Set<Map.Entry<K, V>> entrySet();

static final class TreeMapEntry<K,V> implements Map.Entry<K,V> {
    K key;
    V value;
    TreeMapEntry<K,V> left;
    TreeMapEntry<K,V> right;
    TreeMapEntry<K,V> parent;
    boolean color = BLACK;
    /**
     * Make a new cell with given key, value, and parent, and with
     * {@code null} child links, and BLACK color.
     */
    TreeMapEntry(K key, V value, TreeMapEntry<K,V> parent) {
        this.key = key;
        this.value = value;
        this.parent = parent;
    }
    /**
     * Returns the key.
     *
     * @return the key
     */
    public K getKey() {
        return key;
    }
    /**
     * Returns the value associated with the key.
     *
     * @return the value associated with the key
     */
    public V getValue() {
        return value;
    }
    /**
     * Replaces the value currently associated with the key with the given
     * value.
     *
     * @return the value associated with the key before this method was
     *         called
     */
    public V setValue(V value) {
        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }
    public boolean equals(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?,?> e = (Map.Entry<?,?>)o;
        return valEquals(key,e.getKey()) && valEquals(value,e.getValue());
    }
    public int hashCode() {
        int keyHash = (key==null ? 0 : key.hashCode());
        int valueHash = (value==null ? 0 : value.hashCode());
        return keyHash ^ valueHash;
    }
    public String toString() {
        return key + "=" + value;
    }
}

public V put(K key, V value) {
    TreeMapEntry<K,V> t = root;
    ......
    int cmp;
    TreeMapEntry<K,V> parent;
    // split comparator and comparable paths
    Comparator<? super K> cpr = comparator;
    if (cpr != null) {
        do {
            parent = t;
            cmp = cpr.compare(key, t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    }
    ......
}

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 final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

......
if ((p = tab[i = (n - 1) & hash]) == null)
    tab[i] = newNode(hash, key, value, null);
......

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;
    // 判斷數(shù)組中沒結(jié)點的話創(chuàng)建結(jié)點然后添加進取    
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        // 判斷結(jié)點的hash和key是相等的話直接改值
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
        // 判斷是紅黑樹的情況（后面會說）
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
        // 判斷是鏈表的情況，循環(huán)遍歷鏈表找到hash和key相同的結(jié)點
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                // 沒找到的情況下創(chuàng)建一個新的結(jié)點放到鏈表末尾
                    p.next = newNode(hash, key, value, null);
                    // 判斷是否需要將鏈表轉(zhuǎn)成紅黑樹
                    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;
                p = e;
            }
        }
        // 找到相同的key，直接替換value然后結(jié)束流程
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // 擴容
    if (++size > threshold)
        resize();
    // 鉤子
    afterNodeInsertion(evict);
    return null;
}

/**
 * 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;
/**
 * 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;

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    ......
    int newCap, newThr = 0;
    if (oldCap > 0) {
        ......
        // 重點是newCap = oldCap << 1
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            ......
    }
    ......
    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)
                // 單結(jié)點情況下的擴容
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                // 紅黑樹情況下的擴容
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                // 鏈表情況下的擴容
                    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;
}

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);
        }
    }
}