public synchronized List<Product> addPreferencesClient(Client client,
			Product product) {
		if (client != null && product!=null && listOfAllPreferences.get(client) != null) {//if there is already a list
			
			List<Product> listAux = listOfAllPreferences.get(client);
			listAux.add(product);
			listOfAllPreferences.put(client, listAux);
			logger.info("Client preferences: " + client.getName()
					+ " added or updated successfully");
			return listOfAllPreferences.get(client);
		}else if(client != null && product!=null && listOfAllPreferences.get(client) == null){//if the list does not exist
			List<Product> listAux = new LinkedList<Product>();
			listAux.add(product);
			listOfAllPreferences.put(client, listAux);
			return listOfAllPreferences.get(client);
			}
		return null;			
		}