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;
        }
    }

// Node<K,V> 數組 
transient Node<K,V>[] table;

//加載因子
final float loadFactor;

//默認加載因子 ，容量達到這個比例時自動擴容
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 數量大于8時，鏈表轉換為紅黑樹形式存儲 
static final int TREEIFY_THRESHOLD = 8;

//即數量小于6時，會將紅黑樹轉換回鏈表，刪除元素時remove
static final int UNTREEIFY_THRESHOLD = 6;

//PUT 時每次都做hash
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
//put  函數核心算法
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null) // 這里的n 表示數組的長度。 
            // hash 
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        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;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e); // 空實現
                return oldValue;
            }
        }
        ++modCount;   // modCount 是java集合中Fail-Fast的底層實現原理
        if (++size > threshold) //擴容
            resize();
        afterNodeInsertion(evict);// 空實現
        return null;
    }
    
    // Callbacks to allow LinkedHashMap post-actions
    void afterNodeAccess(Node<K,V> p) { }
    void afterNodeInsertion(boolean evict) { }
    void afterNodeRemoval(Node<K,V> p) { }

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)
              //拋出異常，Fail-Fast
                throw new ConcurrentModificationException(); 
        }
    }
}

//擾動函數：促使元素位置分布均勻，減少碰撞幾率
static final int hash(Object key){
        int h;
        // 如果key == null  返回的值是 0 
        // 如果key !=null  
        // 首先計算 key 的 hashcode 值賦值給 h ,
        // 然后與h 無符號右移16位后的二進制按位異或預算得到最后hash值
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
} 

//get 實現
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
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;
}