Java源码解析HashMap的resize函数

  /**
   * Initializes or doubles table size. If null, allocates in
   * accord with initial capacity target held in field threshold.
   * Otherwise, because we are using power-of-two expansion, the
   * elements from each bin must either stay at same index, or move
   * with a power of two offset in the new table.
   * @return the table
   **/

  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;
    if (oldTab != null) {
      for (int j = 0; j < oldCap; ++j) {
        Node<K,V> e;
        if ((e = oldTab[j]) != null) {
          oldTab[j] = null;
          if (e.next == null)
            newTab[e.hash & (newCap - 1)] = e;
          else if (e instanceof TreeNode)
            ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
          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) {
                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;
  }