The problem In Java, imagine you have a list of User objects, each encapsulates the user’s id, first name, last name and age. Then you want to call a web service UserService.deleteUsersByIds(List<Integer> userIds) to delete the users from your data store. It doesn’t sound too hard, does it? All you need to do is to transform you List<User> to List<Integer>. So you go ahead and write the following code: