@Override protected void deleteBehind(final int n,final Iterator<byte[]> keys){
  final Lock lock=writeLock();
  lock.lock();
  try {
    super.deleteBehind(n,keys);
  }
  finally {
    unlock(lock);
  }
}

@ Override protected void deleteBehind ( final int n , final Iterator < byte [ ] > keys ) { final Lock lock = writeLock ( ) ; lock . lock ( ) ; try { super . deleteBehind ( n , keys ) ; } finally { unlock ( lock ) ; } }
extended to acquire the exclusive write lock .
