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学习笔记 ArrayList / List 初始化：
List list = Arrays.asList(1,2);

Array 初始化： int[]{map.get(target-nums[i]),nums[i]};

HashMap 源码： 为啥要这么多中间变量呢？ tab, first, e; n,k 感觉诚心让我们不容易看懂源码:-) 这些中间变量的作用是存储取出的值？ tab = table 有啥作用，为啥不能直接在table上操作？

putVal时，如果hash key 相同的值超过7个时变为TreeNode(红黑树） if (binCount >= 7) { this.treeifyBin(tab, hash); }

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;
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    HashMap.Node[] tab;
    int n;
    if ((tab = this.table) == null || (n = tab.length) == 0) {
        n = (tab = this.resize()).length;
    }

    Object p;
    int i;
    if ((p = tab[i = n - 1 & hash]) == null) {
        tab[i] = this.newNode(hash, key, value, (HashMap.Node)null);
    } else {
        Object e;
        Object k;
        if (((HashMap.Node)p).hash == hash && ((k = ((HashMap.Node)p).key) == key || key != null && key.equals(k))) {
            e = p;
        } else if (p instanceof HashMap.TreeNode) {
            e = ((HashMap.TreeNode)p).putTreeVal(this, tab, hash, key, value);
        } else {
            int binCount = 0;

            while(true) {
                if ((e = ((HashMap.Node)p).next) == null) {
                    ((HashMap.Node)p).next = this.newNode(hash, key, value, (HashMap.Node)null);
                    if (binCount >= 7) {
                        this.treeifyBin(tab, hash);
                    }
                    break;
                }

                if (((HashMap.Node)e).hash == hash && ((k = ((HashMap.Node)e).key) == key || key != null && key.equals(k))) {
                    break;
                }

                p = e;
                ++binCount;
            }
        }

        if (e != null) {
            V oldValue = ((HashMap.Node)e).value;
            if (!onlyIfAbsent || oldValue == null) {
                ((HashMap.Node)e).value = value;
            }

            this.afterNodeAccess((HashMap.Node)e);
            return oldValue;
        }
    }

    ++this.modCount;
    if (++this.size > this.threshold) {
        this.resize();
    }

    this.afterNodeInsertion(evict);
    return null;
}