@Override
	public List<User> getUsersByLastName(String lastName) {
		if(lastName == null){
			throw new NullPointerException("Last name can not be null.");
		}
		List<User> result = new ArrayList<User>();
		
		List<User> allUsers = this.getUsers();

		for (int i = 0; i < allUsers.size(); i++) {
			if (allUsers.get(i).getLastName() == lastName) {
				result.add(allUsers.get(i));
			}
		}
		return result;
		
	}