java8中的HashMap原理详解

    //Node数组
    transient Node<K,V>[] table;
     //当前哈希表中k-v对个数，实际就是node的个数
    transient int size;
    //修改次数
    transient int modCount;
    //元素阈值
    int threshold;
    //负载因子
    final float loadFactor;

    static class Node<K,V> implements Map.Entry<K,V> {
        //hash值
        final int hash;
        final K key;
        V value;
        //hash & (capacity - 1) 相同的Node会形成一个链表
        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 V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

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

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; //如果数组为空，先初始化一下
        if ((p = tab[i = (n - 1) & hash]) == null) //如果对应的数组为空的话，那么就直接new一个node然后塞进去
            tab[i] = newNode(hash, key, value, null);
        else { //如果有值，说明发生了冲突，那么就先用拉链法来处理冲突
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p; //如果头结点的key和要插入的key相同，那么就说明找到了之前插入的节点
            else if (p instanceof TreeNode) //如果链表转成了红黑树
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) { //如果之前没有put过这个节点，那么就new一个新的节点
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) 
                        //另外要检查一下当前链表的长度，如果超过8那么就将链表转化成红黑树
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        //如果找到了之前的节点，那么就跳出
                        break;
                    p = e;
                }
            }
            if (e != null) {
                V oldValue = e.value; 
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e); //在当前类中NOOP
                return oldValue;
            }
        }
        ++modCount;
        //如果当前元素数量大于门限值，就要resize整个hash表，实际上就是把数组扩大一倍，然后将所有元素重新塞到新的hash表中
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict); //在该类中NOOP
        return null;
    }

final Node<K, V> [] resize() {
......
//去掉了一些代码，只关注最核心的node迁移
//resize会新建一个数组，数组的长度是原来数组长度的两倍
    for (int j = 0; j < oldCap; ++j) {//遍历原来的数组
        Node<K,V> e;
        if ((e = oldTab[j]) != null) {
            oldTab[j] = null;
            if (e.next == null)
                newTab[e.hash & (newCap - 1)] = e; //如果没有形成链表的话，就直接塞到新的hash表中
            else if (e instanceof TreeNode)
                ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); //红黑树操作？？
            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) { //如果hash值小于oldCap的时候，那么就还在原来那个数组的位置，就把这个节点放到low链表中
                        if (loTail == null)
                            loHead = e;
                        else
                            loTail.next = e;
                        loTail = e;
                    }
                    else { //否则的话就是因为扩展数组长度，就把原来的节点放到high链表中
                        if (hiTail == null)
                            hiHead = e;
                        else
                            hiTail.next = e;
                        hiTail = e;
                    }
                } while ((e = next) != null);
                if (loTail != null) {
                    loTail.next = null;
                    newTab[j] = loHead; //low链表还放在原来的位置
                }
                if (hiTail != null) {
                    hiTail.next = null;
                    newTab[j + oldCap] = hiHead; //high链表放到j+oldCap位置上
                }
            }
        }
    }
}

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 && ((k = first.key) == key || (key != null && key.equals(k)))) //如果节点不为空而且头结点与查找的key相同就返回
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);//从红黑树中查找
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null); //遍历链表查找key相同的node
        }
    }
    return null;
}