code: public void clear() {
          Node<K,V>[] tab;
          modCount++;
          if ((tab = table) != null && size > 0) {
              size = 0;
              for (int i = 0; i < tab.length; ++i)
                  tab[i] = null;
          }
      }
docs: Removes all of the mappings from this map.
  The map will be empty after this call returns.