HashMap 源码分析

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简介

HashMap 底层数据结构是数组、哈希表。数组称之为哈希桶,它是线程不安全的。

允许key为null,value为null。

遍历时无序。

在JDK8中,当链表长度达到8,会转化成红黑树,以提升它的查询、插入效率。

结构

HashMap继承AbstractMap<K,V>,实现了Map,Cloneable, Serializable。如下:

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

  • 继承AbstractMap,实现了Map接口:提供了基本功能(size、isEmpty、containsKey、containsValue等到)。
  • 实现Cloneable:实现clone()方法,实现克隆。

数据结构

HashMap中有两种数据结构,一种链表,一种红黑树。

链表

    static class Node<K,V> implements Map.Entry<K,V> {
        // hash值
        final int hash;
        // key
        final K key;
        // value
        V value;
        // 下一节点
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }

        // ^ : 位异或运算。二进制数,从高位到低位逐一比较,相同为0,不同为1。
        // 每个节点的hash值是通过key和value的hash值亦或得到。
        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        // 设置新值,覆盖旧值,同时返回旧值
        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        // 比较是否是同一个对象
        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;
        }
    }

总结:通过分析得知,这是一个单链表;每个节点的hash值是通过key和value的hash值亦或得到。

红黑树

    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // 红黑树的链接
        TreeNode<K,V> left;
        TreeNode<K,V> right;
        TreeNode<K,V> prev;    // 需要取消链接后删除
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }
    //只是部分代码,缺少一个右大括号

常量值

    /**
     * The default initial capacity - MUST be a power of two.
     * - 默认的初始容量(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.
     * MUST be a power of two <= 1<<30.
     * - << : 左移运算符,eg:1<<1 = 2^1 ; 1<<30 = 2^30
     * - 最大容量,如果更高的值是由任何一个带有参数的构造函数隐式指定的,则使用该值。必须是2的幂<= 1<<30。
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     * - 默认加载因子,在构造函数中没有指定时使用的负载因子
     */
    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.
     * -  桶的树化阈值:链表转成红黑树的阈值
     * - 当冲突的元素数增加到8时,链表变为树。
     */
    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.
     * - 桶的链表还原阈值:红黑树转为链表的阈值。
     * - 当减少至6时,树切换为链表。
     */
    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.
     * - 最小树形化容量阈值:当哈希表中的容量 > 该值时,才允许树形化链表 (即 将链表 转换成红黑树)
     * - 否则,若桶内元素太多时,则直接扩容,而不是树形化
     * - 为了避免进行扩容、树形化选择的冲突,这个值不能小于 4 * TREEIFY_THRESHOLD
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

字段

    /**
     * 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.)
     * - 哈希桶,存放链表。 长度是2的N次方,或者初始化时为0.
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     * - 数据转换成set的另一种存储形式,主要用于迭代功能。
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     * - 列表大小:此映射中包含的键-值映射的数目。
     */
    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).
     * - 记录被修改的次数
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    // 扩容阈值:要调整大小的下一个大小值(容量*负载因子)。
    int threshold;

    /**
     * The load factor for the hash table.
     * - 负载因子,可计算出当前table长度下的扩容阈值:threshold = loadFactor * table.length。
     * @serial
     */
    final float loadFactor;

构造方法

HahsMap提供了四种构造方法

HashMap()

无参构造:构造一个空的HashMap,具有默认的初始容量16和默认的负载因子0.75。

    /**
     * Constructs an empty HashMap with the default initial capacity
     * (16) and the default load factor (0.75).
     * - 构造一个空的HashMap,具有默认的初始容量16和默认的负载因子0.75。
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
HashMap(int initialCapacity, float loadFactor)

使用指定的初始容量和负载因子构造一个空的HashMap。

    /**
     * Constructs an empty HashMap with the specified initial
     * capacity and load factor.
     * - 使用指定的初始容量和负载因子构造一个空的HashMap。
     * @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) {
        // 初始容量 < 0 抛出异常
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        //初始容量大于对打容量,取最大容量
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        // 负载因子 <= 0 或 负载因子为空
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        // 赋值
        this.loadFactor = loadFactor;
        // 新的扩容临界值
        this.threshold = tableSizeFor(initialCapacity);
    }

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

>>> : 无符号右移详情 →Java 按位运算符

HashMap(int initialCapacity)

构造一个空的HashMap,具有指定的初始容量和缺省负载因子0.75。

    /**
     * Constructs an empty HashMap with the specified initial
     * capacity and the default load factor (0.75).
     * - 构造一个空的HashMap,具有指定的初始容量和缺省负载因子0.75。
     * @param  initialCapacity the initial capacity.
     * - 初始容量
     * @throws IllegalArgumentException if the initial capacity is negative.
     * - 抛出异常
     */
    public HashMap(int initialCapacity) {
        // 使用HashMap(int initialCapacity, float loadFactor)构造,创建
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
HashMap(Map<? extends K, ? extends V> m)

使用与指定的Map构造新的HashMap,新HashMap使用默认负载因子(0.75)和足够容纳指定Map中的元素的初始容量创建的。

    /**
     * Constructs a new HashMap with the same mappings as the
     * specified  Map .  The HashMap is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified  Map .
     *
     * @param   m the map whose mappings are to be placed in this map
     * - 这个Map的元素将被放在这个Map中
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        // 默认负载因子 0.75f
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

    /**
     * - 哈希桶,存放链表。 长度是2的N次方,或者初始化时为0.
     */
    transient Node<K,V>[] table;

    /**
     * Implements Map.putAll and Map constructor.
     *
     * @param m the map
     * @param evict false when initially constructing this map, else true (relayed to method afterNodeInsertion).
     * - 在最初构造这个映射时为false,否则为true。
     */
    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        // Map的大小是不是大于0(等于0就没必要初始化了)
        if (s > 0) {
            // 通过构造器新建一个HashMap,所以table是null(table是Node数组)
            if (table == null) { // pre-size
                //计算阀值,判断是否大于最大容量,如果没超过则还是原阀值ft,将ft赋给t。
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ? (int)ft : MAXIMUM_CAPACITY);
                //判断阀值t是否大于原阀值(原阀值是16,默认容量是16),如果大于原阀值,重新计算tableSizeFor(t)。
                if (t > threshold)
                    threshold = tableSizeFor(t);
            }
            else if (s > threshold)
                //构造调用,先不做解释
                resize();
            //
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                // putVal()方法先不看,主要功能是创建一个节点,把这个节点放到了tab中
                putVal(hash(key), key, value, false, evict);
            }
        }
    }

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

普通方法

size()

返回此Map中的键值对个数。

isEmpty()

如果此Map不包含键值映射,则返回true

public boolean isEmpty() { return size == 0; }

get(Object key)

返回指定key对应的value,如果没有改键值对,则返回 null。

    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.
     * - 实现于 Map.get 方法 和 相关方法
     * @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;
        // 判断Node[]不等与空,并且第一个元素不等于空
        if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {
            // 判断是否是第一个元素
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            //判断下一个元素不等于空
            if ((e = first.next) != null) {
                //判断是否是红黑树
                if (first instanceof TreeNode)
                    // 红黑树循环遍历获取ke对应的value
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                // 链表循环遍历获取ke对应的value
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

containsKey(Object key)

判断指定key是否存在,如果存在则返回true,否则返回false。

    public boolean containsKey(Object key) {
        return getNode(hash(key), key) != null;
    }

    /**
     * Implements Map.get and related methods.
     * - 实现于 Map.get 方法 和 相关方法
     * @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;
        // 判断Node[]不等与空,并且第一个元素不等于空
        if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {
            // 判断是否是第一个元素
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            //判断下一个元素不等于空
            if ((e = first.next) != null) {
                //判断是否是红黑树
                if (first instanceof TreeNode)
                    // 红黑树循环遍历获取ke对应的value
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                // 链表循环遍历获取ke对应的value
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

containsValue(Object value)

判断指定value是否存在,如果存在则返回true,否则返回false。

    public boolean containsValue(Object value) {
        Node<K,V>[] tab; V v;
        //判断table不等与空
        if ((tab = table) != null && size > 0) {
            //循环遍历table数组
            for (int i = 0; i < tab.length; ++i) {
                //循环遍历链表
                for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                    //判断value是否存在,存在返回true
                    if ((v = e.value) == value || (value != null && value.equals(v)))
                        return true;
                }
            }
        }
        return false;
    }

clear()

删除Map中的元素

    public void clear() {
        Node<K,V>[] tab;
        //集合修改次数加1
        modCount++;
        //循环遍历Node数组
        if ((tab = table) != null && size > 0) {
            //长度置空
            size = 0;
            //循环遍历,链表置空
            for (int i = 0; i < tab.length; ++i)
                tab[i] = null;
        }
    }

put(K key, V value)

将指定键与值进行关联,并存储。如果键已存在,则新value值覆盖旧value值。

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict)

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {
        Node<K,V>[] tab;
        Node<K,V> p;
        int n, i;
        // 如果table为空,则进行必要字段的初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;// 获取长度(16)
        // 如果根据hash值获取的结点为空,则新建一个结点
        if ((p = tab[i = (n - 1) & hash]) == null)  // 此处 & 代替了 % (除法散列法进行散列)
            // 这里的p结点是根据hash值算出来对应在数组中的元素
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e;
            K k;
            // 如果新插入的结点和table中p结点的hash值,key值相同的话
            if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //如果是红黑树结点,则进行红黑树插入
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    //代表这个单链表只有一个头部结点,则直接新建一个结点即可
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        // 链表长度大于8时,将链表转红黑树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    // 及时更新p
                    p = e;
                }
            }
             // 如果存在这个映射就覆盖
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                //判断是否允许覆盖,并且value是否为空
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e); //回调以允许LinkedHashMap后置操作
                return oldValue;
            }
        }
        // 更改操作次数
        ++modCount;
        // 大于临界值
        if (++size > threshold)
            // 将数组大小设置为原来的2倍,并将原先的数组中的元素放到新数组中
            // 因为有链表,红黑树之类,因此还要调整他们
            resize();
        afterNodeInsertion(evict);  //回调以允许LinkedHashMap后置操作
        return null;
    }

resize()

    /**
     *初始化或者扩容之后元素调整
     */
    final Node<K,V>[] resize() {
        // 获取旧元素数组的各种信息
        Node<K,V>[] oldTab = table;
        // 旧元素的长度
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        // 旧元素的临界值
        int oldThr = threshold;
        // 定义新元素的长度及临界值
        int newCap, newThr = 0;
        // 如果旧元素不为空
        if (oldCap > 0) {
            // 如果旧元素长度达到最大值,则修改临界值为Integer.MAX_VALUE
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 下面就是扩容操作(2倍)
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)
                // threshold也变为二倍
                newThr = oldThr << 1; // double threshold
        }
        
        else if (oldThr > 0) // 初始容量设置为旧元素的临界值
            newCap = oldThr;
        else {               // threshold为0,则使用默认值
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {  // 如果临界值还为0,则设置临界值
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr; // 更新填充因子
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {    // 调整数组大小之后,需要调整红黑树或者链表的指向
            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)  // 红黑树调整
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        // 链表调整
                        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;
    }

treeifyBin(Node<K,V>[] tab, int hash)

    // 链表转双向链表操作
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;  
        // 如果元素总个数小于64,则继续进行扩容,结点指向调节
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        // 先找到那个链表的头
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                //创建红黑树根结点
                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)
                // 此处才是真正的转为红黑树
                hd.treeify(tab);
        }
    }

treeify(Node<K,V>[] tab)

        //将链表中每个值进行红黑树插入操作
        final void treeify(Node<K,V>[] tab) {
            TreeNode<K,V> root = null;
            // TreeNode<K,V> x = this  相当于初始化了一个结点
            for (TreeNode<K,V> x = this, next; x != null; x = next) {
                next = (TreeNode<K,V>)x.next;
                // 初始化Root
                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 &&
                            // comparableClassFor(k) 返回 k 类型的比较器
                                  (kc = comparableClassFor(k)) == null) ||
                            // compareComparables(kc, k, pk) 返回p,pk比较的结果
                                 (dir = compareComparables(kc, k, pk)) == 0)
                            // tieBreakOrder(k, pk) 比较两个hash码
                            dir = tieBreakOrder(k, pk);
                        // 此处进行红黑树操作
                        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;
                            // 平衡调节
                            root = balanceInsertion(root, x);
                            break;
                        }
                    }
                }
            }
            // 确保给定的根是根结点
            moveRootToFront(tab, root);
        }

remove(Object key)

如果存在,则从此映射中删除指定键的映射。

    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.
     * - 实现了与remove相关操作
     * @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;
        //table不为空
        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;
            //如果要删除的结点和table中p结点的hash值,key值相同的话
            if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                //如果是红黑树结点,则进行红黑树查找key对应的value
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    // 链表循环遍历获取ke对应的value
                    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)))) {
                //如果是红黑树,则进行红黑树删除
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p) //头元素的删除
                    tab[index] = node.next;
                else    //非头元素的删除
                    p.next = node.next;
                //操作的次数
                ++modCount;
                //长度减一
                --size;
                afterNodeRemoval(node); //回调以允许LinkedHashMap后置操作
                return node;
            }
        }
        return null;
    }