jdk8的hashmap中扩容方法源码注释

关于树的以后再更新

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;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                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"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        // 以上是对新数组的长度和阈值进行判断设置
        // ----------------------------
        // 以下是将老数组中的元素放入新数组中
        if (oldTab != null) {
        // 如果老数组中有数组准备进行扩容
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                // 遍历到数组的元素不为空时进行拷贝,先将老数组的引用去掉，因为上边缓存了，个人觉得如果这时候发生gc可能直接回收老数组,但也是得进行最后的元素拷贝了
                    oldTab[j] = null;
                    if (e.next == null)
                    // 老数组中的元素没有进行拉链法，直接放入新数组中
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                    // 老数组的元素是个红黑树
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                    // 老数组中的元素是链表
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            // 赋值链表头结点的next节点
                            next = e.next;
                            //当老链表中的哈希冲突，有的节点可能跑到下面的else里了，当前位置空了，就可以将一个节点赋值过来，还不改变索引位置
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            // 将老的链表分成两个小的链表，挂在老索引位移了新索引长度的位置
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            //完善链表尾部
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            // 新索引位置
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }