public synchronized void removeBooks(Set<Integer> isbnSet)
			throws BookStoreException {

		if (isbnSet == null) {
			throw new BookStoreException(BookStoreConstants.NULL_INPUT);
		}
		for (Integer ISBN : isbnSet) {
			if (BookStoreUtility.isInvalidISBN(ISBN))
				throw new BookStoreException(BookStoreConstants.ISBN + ISBN
						+ BookStoreConstants.INVALID);
			if (!bookMap.containsKey(ISBN))
				throw new BookStoreException(BookStoreConstants.ISBN + ISBN
						+ BookStoreConstants.NOT_AVAILABLE);
		}

		for (int isbn : isbnSet) {
			bookMap.remove(isbn);
		}
	}