?> map, float lf) {
            unsafe.putFloat(map, LF_OFFSET, lf);
        }
    }

    /* ------------------------------------------------------------ */
    // 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);
            }
        }

        public final boolean hasNext() {
            return next != null;
        }

        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> {
        public final K next() { return nextNode().key; }
    }

    final class ValueIterator extends HashIterator
        implements Iterator<V> {
        public final V next() { return nextNode().value; }
    }

    final class EntryIterator extends HashIterator
        implements Iterator<Map.Entry<K,V>> {
        public final Map.Entry<K,V> next() { return nextNode(); }
    }

    /* ------------------------------------------------------------ */
    // spliterators

    static class HashMapSpliterator<K,V> {
        final HashMap<K,V> map;
        Node<K,V> current;          // current node
        int index;                  // current index, modified on advance/split
        int fence;                  // one past last index
        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
    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
    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
    TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
        return new TreeNode<>(hash, key, value, next);
    }

    // For treeifyBin
    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.
    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

    /**
     * Tree节点结构。LinkedHashMap延伸。结构(又扩展Node)，因此可以用作常规节点或链接节点的扩展。
     */
    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // 红黑树链接，父节点
        TreeNode<K,V> left; // 左子树 
        TreeNode<K,V> right; // 右子树
        TreeNode<K,V> prev;    // 需要在删除后解除链接
        boolean red; // 判断颜色的标识
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }

        /**
         * 返回包含此节点的树的根。
         */
        final TreeNode<K,V> root() {
            for (TreeNode<K,V> r = this, p;;) {
                if ((p = r.parent) == null)
                    return r;
                r = p;
            }
        }

        /**
         * 确保给定的根是其桶的第一个节点。
         */
        static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
            int n;
            if (root != null && tab != null && (n = tab.length) > 0) {
                int index = (n - 1) & root.hash;
                TreeNode<K,V> first = (TreeNode<K,V>)tab[index];
                if (root != first) {
                    Node<K,V> rn;
                    tab[index] = root;
                    TreeNode<K,V> rp = root.prev;
                    if ((rn = root.next) != null)
                        ((TreeNode<K,V>)rn).prev = rp;
                    if (rp != null)
                        rp.next = rn;
                    if (first != null)
                        first.prev = root;
                    root.next = first;
                    root.prev = null;
                }
                assert checkInvariants(root);
            }
        }

        /**
         * 用给定的散列和键查找从根p开始的节点。
         * kc参数在第一次使用比较键时缓存comparableClassFor(key)。
         */
        final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
            TreeNode<K,V> p = this;
            do {
                int ph, dir; K pk;
                TreeNode<K,V> pl = p.left, pr = p.right, q;
                if ((ph = p.hash) > h)
                    p = pl;
                else if (ph < h)
                    p = pr;
                else if ((pk = p.key) == k || (k != null && k.equals(pk)))
                    return p;
                else if (pl == null)
                    p = pr;
                else if (pr == null)
                    p = pl;
                else if ((kc != null ||
                          (kc = comparableClassFor(k)) != null) &&
                         (dir = compareComparables(kc, k, pk)) != 0)
                    p = (dir < 0) ? pl : pr;
                else if ((q = pr.find(h, k, kc)) != null)
                    return q;
                else
                    p = pl;
            } while (p != null);
            return null;
        }

        /**
         * 对根节点调用find
         */
        final TreeNode<K,V> getTreeNode(int h, Object k) {
            return ((parent != null) ? root() : this).find(h, k, null);
        }

        /**
         * 当hashCodes相等且不可比较时，用于排序插入的Tie-breaking实用程序。
         * 我们不需要一个总顺序，只需要一个一致的插入规则来保持跨再平衡的等价性。不必要的断线进一步简化了测试。
         */
        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;
        }

        /**
         * 形成从该节点链接的节点的树
         */
        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) {
                    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);
        }

        /**
         * 返回非treenode的列表，替换从该节点链接的那些节点
         */
        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;
        }

        /**
         * putVal的树版本
         */
        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 || (k != null && k.equals(pk)))
                    return p;
                else if ((kc == null &&
                          (kc = comparableClassFor(k)) == null) ||
                         (dir = compareComparables(kc, k, pk)) == 0) {
                    if (!searched) {
                        TreeNode<K,V> q, ch;
                        searched = true;
                        if (((ch = p.left) != null &&
                             (q = ch.find(h, k, kc)) != null) ||
                            ((ch = p.right) != null &&
                             (q = ch.find(h, k, kc)) != null))
                            return q;
                    }
                    dir = tieBreakOrder(k, pk);
                }

                TreeNode<K,V> xp = p;
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    Node<K,V> xpn = xp.next;
                    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;
                    moveRootToFront(tab, balanceInsertion(root, x));
                    return null;
                }
            }
        }

        /**
         * 删除在此调用之前必须存在的给定节点。
         * 这比典型的红黑删除代码更混乱，因为我们不能将内部节点的内容与由在遍历期间可独立访问的“next”指针固定的叶继承节点交换。
         * 所以我们交换树形连杆。如果当前树的节点太少，则将该二进制文件转换回普通二进制文件。
         * (测试触发2到6个节点，这取决于树的结构)。
         */
        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
                || (movable
                    && (root.right == null
                        || (rl = root.left) == null
                        || rl.left == null))) {
                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);
        }

        /**
         * 将树仓中的节点拆分为上树仓和下树仓，如果太小则拆分为非树仓。
         * 只能从resize中调用;参见上面关于分割位和索引的讨论。
         */
        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)
                    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);
                }
            }
        }

        /* ------------------------------------------------------------ */
        // 红黑树方法，全部改编自CLR

        static <K,V> TreeNode<K,V> rotateLeft(TreeNode<K,V> root,
                                              TreeNode<K,V> p) {
            TreeNode<K,V> r, pp, rl;
            if (p != null && (r = p.right) != null) {
                if ((rl = p.right = r.left) != null)
                    rl.parent = p;
                if ((pp = r.parent = p.parent) == null)
                    (root = r).red = false;
                else if (pp.left == p)
                    pp.left = r;
                else
                    pp.right = r;
                r.left = p;
                p.parent = r;
            }
            return root;
        }

        static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,
                                               TreeNode<K,V> p) {
            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;
        }

        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) {
                    x.red = false;
                    return x;
                }
                else if (!xp.red || (xpp = xp.parent) == null)
                    return root;
                if (xp == (xppl = xpp.left)) {
                    if ((xppr = xpp.right) != null && xppr.red) {
                        xppr.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    }
                    else {
                        if (x == xp.right) {
                            root = rotateLeft(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        if (xp != null) {
                            xp.red = false;
                            if (xpp != null) {
                                xpp.red = true;
                                root = rotateRight(root, xpp);
                            }
                        }
                    }
                }
                else {
                    if (xppl != null && xppl.red) {
                        xppl.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    }
                    else {
                        if (x == xp.left) {
                            root = rotateRight(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        if (xp != null) {
                            xp.red = false;
                            if (xpp != null) {
                                xpp.red = true;
                                root = rotateLeft(root, xpp);
                            }
                        }
                    }
                }
            }
        }

        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)
                    return root;
                else if ((xp = x.parent) == null) {
                    x.red = false;
                    return x;
                }
                else if (x.red) {
                    x.red = false;
                    return root;
                }
                else if ((xpl = xp.left) == x) {
                    if ((xpr = xp.right) != null && xpr.red) {
                        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)) {
                            xpr.red = true;
                            x = xp;
                        }
                        else {
                            if (sr == null || !sr.red) {
                                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) {
                                xp.red = false;
                                root = rotateLeft(root, xp);
                            }
                            x = root;
                        }
                    }
                }
                else { // symmetric
                    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;
                        }
                    }
                }
            }
        }

        /**
         * 递归不变检验
         */
        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;
        }
    }
}