docs: @SuppressWarnings("unchecked")
      public Iterator<K> keys()
      {
          int size = _size;
  
          if (size == 0)
          {
              return null;
          }
          ArrayList<K> keyList = new ArrayList<K>();
          int i = (size - 1) * 2;
          while (i >= 0)
          {
              keyList.add((K) _array[i]);
              i = i - 2;
          }
          return keyList.iterator();
      }