How can I collect a Java 8 stream into a Guava ImmutableCollection?

I would like to do the following:

List<Integer> list = IntStream.range(0, 7).collect(Collectors.toList());

but in a way that the resulting list is an implementation of Guava's ImmutableList.

I know I could do

List<Integer> list = IntStream.range(0, 7).collect(Collectors.toList());
List<Integer> immutableList = ImmutableList.copyOf(list);

but I would like to collect to it directly. I've tried

List<Integer> list = IntStream.range(0, 7)
    .collect(Collectors.toCollection(ImmutableList::of));

but it threw an exception:

java.lang.UnsupportedOperationException at com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:96)

Solution 1:

The toImmutableList() method in the accepted answer of Alexis is now included in Guava 21 and can be used as:

ImmutableList<Integer> list = IntStream.range(0, 7)
    .boxed()
    .collect(ImmutableList.toImmutableList());

Edit: Removed @Beta from ImmutableList.toImmutableList along with other frequently used APIs in Release 27.1 (6242bdd).

Solution 2:

This is where the collectingAndThen collector is useful:

List<Integer> list = IntStream.range(0, 7).boxed()
                .collect(collectingAndThen(toList(), ImmutableList::copyOf));

It applies the transformation to the List you just built; resulting in an ImmutableList.

Or you could directly collect into the Builder and call build() at the end:

List<Integer> list = IntStream.range(0, 7)
                .collect(Builder<Integer>::new, Builder<Integer>::add, (builder1, builder2) -> builder1.addAll(builder2.build()))
                .build();

If this option is a bit-verbose to you and you want to use it in many places, you can create your own collector:

class ImmutableListCollector<T> implements Collector<T, Builder<T>, ImmutableList<T>> {
    @Override
    public Supplier<Builder<T>> supplier() {
        return Builder::new;
    }

    @Override
    public BiConsumer<Builder<T>, T> accumulator() {
        return (b, e) -> b.add(e);
    }

    @Override
    public BinaryOperator<Builder<T>> combiner() {
        return (b1, b2) -> b1.addAll(b2.build());
    }

    @Override
    public Function<Builder<T>, ImmutableList<T>> finisher() {
        return Builder::build;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return ImmutableSet.of();
    }
}

and then:

List<Integer> list = IntStream.range(0, 7)
                              .boxed()
                              .collect(new ImmutableListCollector<>());

Just in case the link disappears in the comments; my second approach could be defined in a static utility method that simply uses Collector.of. It's simpler than creating your own Collector class.

public static <T> Collector<T, Builder<T>, ImmutableList<T>> toImmutableList() {
    return Collector.of(Builder<T>::new, Builder<T>::add, (l, r) -> l.addAll(r.build()), Builder<T>::build);
}

and the usage:

 List<Integer> list = IntStream.range(0, 7)
                               .boxed()
                               .collect(toImmutableList());