code: final void treeifyBin(Node<K,V>[] tab, int hash) {
          int n, index; Node<K,V> e;
          if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
              resize();
          else if ((e = tab[index = (n - 1) & hash]) != null) {
              TreeNode<K,V> hd = null, tl = null;
              do {
                  TreeNode<K,V> p = replacementTreeNode(e, null);
                  if (tl == null)
                      hd = p;
                  else {
                      p.prev = tl;
                      tl.next = p;
                  }
                  tl = p;
              } while ((e = e.next) != null);
              if ((tab[index] = hd) != null)
                  hd.treeify(tab);
          }
      }
docs: Replaces all linked nodes in bin at index for given hash unless
  table is too small, in which case resizes instead.