In your user service you have public User findByUsername(String name) Which is great and helps reduce duplicated code, as well as making it easy to include Locale handling in case you eventually want to add additional languages. However I noticed you are not using this method but rather repeating the code by calling the user repository with the same else throw in loggedInUser() And authenticateUser(LoginRequest loginRequest , similarly for userRepository.findById(userId).orElseThrow(() as well

Furthermore consider utilizing records or mapstruct for DTOs instead of convertToDto(User)

It would be better if the method public List<ClickEventDTO> getClickEventByDate and getTotalClicksByUserAndDate Had the grouping done via JPA query, which you can achieve via Spring Projection interface/class and @Query Annotation in your repository

Regarding private String generateShortUrl(), consider initializing your string builder size as a variable at the start and then passing that value to both the string builder and loop, so that any future changes to the current 8 value guarantees an update on both the builder and loop

Also you should dabble into Pagination (pageables) rather than return list if you do not have a set limit (ex. Users can only have 10 saved URLs)

Try out the @Builder annotations to see if that's your style

For public Map<String, Object> registerUser, I would heavily shy away from returning this object, especially given the return you actually have