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 = ?

`SUBSCRIBER_ALLOWED_NUMBER_CD = Stream.concat(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD.stream(),INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD.stream()) .collect(Collectors.toSet());` — Hadi J, Aug 03 '20 at 10:10
*Why* is this getting downvoted? Just look at the weak answers - this trivial problem has no neat JDK-only solution. — drekbour, Aug 03 '20 at 10:19
Does this answer your question? [Is there a better way to combine two string sets in java?](https://stackoverflow.com/a/30139228/4207306) — Eklavya, Aug 03 '20 at 10:40

score 4 · Answer 1 · answered Aug 03 '20 at 10:12

Stream#concat is useful too

Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD  =  
         Stream.concat(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD.stream(),
                       INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD.stream())
                .collect(Collectors.toSet());

Youcef LAIDANI · Answer 2 · 2020-08-03T10:43:22.237

2

If you want to use streams:

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

Or, in your case, and as mentioned in comment, you can use Collectors.toUnmodifiableSet() :

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

edited Aug 03 '20 at 10:43

answered Aug 03 '20 at 10:11

Youcef LAIDANI

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I like this one the most but improve with `Collectors.toImmutableSet()` (introduced in Java 10) – drekbour Aug 03 '20 at 10:16
1

I meant `Collectors.toUnmodifiableSet()` – drekbour Aug 03 '20 at 10:24
Thank you @drekbour, this is interesting also :) – Youcef LAIDANI Aug 03 '20 at 10:43

score 2 · Answer 3 · answered Aug 03 '20 at 10:22

Just to point out that Guava still handles this case more more readably (and probably much more performant) than the JDK-only solutions:

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = ImmutableSet.builder()
    .addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD)
    .addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
    .build();

Tim Biegeleisen · Answer 4 · 2020-08-03T10:07:22.757

1

Any reason why you can't create a third set and just add both of the two final sets?

Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = new HashSet<>();
SUBSCRIBER_ALLOWED_NUMBER_CD.addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD);
SUBSCRIBER_ALLOWED_NUMBER_CD.addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD);

The first two sets' being final just means that you can't modify those sets. It does not prevent you from reading the sets into another new collection.

edited Aug 03 '20 at 10:07

answered Aug 03 '20 at 10:05

Tim Biegeleisen

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`Set SUBSCRIBER_ALLOWED_NUMBER_CD = new HashSet<>(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD);SUBSCRIBER_ALLOWED_NUMBER_CD.addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD);` – Hadi J Aug 03 '20 at 10:06

score 1 · Accepted Answer · answered Aug 03 '20 at 10:11

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.

Since this is joining two sets created via `Set.of(…)`, the OP might want to use the collector `Collectors.toUnmodifiableSet()` instead of `Collectors.toSet()`, to get a set with the same semantics (wrapping it in `Collections.unmodifiableSet(…)` becomes unnecessary then). Likewise, the alternative might use `Set.copyOf(…)` instead of `Collections.unmodifiableSet(…)`. It may cause a little higher initialization time, which is compensated with potentially higher performance in the subsequent runtime. — Holger, Aug 03 '20 at 11:39

How to merge two unmodifiable Sets in Java?

5 Answers5