How to merge two unmodifiable Sets in Java?

Question

How to merge two unmodifiable static final sets?

public static final Set<Long> ORG_SUBSCRIBER_ALLOWED_NUMBER_CD = Set.of(COMPANY_GST, GOVERNMENT_BODY_GST);

public static final Set<Long> INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD = Set.of(BUSINESS_PAN, INDIVIDUAL_PAN);

I want to combine the above static final sets into one set (one-statement initialization), because it's a Class variable

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = ?

Answer 1

A one-statement initialization, if preferred:

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = 
     Collections.unmodifiableSet(
        Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, 
                  INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
              .flatMap(Set::stream)
              .collect(Collectors.toSet()));

Or, perhaps more readable:

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD;
static {
    Set<Long> all = new HashSet<>();
    all.addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD);
    all.addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD);
    
    SUBSCRIBER_ALLOWED_NUMBER_CD = Collections.unmodifiableSet(all);
}

Ignore the Collections.unmodifiableSet call if the third set is not expected to be unmodifiable.