HashMap源码解析——基于JDK1.8

前言

HashMap数据结构由数组和链表(超过一定数量转换为红黑树)组成，在进行增删查等操作时，首先要定位到元素的所在表的位置，之后再从链表中定位该元素。具体找到表下标的方法是(n - 1) & hash，其中n代表表的长度。

HashMap数据结构如下(确定下标用的是%取余的方式)：

1 常量介绍

    /**
     * 默认容量16
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 最大的容量
     * 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.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 默认的负载因子
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * 由链表转换为树时桶的阈值,该值必须大于2，并且至少应该是8，以配合在树移除结点收缩变回普通桶的假设
     * 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.
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * 在resize操作中解除树化的桶的最小阈值
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * 桶树化时table最小的容量(否则如果很多结点在桶中,table将进行扩容)(桶中的结点个数已经达到树化的阈	  *	值,但是table的容量没有达到MIN_TREEIFY_CAPACITY,将进行扩容)
     * 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.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

    /**
     * table是使用的时候才初始化,需要的时候扩容.长度是2的幂指数(有事也是0)
     * 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(引导) mechanics that are currently not needed.)
     */
    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;

    /**
     * map中存储的键值对数量
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * 改变hashMap中存储的映射数量,或者改变HashMap的结构,值都会改变
     * 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).
     */
    transient int modCount;

    /**
     * 进行扩容的阈值
     * 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.)
    int threshold;

    /**
     * 负载因子
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

2 方法解析

2.1 构造方法

	/**
	 * 使用默认初始化的容量16与负载因子
     * Constructs an empty HashMap with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    /**
     * 使用指定初始化容量
     * Constructs an empty HashMap 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);
    }

    /**
     * 指定初始化容量和负载因子
     * Constructs an empty HashMap 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);
    }

tableSizeFor

    /**
     * 得到大于n的2的整数次幂(对不规范的输入进行处理)
     * Returns a power of two size for the given target capacity.
     */
    static final int tableSizeFor(int cap) {
        //注意这里-1操作
        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;
    }

将n的二进制出现1的最高位后面所有位都实现置1(用4次或等(|=)运算实现)，最后+1得到大于n的整数次幂。因为int类型为32位，极限就是将32位全置为1。

举个栗子：

    /**
     * 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
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

putMapEntries

    /**
     * Implements Map.putAll and Map constructor
     *
     * @param m the map
     * @param evict false when initially constructing this map, else
     * true (relayed to method afterNodeInsertion).
     */
    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        if (s > 0) {
            //如果table为空
            if (table == null) { // pre-size
                //计算容量是否超过最大容量
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                         (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)
                    //得到大于等于t的2的整数次幂
                    threshold = tableSizeFor(t);
            }
            //如果不为空且s>当前容量,扩容
            else if (s > threshold)
                resize();
            
            //循环添加进相应的位置
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);
            }
        }
    }

2.2 常用方法

2.2.1 put(K,V)方法

	/**
	 * 将指定值与此map中的指定键关联,如果map中存在键,旧的值将被替换
     * 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 key, or
     *         null if there was no mapping for key.
     *         (A null return can also indicate that the map
     *         previously associated null with key.)
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

hash

    /**
     * 将高16位与低16位异或,目的是减少碰撞
     * 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(合并) impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

putVal

    /**
     * Implements Map.put and related methods
     *
     * @param hash hash for key 键的hash值
     * @param key the key 键
     * @param value the value to put 值
     * @param onlyIfAbsent if true, don't change existing value (为真时，不会改变存在的值)
     * @param evict if false, the table is in creation mode.(为假时,创建模式)
     * @return previous value(上一个值), or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //如果是第一次，调用resize初始化
        //如果table!=null(存在)，进行第二个条件判断，将table的长度赋予n
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //注意算下标的方法：(n - 1) & hash
        //如果相应下标下结点为空，直接添加
        if ((p = tab[i = (n - 1) & hash]) == null)
            // Create a regular (non-tree) node 创建一个一般的非树结点
            tab[i] = newNode(hash, key, value, null);
        //如果相应下标下结点不为空
        else {
            Node<K,V> e; K k;
            //如果hash相同,key相同
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //如果是红黑树结点
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //如果是链表结点，hash相同,但是key不同
            else {
                for (int binCount = 0; ; ++binCount) {
                    //将新加入的结点加在链表末尾
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //如果>=8-1,转变为红黑树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    //如果hash相同，键相同
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            
            if (e != null) { // existing mapping for key 存在键相同的结点
                V oldValue = e.value;
                //如果为true就不改变存在的值
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

resize

 	/**
 	 * 初始化或者加倍table的大小。如果为null，分配初始化容量16，否则就加倍，每个元素的下标不变或者移动
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
        //保存旧的（当前的）table
        Node<K,V>[] oldTab = table;
        //如果为空，旧的容量为0，否则就是原本table的长度
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //保存旧的（当前的）阈值
        int oldThr = threshold;
        int newCap, newThr = 0;
        //如果旧的容量大于0
        if (oldCap > 0) {
            //如果>=最大容量
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //如果2*oldCap小于最大容量与oldCap>=16
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold 加倍
        }
        //如果旧的阈值大于0
        //不满足上面的情况(初始化容量时<16)，将存储在threshold中的值赋予newCap(容量)
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        //容量<=0与阈值<=0,使用默认值初始化
        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;
        //如果旧的table不为空
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    //将旧值清空
                    oldTab[j] = null;
                    //next为空，该位置只有一个结点(值)
                    if (e.next == null)
                        //重新根据hash与新的容量计算下标
                        newTab[e.hash & (newCap - 1)] = e;
                    //如果是树结点
                    else if (e instanceof TreeNode)
                        //根据(e.hash & oldCap)分为两个，如果哪个数目不大于			//UNTREEIFY_THRESHOLD，就转为链表(与下面相同)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        //分成低链表和高链表，分别指向头与尾
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        //遍历链表，根据e.hash & oldCap的值将同一桶中的元素分成两个链表
                        do {
                            next = e.next;
                            //低链表，索引下标不变
                            if ((e.hash & oldCap) == 0) {
                                //第一次尾结点为空，头结点指向当前结点
                                if (loTail == null)
                                    loHead = e;
                                //第二次及以后，将当前尾结点与当前结点连接起来
                                else
                                    loTail.next = e;
                                //尾结点指向当前结点
                                loTail = e;
                            }
                            //高链表
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        //将低链表的尾结点的next置为null，替换原本索引下标下的链表
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        //将高链表的尾结点的next置为空，放在新的索引下标下
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

分成的两个高低链表索引下标是否变化,具体是要看原本hash值在新增加(扩容为原来的2倍后,进位)的位上是0还是1，如果是0索引下标就不变(低链表)，如果是1索引下标就变化(原来的索引下标 + oldCap)。

原因：因为表的容量是2的幂指数，原本的下标是通过hash&(容量-1)计算的。假设原本容量是8(0000,1000),hash值是19(0001,0011)。hash&(容量-1)[(0001,0011&0000,0111)]等于3。当容量变为原来的两倍时16(0001,0000),再次相与(0001,0011&0000,1111),值依然为3(原因是hash指在容量新增加的位上是0，所以索引下标不变，反之增加的就是原本的旧容量)

treeifyBin

    /**
     * 替换指定哈希表的索引处的所有链接节点，除非表太小(未达到表的MIN_TREEIFY_CAPACITY 64)，否则将改	   * 为resize扩容操作
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        //如果tab为null或者长度小于64,进行resize操作
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        //下面的操作形成红黑树
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

2.2.2 get(Object)方法

    /**
     * 如果map中存在则返回指定键的值，如果不存在则返回null
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * 
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.)
     *
     * 返回null也可能是存在指定的key，但是value为null，使用containsKey分辨
     * 
A return value of {@code null} does not necessarily
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

getNode

    /**
     * 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;
        //如果table不为空，长度>0，table中hash对应下标的第一个结点不为null，有值
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            //第一个结点的hash和key都匹配，找到，返回
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            //第一个结点不匹配，且后面还存在结点，遍历寻找
            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;
    }

2.2.3 remove(Object)方法

     /**
     * 如果存在，则从此映射中删除指定键的映射
     * 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 key, or
     *         null if there was no mapping for key.
     *         (A null return can also indicate that the map
     *         previously associated null with key.)
     */
    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }

removeNode

    /**
     * Implements Map.remove and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to match if matchValue, else ignored
     * @param matchValue if true only remove if value is equal
     * @param movable if false do not move other nodes while removing
     * @return the node, or null if none
     */
    final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        //p表示头结点或者将要被删除的结点的上一个结点
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        //如果table不为空，长度>0，table中hash对应下标的第一个结点不为null，有值
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            //node指将要被删除的结点
            Node<K,V> node = null, e; K k; V v;
            //与上面相同
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            //链表头结点不等，且后面还有结点
            else if ((e = p.next) != null) {
                //树节点
                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)))) {
                            //赋值给node
                            node = e;
                            break;
                        }
                        //赋值给p,方便后面操作
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                //如果是树节点
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                //如果头结点是将要被删除的结点
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

2.2.4 keySet()方法

    /**
     * 返回包含在此map中的键的set视图
     * 如果在对集合进行迭代时修改了映射(除了通过迭代器自己的remove操作)，迭代的结果是未定义的(注意)
     * 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 remove operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * Iterator.remove, Set.remove,
     * removeAll, retainAll, and clear
     * operations.  It does not support the add or addAll
     * operations.
     *
     * @return a set view of the keys contained in this map
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        //如果ks为null,创建新的KeySet类
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

keySet类

    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();
            }
        }
    }

2.2.5 entrySet()方法

    /**
     * 与上面相同
     * 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 remove operation, or through the
     * setValue 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 Iterator.remove,
     * Set.remove, removeAll, retainAll and
     * clear operations.  It does not support the
     * add or addAll 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;
    }

EntrySet类

    final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public final int size()                 { return size; }
        public final void clear()               { HashMap.this.clear(); }
        //返回迭代对象
        public final Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }
        //是否存在
        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);
            //这里的equals,当e为Map.Entry时,比较键与值是否相同
            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();
            }
        }
    }

2.2.6 values()方法

    /**
     * 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 remove operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the Iterator.remove,
     * Collection.remove, removeAll,
     * retainAll and clear operations.  It does not
     * support the add or addAll operations.
     *
     * @return a view of the values contained in this map
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

Values类

    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();
            }
        }
    }

2.2.7 clear()方法

    /**
     * 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) {
            //置0
            size = 0;
            //遍历置空
            for (int i = 0; i < tab.length; ++i)
                tab[i] = null;
        }
    }