}

  /**
   * Implementation for the EntryIterator, which is used to build Key and Value iterators.
   *
   * <p>Expiration is only checked on hasNext(), so as to ensure that a next() call never returns
   * null when hasNext() has already been called.
   */
  class EntryIterator implements Iterator<Entry<K, V>> {
    Iterator<Entry<K, Timestamped<V>>> iterator;
    Entry<K, Timestamped<V>> lastEntry;