assignment-mistake.java

@Resource(name = "userCache")
private Map<User, Id> userCache;

private UserStats getUser(String userID) {
    String login = userCache
            .entrySet()
            .stream()
            .filter(u -> userID.equals(u.getValue().getId()))
            .map(u -> u.getKey().getLogin())
            .collect(Collectors.joining());

    return new UserStats().userID(userID).login(login);
}

I don't remember context, nor assignment anymore - it's almost 3 years. But the point is to switch keys and values, so use id as a key and user as a value.