code: private boolean containsNullValue() {
          Entry<K,V>[] tab = getTable();
          for (int i = tab.length; i-- > 0;)
              for (Entry<K,V> e = tab[i]; e != null; e = e.next)
                  if (e.value==null)
                      return true;
          return false;
      }
docs: Special-case code for containsValue with null argument