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

Question

I would like to do the following:

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

but in a way that the resu

Answer 1

This is where the collectingAndThen collector is useful:

List 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 list = IntStream.range(0, 7)
                .collect(Builder::new, Builder::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 implements Collector, ImmutableList> {
    @Override
    public Supplier> supplier() {
        return Builder::new;
    }

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

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

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

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

and then:

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

and the usage:

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