hashMap解读1

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package com.xinye.web.controller.redandblack;/*
                                                 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
                                                 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
                                                 */

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.AbstractMap;
import java.util.Map;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;

/**
 * 基于哈希表的Map接口实现lei 。
 * 该类实现提供了所有可选的map操作且允许null值和null键:
 * (HashMap 类大致等同于hashtable,除了它是不同步(线程不安全)且允许为空。)
 * 此类不保证先后的顺序是无须的;特别是,它不能保证会一直保持不变(扩容的时候顺序会产生变化)。)
 * 该类提供的的基本方法get和put,假设将元素 hash后适当地分散在存储桶(buckets)中。迭代结束查看集合视图需要的时间
 * 与HashMap实例的“容量”(存储桶数)加上其大小(键值映射数)成比例。因此,这俩参数对迭代性能是影响很大。
 * HashMap的实例有两个参数影响 性能:初始容量和负载系数。这个初始容量是哈希表中的存储桶数,
 * 初始容量只是创建哈希表时的容量。负载因子是在哈希表的容量自动增加之前,允许哈希表获得的满容量的度量。
 * 当哈希表中的条目数超过加载因子和 在当前容量下,哈希表将被重新灰化(即重建内部数据结构),
 * 重构号哈希表的存储桶数(size)大约是之前的存储桶数的两倍。
 * ----------------------------------------------------------------------------
 * 一般来说,默认的荷载系数(.75)提供了一个很好的时间和空间成本之间的权衡。较高的值会减少空间开销,
 * 但会增加查找成本(反映在HashMap类的大多数操作中,包括get和put)。中的预期条目数在设置初始容量时,
 * 应考虑map及其负载系数,以尽量减少再灰化操作的次数。如果使用量除以初始容量小于负载系数 ,将不会发生再扩容操作!
 * 如果许多映射要存储在HashMap中 例如,创建具有足够大容量的映射将允许更有效地存储映射,
 * 而不是让它根据需要执行自动重新灰化以扩展表。请注意,使用多个具有相同{@code hashCode()}的键
 * (hash冲突:那么他们确定的索引位置就相同,这时判断他们的key是否相同,如果不相同,这时就是产生了hash冲突)
 * 肯定会降低任何哈希表的性能。为了改善影响,当键是{@link Comparable}时(当键实现了Comparable类的compareTo方法),
 * 这个类可以使用键之间的比较顺序来帮助打破联系提高性能。
 * ----------------------------------------------------------------------------
 * 请注意,此实现不同步(线程不安全)。
 * 如果多个线程同时访问哈希映射,并且至少有一个线程在结构上修改了该映射,则必须在外部对其进行同步。
 * (结构修改是添加或删除一个或多个映射的任何操作;仅更改与实例已包含的键相关联的值不是结构修改。)
 * 这通常通过在自然封装映射的某个对象上进行同步来完成。
 * 如果不存在此类对象,则应使用{@link Collections#synchronizedMap Collections.synchronizedMap}
 * 方法“包装”映射。最好在创建时执行此操作,以防止意外地对映射进行非同步访问:
 * Map m=Collections.synchronizedMap(new HashMap(...));
 * ----------------------------------------------------------------------------
 * 这个类的所用的“collection”里的的迭代器都是快速失败的:如果在迭代器创建之后的任何时候对映射进行了结构上的修改,
 * 除了通过迭代器自己的remove方法之外,迭代器将抛出{@link ConcurrentModificationException}。
 * 因此,在面对并发修改时,迭代器会快速而干净地失败,而不是在将来某个不确定的时间冒着任意的、不确定的行为的风险。
 * ----------------------------------------------------------------------------
 * 注意,不能保证迭代器的fail-fast行为(fail-fast机制),因为通常情况下,在存在不同步的并发修改的情况下,
 * 不可能做出任何硬保证。 fail-fast机制在尽最大努力的基础上抛出ConcurrentModificationException。
 * 因此,编写依赖于此异常的程序以确保其正确性是错误的:迭代器的快速失败行为应该只用于检测错误
 */
public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable {

    private static final long serialVersionUID = 362498820763181265L;

    /*
     * map一般作为一个个桶组成的hash表,当数量很多的时候,会转变成TreeNodes(树节点),这样结构上类似TreeMap.
     * TreeNodes可能进行了转化,使用起来和其他非TreeNodes一样,但是提供了较快速度的遍历效率.
     * 然而大多数场景下出现很多元素拥挤的情况不会出现,可是检查是否是tree bins将会在使用各个方法时消耗性能.
     * 那就要看这个判断的性能是不是需要很大消耗了.
     * 我们知道hashmap的实现时数组+链表,在链表拥挤情况时,将它传变成树,有助于查询,但是如果是加减元素就不好说了.
     * -------------------------
     * Tree bins排序核心依赖hashCode,这里说的排序其实就是算出自己在数组中的下标,
     * 如果有两个元素都class C implements Comparable<C>,compareTo 方法会被用于排序.
     * (我们使用反射去见这个类型,方法:comparableClassFor)
     * 无论在不同hash值或和排序的情况下都证明算法复杂度是 O(log n),所以tree bins 带来的复杂度是值得的.
     * 因此,即时在hashCode出来的值不够充分的分散,因为是树的原因,性能变差的过程也会比较平滑.
     * --------------------------
     * 当一个桶里有足够多的节点是才会将结构转成tree,目前TREEIFY_THRESHOLD默认设置为8,当变少的时候,也会转换为
     * 原来平的链表结构.如果hashCodes是均匀分散的,这种转成tree基本用不到. 理想的分布应该是泊松分布.
     * 这里有点难理解,查了很多资料,有了以下详细解释: 这里提到泊松分布,可以看wiki,也可以看下推荐的博文:
     * http://www.ruanyifeng.com/blog/2015/06/poisson-distribution.html
     * 在文档上无法用数学公式和图片,所以下面对注释众提到的公式进行解析:
     * exp : 指数函数
     * pow : 乘方运算
     * factorial : 阶乘
     * (exp(-0.5) * pow(0.5, k) / factorial(k)) 这个公式是可以对应到泊松分布的公式的.
     * 这个0.5的意思是表示在这里假定元素数量占桶数量的百分50,而threshold是0.75,元素在某个桶里的概率是0.5.
     * 所以我们以这个概率为基础数据算出,桶里有1-8个元素的概率,如数据.当有8个元素在一个桶里时的概率非常低,
     * 在这里也解释了,如果出现需要将链表转成树的情况出现,已经表示不合理的场景出现了.
     * --------------
     * 一般树的根是第一个加入的node,也有其他情况,比如remove掉了root,不过可以重新分配出root.
     * 这里加一下信息:
     * redis中,在处理这种情况时是把新加入的元素放在链表的头部,在它的场景里最近加入的元素越容易被用到
     * 所有的内部方法都可以接受一个hashcode来做为参数,如此内部调用的时候完全可以通过这个参数而不需要重新计算
     * hashCodes.大部分内部方法也接受一个tab参数,一般这个有是现在的表的,在resizing或converting的时候也有可能代表新表或老表的.
     */

    /**
     * 默认初始capacity,必须是2的幂.
     */
    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.
     * capacity最大值2的30幂次
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 负载因子
     */
    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.
     * 当一个链表上的元素到8个的时候,会转成树结构
     */
    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.
     * 当元素减小到6个时会从树转成链表
     */
    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.
     * 当发生链表转树这种情况,需要满足capacity必须大于等于64(8的四倍)
     * * 容量大于这个值时,表中的桶才能进行树形化
     */
    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.)
     * 这个就是核心数据结构,一个node对应一个key-value元素,hash表示自己在哪个桶里的,next表示链表结构.
     */
    static class Node<K, V> implements Map.Entry<K, V> {

        final int hash;
        final K key;
        V value;
        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;
        }

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

    /**
     * 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.
     * 代码中是将key的hashCode和高16位进行了异或操作
     * 注意到我们table的长度必然为2的幂,这里有一点要注意在取模的操作里如果是和素数(质数)取模比和合数取模冲突的
     * 概率要低.
     * 合数既然可以由自身以外的数除尽,哪些可以相乘得到这个合数,这些乘数或乘数的倍数,都是潜在引起冲突的值.
     * 所以作者解释了把高位的16位下移,做一个异或操作(XOR),保证了高位参与hash值取模时参加计算,这是在权衡了速度,
     * 质量和实用性上进行的妥协.
     */
    static final int hash(Object key) {

        int h;
        // 对key的hashCode得到值再修饰一下
        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.
     * 如果实现了Comparable,返回x的实际类型,也就是Class<C>,否则返回null.
     * 例子:public class AppVersion implements Comparable<AppVersion>
     */
    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));
    }

    /**
     * 返回一个2的幂大小的数,这个数比cap大.
     */
    static final int tableSizeFor(int cap) {
        /**
         * 先解释或运算|=:
         * int a = 5; // 0000 0101
         * int b = 3; // 0000 0011
         * a |= b; // 0000 00111
         */

        /**
         * 再解释且运算|=:
         * int a = 5; // 0000 0101
         * int b = 3; // 0000 0011
         * a &= b; // 0000 0001
         */
        /**
         * 二级制中,与高位相对,表示二进制数字右边部分。
         */

        // cap的二进制里低位全部转成1
        // 解释一个:n |= n >>> 1 ==> n = n>>>1 | n
        // 假设n= 0001 xxxx xxxx xxxx
        // 计算:0001 xxxx xxxx xxxx | 0000 1xxx xxxx xxxx => 0001 1xxx xxxx xxxx
        // 此时最高位就是两个连续的1,然后操作n |= n >>> 2,那么就变成 0001 111x xxxx xxxx
        // 所以变1的节奏个数是:1 2 4 8 16 相加 31 刚好足够把32位的一个值低位全部变成1.
        // 只不过cap最大也就是2的30次
        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.)
     * 所以我们说hashmap的核心数据结构就是一个装着node的数组 我们注意到字段使用transient修饰,不参与序列化,
     * 可是hashmap继承Serializable.原因是hashcode操作依赖jvm所处的环境因素,不同环境可能有不同的hash值,
     * 做一现成存储的内容既是序列化也无法通用.所以hashmap自己实现了writeObject和readObject
     * 这里就需要知道java在序列化和反序列化一个类时是先调用writeObject和readObject,如果没有默认调用的
     * 是ObjectOutputStream的defaultWriteObject以及ObjectInputStream的defaultReadObject方法
     */
    transient Node<K, V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K, V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     * 记录有多少元素存进来了
     */
    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).
     * 前面提到过在迭代的时候如果改变了map的结构是要抛异常的,这个数用于记录改变的次数.
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     * 判断什么时候可以resize了
     *
     * @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.)
    int threshold;

    /**
     * The load factor for the hash table.
     * 负载因子
     * 
     * @serial
     */
    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
     */
    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>.
     * 参数为一个map的构造函数,新的HashMap负载因子为0.75,参数不能为null
     * 
     * @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;
        putMapEntries(m, false);
    }

    /**
     * Implements Map.putAll and Map constructor
     * putAll也调用这个方法.evict为false时代表构造函数调用
     *
     * @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
                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) // 提前做了一次resize
                resize();
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                // 调用内部put方法 hash(key)方法先处理下key
                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
     */
    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
     */
    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.
     * 获取key对应的value,这里返回null不一定代表map里没有这个元素,可能是value本来就是null.
     *
     * @see #put(Object, Object)
     */
    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))))
                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;
    }

    /**
     * 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.
     */
    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>.)
     */
    public V put(K key, V value) {

        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods
     * put方法调用.
     * onlyIfAbsent参数用于putIfAbsent方法调用时使用true,表示是否替换
     *
     * @paramhash后的key值
     * @param 原来的key
     * @param value值
     * @param 如果为true,则不更改现有值(key重复不覆盖原有的值)
     * @param 钩子方法,这在HashMap中是个空方法,但是在其子类LinkedHashMap中会被Override
     * @return 返回null 或者上一次的值
     */
    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)
            n = (tab = resize()).length;// 若当前哈希数组table的长度为0,则进行扩容

        if ((p = tab[i = (n - 1) & hash]) == null)// 确定输入的hash在哈希数组中对应的下标i
            // 若数组该位置之前没有被占用,则新建一个节点放入,插入完成。
            tab[i] = newNode(hash, key, value, null);
        else {// 桶内已经有元素情况
            Node<K, V> e;
            K k;
            if (p.hash == hash &&
                    ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;// 放入元素和头元素相同,进行替换

            else if (p instanceof TreeNode)// 不相同,则判断是否为TreeNode

                /**
                 * 若该位置的第一个节点p为TreeNode类型,说明这里存放的是一棵红黑树,p为根节点。
                 * 于是交给putTreeVal方法来完成后续操作,该方法下文会有详述
                 **/

                e = ((TreeNode<K, V>) p).putTreeVal(this, tab, hash, key, value);
            else {
                // 走到这里,说明p不匹配且是一个链表的头结点,该遍历链表了
                // 链表的情况,这里是先进行循环,在循环的过程中判断出元素超过TREEIFY_THRESHOLD则进行treeifyBin操作
                for (int binCount = 0;; ++binCount) {
                    /** e指向p的下一个节点 **/
                    if ((e = p.next) == null) {
                        // 当next是null的时候就是尾部了,这里就是把新放入的元素加到链表尾部的操作
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            // treeifyBin操作 转换成tree结构

                            /**
                             * 若插入后,该桶中的节点个数已达到了树化阈值
                             * 则对该桶进行树化。该部分源码下文会有详述
                             **/

                            treeifyBin(tab, hash);
                        break;
                    }
                    // 这里判断已经有相同key的元素
                    if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                        /**
                         * 匹配成功,我们需要用新的value来覆盖e节点
                         **/
                        break;

                    p = e; // 循环继续
                }
            }
            // 若执行到此时e不为空,则说明在map中找到了与key相匹配的节点e
            if (e != null) { // existing mapping for key
                V oldValue = e.value;// 暂存e节点当前的值为oldValue
                // 这里处理onlyIfAbsent,先新建一个node,然后再判断onlyIfAbsent,来决定是否替换原来的元素.
                // 注意如果原来的元素的value是会替换掉的!
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                // 钩子方法 LinkedHashMap使用
                afterNodeAccess(e);
                return oldValue;
            }
        }
        /**** --执行到此处说明没有匹配到已存在节点,一定是有新节点插入-- ****/
        ++modCount; // 结构操作数加一
        // 触发resize
        if (++size > threshold)
            resize();// 插入后,map中的节点数加一,若此时已达阈值,则扩容
        afterNodeInsertion(evict);// 同样的钩子方法,通知子类有新节点插入
        return null;// 同样的钩子方法,通知子类有新节点插入
    }

    /**
     * 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.
     * 初始化或倍增table的长度,因为长度遵守2的幂,所以元素的在resize后的新位置要么在远处要么移动2的幂次位置.
     * resize是map核心算法之一,它决定这map在扩容时的性能.如果是一个膨胀速度快的map,对resize的要求就很高了.
     *
     * @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) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                    oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        } else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else { // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int) (DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float) newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float) MAXIMUM_CAPACITY ? (int) ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({ "rawtypes", "unchecked" })
        Node<K, V>[] newTab = (Node<K, V>[]) new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K, V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K, V>) e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K, V> loHead = null, loTail = null;
                        Node<K, V> hiHead = null, hiTail = null;
                        Node<K, V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            } else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

    /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     * 将链表转成树结构,如果table还很小,就用resize操作.
     */
    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(); // 若table数组为空或其容量小于最小树化值,则用扩容取代树化
        else if ((e = tab[index = (n - 1) & hash]) != null) { // 定位到hash对应的桶位,头结点记为e
            TreeNode<K, V> hd = null;
            TreeNode<K, V> tl = null; // 声明两个指针分别指向链表头尾节点
            do {
                TreeNode<K, V> p = replacementTreeNode(e, null); // 将Node类型的节点e替换为TreeNode类型的p
                if (tl == null)
                    hd = p; // 若当前链表为空,则赋值头指针为p
                else {
                    p.prev = tl; // 否则将p添加到链表尾部
                    tl.next = p;
                }
                tl = p; // 后移尾指针
            } while ((e = e.next) != null); // 循环继续

            if ((tab[index] = hd) != null) // 将链表头节点放入table的index位置
                hd.treeify(tab); // 通过treeify方法将链表树化
        }
    }

    /**
     * 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
     */
    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>.)
     */
    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
     * 提供内remove方法使用
     *
     * @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) {
            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) {
                // tree情况
                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;
    }

    /**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
     */
    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;
        }
    }

    /**
     * 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
     */
    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;
    }

    /**
     * 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
     */
    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)
                        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
     */
    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)
                        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
     */
    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);// table长度
        s.writeInt(size);// 只需要写入全部元素,部需要记录table上无元素的情况
        internalWriteEntries(s);// 写入元素
    }

    /**
     * Reconstitute the {@code HashMap} instance from a stream (i.e.,
     * deserialize it).
     * 反序列化使用,在反序列化时系统会调用到这个方法.依次读出writeObject写入的内容
     */
    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);
            }
        }

        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.
    // 全部元素
    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.
     * 树结构节点,继承LinkedHashMap.Entry
     */
    static final class TreeNode<K, V> extends LinkedHashMap.Entry<K, V> {

        // 父,左右子,颜色
        TreeNode<K, V> parent; // red-black tree links
        TreeNode<K, V> left;
        TreeNode<K, V> right;
        TreeNode<K, V> prev; // needed to unlink next upon deletion
        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.
         */
        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.
         */
        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);
            }
        }

        /**
         * Finds the node starting at root p with the given hash and key.
         * The kc argument caches comparableClassFor(key) upon first use
         * comparing keys.
         */
        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;
        }

        /**
         * Calls find for root node.
         * 查找树中元素 -> 从root开始
         */
        final TreeNode<K, V> getTreeNode(int h, Object k) {

            // root的parent==null
            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.
         * 两节点hashcode相同无法排序时,用System.identityHashCode再进行依次比较
         * identityHashCode 使用内存地址进行hashCode
         */
        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
         *         真正转变操作
         */
        // 这是TreeNode类的实例方法,以调用节点this为根节点,将链表树化
        final void treeify(Node<K, V>[] tab) {

            TreeNode<K, V> root = null; // 声明root变量以记录根节点
            for (TreeNode<K, V> x = this, next; x != null; x = next) { // 从调用节点this开始遍历
                next = (TreeNode<K, V>) x.next; // 暂存链表中的下一个节点,记为next
                x.left = x.right = null; // 当前节点x的左右子树置空
                if (root == null) {
                    x.parent = null; // 若root仍为空,则将x节点作为根节点
                    x.red = false; // 红黑树特性之一:根节点为黑色
                    root = x; // 赋值root
                } else { // 否则的话需将当前节点x插入到已有的树中
                    K k = x.key;
                    int h = x.hash;
                    Class<?> kc = null;
                    // 第二层循环,从根节点开始寻找适合x插入的位置,并完成插入操作。
                    // putTreeVal方法的实现跟这里十分相似。
                    for (TreeNode<K, V> p = root;;) {
                        int dir, ph;
                        K pk = p.key;
                        if ((ph = p.hash) > h) // 若x的hash值小于节点p的,则往p的左子树中继续寻找
                            dir = -1;
                        else if (ph < h) // 反之在右子树中继续
                            dir = 1;
                        // 若两节点hash值相等,且key不可比,则利用System.identityHashCode方法来决定一个方向
                        else if ((kc == null && (kc = comparableClassFor(k)) == null) ||
                                (dir = compareComparables(kc, k, pk)) == 0)
                            dir = tieBreakOrder(k, pk);

                        TreeNode<K, V> xp = p; // 将当前节点p暂存为xp
                        // 根据上面算出的dir值将p向下移向其左子树或右子树,若为空,则说明找到了合适的插入位置,否则继续循环
                        if ((p = (dir <= 0) ? p.left : p.right) == null) {
                            // 执行到这里说明找到了合适x的插入位置
                            x.parent = xp; // 将x的parent指针指向xp
                            if (dir <= 0) // 根据dir决定x是作为xp的左孩子还是右孩子
                                xp.left = x;
                            else
                                xp.right = x;
                            // 由于需要维持红黑树的平衡,即始终满足其5条性质,每一次插入新节点后都需要做平衡操作
                            // 这个方法的源码我们在<<红黑树(Red-Black Tree)解析>>一文中已有详细分析,此处不再重复
                            root = balanceInsertion(root, x);
                            break; // 插入完成,跳出循环
                        }
                    }
                }
            }
            // 由于插入后的平衡调整可能会更换整棵树的根节点,
            // 这里需要通过moveRootToFront方法确保table[index]中的节点与插入前相同
            moveRootToFront(tab, root);
        }

        /**
         * Returns a list of non-TreeNodes replacing those linked from
         * this node.
         */
        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.
         */
        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;
                }
            }
        }

        /**
         * 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) {
                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.
         * 修剪或转成链表
         *
         * @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
         */
        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);
                }
            }
        }

        /* ------------------------------------------------------------ */
        // Red-black tree methods, all adapted from CLR

        // 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;
        }

        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;
                        }
                    }
                }
            }
        }

        /**
         * Recursive invariant check
         */
        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;
        }
    }

}

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