Java 8 stream.sorted with comparator in sets

Question

I have a set for sorting (using comparators), and I don’t know which version to choose:

version 1:

public static void sort(Set<User> users) { users = users.stream() .sorted(sort_gender.thenComparing(sort_age)) .collect(Collectors.toCollection(LinkedHashSet::new)); } 

version 2:

 public static Set<User> sort(Set<User> users) { return users.stream() .sorted(sort_gender.thenComparing(sort_age)) .collect(Collectors.toCollection(LinkedHashSet::new)); } 

version 3:

 public static void sort(Set<User> users) { users.stream() .sorted(sort_gender.thenComparing(sort_age)) .collect(Collectors.toSet()); } 

version 4

 public static List<User> sort(Set<User> users){ List<User> list = new ArrayList<>(users); list.sort(sort_gender.thenComparing(sort_age)); return list; } 

All versions sort the set and return the sorted set. I know only the associated HashSet can keep order.

Which one to choose , I only want to sort the properties of the input properties and return it, so that version 1 is best for this case? (For all cases, I want the input user links to be the same as the output users.)

EDIT: I think I will choose version 4.

Answer 1

I would add a 4th method (if you're ok, to change this method to return a sorted Set )

  users.stream() .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing...))) 

I would return a SortedSet to make it clear to the caller that this is really sorted.

If you can not do:

 SortedSet<User> sorted = new TreeSet<>(Comparator.comparing...) sorted.addAll(users); 

Answer 2

Version one really does nothing. You change the link for the users parameter, but do not change the set, which is passed as an argument and returns nothing.

Version 2 is working correctly.

The third version tries to save a sorted set in a set that does not support order. This is effectively no different from returning the kit you are given. From JavaDoc for toSet :

There are no guarantees regarding the type, variability, serializability or thread safety of the return set.