A Collector that uses an intermediate Stream.Builder to collect to a new stream, ensuring that the source stream has been fully consumed.

StreamCollectors.java

package net.biesemeyer.streams;

import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Stream;

public class StreamCollectors {
    private StreamCollectors() { }

    public static <T> Collector<T, Stream.Builder<T>, Stream<T>> toStream() {
        return new ToStreamCollector<>();
    }

    static class ToStreamCollector<T> implements Collector<T, Stream.Builder<T>, Stream<T>> {

        @Override
        public Supplier<Stream.Builder<T>> supplier() {
            return Stream::builder;
        }

        @Override
        public BiConsumer<Stream.Builder<T>, T> accumulator() {
            return Stream.Builder::accept;
        }

        @Override
        public BinaryOperator<Stream.Builder<T>> combiner() {
            return (left,right) -> {
                right.build().forEach(left);
                return left;
            };
        }

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

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