Streams: Collectors.toSet gives no ordering guarantee, unlike toList

toList preserves encounter order in its result, but toSet is documented to make no promise about iteration order, so only the size or membership should be relied on, not the order.

Code

List<Integer> nums = List.of(1, 2, 3, 4, 5);
List<Integer> asList = nums.stream().collect(Collectors.toList());
System.out.println("toList preserves order: " + asList);
Set<Integer> asSet = nums.stream().collect(Collectors.toSet());
System.out.println("toSet size (order not guaranteed): " + asSet.size());

Output

toList preserves order: [1, 2, 3, 4, 5]
toSet size (order not guaranteed): 5