private static ConcurrentHashMap<Long, UserNode> readUserMap(
			String directory) throws IOException, ClassNotFoundException {
		FileInputStream fin;
		try {
			fin = new FileInputStream(directory);
		} catch (FileNotFoundException e) {
			writeHashMap(directory, new ConcurrentHashMap<Long, UserNode>());
			fin = new FileInputStream(directory);
		}

		ObjectInputStream ois = new ObjectInputStream(fin);
		@SuppressWarnings("unchecked")
		ConcurrentHashMap<Long, UserNode> map = (ConcurrentHashMap<Long, UserNode>) ois
				.readObject();
		ois.close();

		return map;
	}