How would you make a java.util.stream.Collector that collects to com.google.common.collect.ImmutableSet?

Question

That seems too tricky for me since ImmutableSet instances are only built with ImmutableSet.Builder instances, which don't implement Collection so you can't just use Collectors.toCollection(ImmutableSet::new) or Collectors.toCollection(ImmutableSet.Builder::new).

Answer 1

This is built into guava now,

ImmutableSet#toImmutableSet

Use like,

something.stream().collect(ImmutableSet.toImmutableSet())

Answer 2

In fact, 3 months after :-), instead of defining a whole class for this, you can use Collector.ofand wrap it in a simple utility method:

public static <T> Collector<T, Builder<T>, ImmutableSet<T>> immutableSetCollector() {
    return Collector.of(Builder<T>::new, Builder<T>::add, (s, r) -> s.addAll(r.build()), Builder<T>::build);
}

and then:

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSet.Builder;
....

ImmutableSet<Point2D> set = 
    Stream.of(new Point2D(1, 2), ...).collect(immutableSetCollector());