public class StreamSetTest {
    public static void main(String[] args) {
        Set<String> set1 = new HashSet<>();
        Set<String> set2 = new HashSet<>();

        set1.addAll(Arrays.asList("aa","bb","cc"));
        set2.addAll(Arrays.asList("aa","bb","cc"));

        System.out.println("set1 before update: " + set1);
        updateSet1(set1);
        System.out.println("set1 before update: " + set1);

        System.out.println("set2 before update: " + set2);
        updateSet2(set2);
        System.out.println("set2 before update: " + set2);

    }

    private static void updateSet1(Set<String> set) {
        set.remove("bb");
        set.add("xx");
    }

    private static void updateSet2(Set<String> set) {
        set = set.stream().map(s-> s.replace("bb",  "xx")).collect(Collectors.toSet());
System.out.println("set inside method: " + set);
    }
}

set1 before update: [aa, bb, cc]
set1 before update: [aa, cc, xx]

set2 before update: [aa, bb, cc]
set inside method: [aa, xx, cc]
set2 before update: [aa, bb, cc]