class UserService {

    Set<User> users = new HashSet<>();

    {
        User user = new User();
        user.username = "adrianvdh";
        user.password = "hello123";
        users.add(user);
    }

    public User authenicate(String username, String password) {
        User foundUser = null
        for (User userInCollection : users) {
            if (userInCollection.username.equals(username) {
                foundUser = userInCollection;
                break;
            }
        }

        if(foundUser == null)
            throw new RuntimeException(String.format("User with username %s could not be found!", username));

        return foundUser;
    }
}