How to initialize HashSet values by construction?

Question

I need to create a Set with initial values.

Set<String> h = new HashSet<String>();
h.add("a");
h.add("b");

Is there a way to do this in one line of code? For instance, it's useful for a final static field.

Answer 1

With Java 9 you can do the following:

Set.of("a", "b");

and you'll get an immutable Set containing the elements. For details see the Oracle documentation of interface Set.

Answer 2

Using Java 10 (Unmodifiable Sets)

Set<String> strSet1 = Stream.of("A", "B", "C", "D")
         .collect(Collectors.toUnmodifiableSet());

Here the collector would actually return the unmodifiable set introduced in Java 9 as evident from the statement set -> (Set<T>)Set.of(set.toArray()) in the source code.

Using Java 9 (Unmodifiable Sets)

Set<String> strSet6 = Set.of("Apple", "Ball", "Cat", "Dog");

Using Java 8 (Modifiable Sets)

Using Stream in Java 8.

Set<String> strSet1 = Stream.of("A", "B", "C", "D")
         .collect(Collectors.toCollection(HashSet::new));

// stream from an array (String[] stringArray)
Set<String> strSet2 = Arrays.stream(stringArray)
         .collect(Collectors.toCollection(HashSet::new));

// stream from a list (List<String> stringList)
Set<String> strSet3 = stringList.stream()
         .collect(Collectors.toCollection(HashSet::new));

Using Java 8 (Unmodifiable Sets)

Using Collections.unmodifiableSet - We can use Collections.unmodifiableSet as:

Set<String> strSet4 = Collections.unmodifiableSet(strSet1);

But it looks slightly awkward and we can write our own collector like this:

class ImmutableCollector {
    public static <T> Collector<T, Set<T>, Set<T>> toImmutableSet() {
        return Collector.of(HashSet::new, Set::add, (l, r) -> {
            l.addAll(r);
            return l;
        }, Collections::unmodifiablSet);
    }
}

And then use it as:

Set<String> strSet4 = Stream.of("A", "B", "C", "D")
             .collect(ImmutableCollector.toImmutableSet());

Using Collectors.collectingAndThen - Another approach is to use the method Collectors.collectingAndThen which lets us perform additional finishing transformations:

import static java.util.stream.Collectors.*;
Set<String> strSet5 = Stream.of("A", "B", "C", "D").collect(collectingAndThen(
   toCollection(HashSet::new),Collections::unmodifiableSet));

If we only care about Set then we can also use Collectors.toSet() in place of Collectors.toCollection(HashSet::new).

---

One point to note is that the method Collections::unmodifiableSet returns an unmodifiable view of the specified set, as per doc. An unmodifiable view collection is a collection that is unmodifiable and is also a view onto a backing collection. Note that changes to the backing collection might still be possible, and if they occur, they are visible through the unmodifiable view. But the method Collectors.unmodifiableSet returns truly immutable set in Java 10.

Answer 3

(ugly) Double Brace Initialization without side effects:

Set<String> a = new HashSet<>(new HashSet<String>() {{
    add("1");
    add("2");
}})

But in some cases, if we mentioned that is a good smell to make final collections unmutable, it could be really useful:

final Set<String> a = Collections.unmodifiableSet(new HashSet<String>(){{
    add("1");
    add("2");
}})

Answer 4

Using Java 8 we can create HashSet as:

Stream.of("A", "B", "C", "D").collect(Collectors.toCollection(HashSet::new));

And if we want unmodifiable set we can create a utility method as :

public static <T, A extends Set<T>> Collector<T, A, Set<T>> toImmutableSet(Supplier<A> supplier) {
        return Collector.of(
                supplier,
                Set::add, (left, right) -> {
                    left.addAll(right);
                    return left;
                }, Collections::unmodifiableSet);
    }

This method can be used as :

 Stream.of("A", "B", "C", "D").collect(toImmutableSet(HashSet::new));

Answer 5

If you have only one initial value in set this would be enough:

Set<String> h = Collections.singleton("a");

Answer 6

Can use static block for initialization:

private static Set<Integer> codes1=
        new HashSet<Integer>(Arrays.asList(1, 2, 3, 4));

private static Set<Integer> codes2 =
        new HashSet<Integer>(Arrays.asList(5, 6, 7, 8));

private static Set<Integer> h = new HashSet<Integer>();

static{
    h.add(codes1);
    h.add(codes2);
}