HashMap

1.8之前的HashMap线程不安全，包含循环链表的问题，get()时候会造成死循环cpu爆满。原因是1.8之前采用的头插法；1.8之后采用尾插法。

1.8 resize()

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        // 新容器size扩大一倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            // 阈值也扩大一倍
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    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"})
    // 开启一个新数组，size为原来两倍
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    // 指向新的table
    table = newTab;
    if (oldTab != null) {
        // 遍历老的数组
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            // 遍历元素链表
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                // 单节点直接用&定位，容量是1000 变成 10000，则&之后要么在原来位置，要么是原来位置*2
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                // 如果改节点已经变成一颗树了，在树中插入
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                   // 新表size是原来一倍，所以出现高位和低位，所以拆出来两条链表了，每个链表都维护头和尾
                    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;
}