Best way to find non intersecting elements of array

Question

I have 2 arrays of User Objects. 1 array contains list of all users and 2nd array contains list of all logged in Users. Which is the best way to find non-logged in users

Answer 1

There are two cases:

1. If you do not have equals and hash code methods implemented in User:

then the simplest way would be to get all UserIdentifier, like databaseId or username. Persist it in a hashed structure, for fast search. Then go through all Users and accept all not contained in this collection.

Create a set of logged user Identifiers:

 Set<Long> loggedUserIdentifiers = loggedUsers.stream()
                                   .map(User::getId) // I have chosen Id
                                   .collect(toSet());

Now you can find all not-logged users:

 List<User> unloggedUsers = allUsers.stream()
                            .filter( user -> !loggedUserIdentifiers.contains(user.getId()))
                            .collect(toSet());

2. If you have equals and hash code implemented, then you could simply remove all logged users from a copy of all Users collection. Just remember to create a copy first, otherwise You will end with only logged and not-logged Users.

   Set<User> allUserCopy = new HashSet<>(allUsers); // works on lists also
   allUserCopy.removeAll(loggedUsers);
   Set<User> notLoggedUsers = Set.copyOf(allUserCopy);
   

Now you will have a collection with only not logged users.