What is the advantage of forking a stream over just using multiple streams?

Question

I am reading java 8 in action and the author references this link: http://mail.openjdk.java.net/pipermail/lambda-dev/2013-November/011516.html

and writes his own stream forker that looks like this:

import java.util.*;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class Main {

    public static void main(String... args) {
        List<Person> people = Arrays.asList(new Person(23, "Paul"), new Person(24, "Nastya"), new Person(30, "Unknown"));
        StreamForker<Person> forker = new StreamForker<>(people.stream())
                .fork("All names", s -> s.map(Person::getName).collect(Collectors.joining(", ")))
                .fork("Age stats", s -> s.collect(Collectors.summarizingInt(Person::getAge)))
                .fork("Oldest", s -> s.reduce((p1, p2) -> p1.getAge() > p2.getAge() ? p1 : p2).get());
        Results results = forker.getResults();

        String allNames = results.get("All names");
        IntSummaryStatistics stats = results.get("Age stats");
        Person oldest = results.get("Oldest");

        System.out.println(allNames);
        System.out.println(stats);
        System.out.println(oldest);
    }

    interface Results {
        <R> R get(Object key);
    }

    static class StreamForker<T> {
        private final Stream<T> stream;
        private final Map<Object, Function<Stream<T>, ?>> forks = new HashMap<>();

        public StreamForker(Stream<T> stream) {
            this.stream = stream;
        }

        public StreamForker<T> fork(Object key, Function<Stream<T>, ?> f) {
            forks.put(key, f);
            return this;
        }

        public Results getResults() {
            ForkingStreamConsumer<T> consumer = build();
            try {
                stream.sequential().forEach(consumer);
            } finally {
                consumer.finish();
            }
            return consumer;
        }

        private ForkingStreamConsumer<T> build() {
            List<BlockingQueue<T>> queues = new ArrayList<>();

            Map<Object, Future<?>> actions =
                    forks.entrySet().stream().reduce(
                            new HashMap<>(),
                            (map, e) -> {
                                map.put(e.getKey(),
                                        getOperationResult(queues, e.getValue()));
                                return map;
                            },
                            (m1, m2) -> {
                                m1.putAll(m2);
                                return m1;
                            }
                    );
            return new ForkingStreamConsumer<>(queues, actions);
        }

        private Future<?> getOperationResult(List<BlockingQueue<T>> queues,
                                             Function<Stream<T>, ?> f) {
            BlockingQueue<T> queue = new LinkedBlockingQueue<>();
            queues.add(queue);
            Spliterator<T> spliterator = new BlockingQueueSpliterator<>(queue);
            Stream<T> source = StreamSupport.stream(spliterator, false);
            return CompletableFuture.supplyAsync(() -> f.apply(source));
        }
    }

    static class ForkingStreamConsumer<T> implements Results, Consumer<T> {
        static final Object END_OF_STREAM = new Object();
        private final List<BlockingQueue<T>> queues;
        private final Map<Object, Future<?>> actions;

        ForkingStreamConsumer(List<BlockingQueue<T>> queues,
                              Map<Object, Future<?>> actions) {
            this.queues = queues;
            this.actions = actions;
        }

        public void finish() {
            accept((T) END_OF_STREAM);
        }

        @Override
        public <R> R get(Object key) {
            try {
                return ((Future<R>) actions.get(key)).get();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

        @Override
        public void accept(T t) {
            queues.forEach(q -> q.add(t));
        }
    }

    static class BlockingQueueSpliterator<T> implements Spliterator<T> {

        private final BlockingQueue<T> q;

        public BlockingQueueSpliterator(BlockingQueue<T> q) {
            this.q = q;
        }

        @Override
        public boolean tryAdvance(Consumer<? super T> action) {
            T t;
            while (true) {
                try {
                    t = q.take();
                    break;
                } catch (InterruptedException e) {
                }
            }

            if (t != ForkingStreamConsumer.END_OF_STREAM) {
                action.accept(t);
                return true;
            }
            return false;
        }

        @Override
        public Spliterator<T> trySplit() {
            return null;
        }

        @Override
        public long estimateSize() {
            return 0;
        }

        @Override
        public int characteristics() {
            return 0;
        }
    }

    static class Person {
        private int age;
        private String name;

        public Person(int age, String name) {
            this.age = age;
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return String.format("Age: %d, name: %s", age, name);
        }
    }
}

How the code written by the author works:

First, we create a StreamForker out of a stream. Then we fork 3 operations, saying what we want to do on that stream in parallel. In our case, our data model is the Person{age, name} class and we want to perform 3 actions:

• Get a string of all names
• Get age statistics
• Get the oldest person

we then call the forker.getResults() method, that applies a StreamForkerConsumer to the stream, spreading its elements into 3 blocking queues, which are then turned into 3 streams and processed in parallel.

My question is, does this approach have any advantage over just doing this:

Future<String> allNames2 =
                CompletableFuture.supplyAsync(() -> people.stream().map(Person::getName).collect(Collectors.joining(", ")));
Future<IntSummaryStatistics> stats2 =
                CompletableFuture.supplyAsync(() -> people.stream().collect(Collectors.summarizingInt(Person::getAge)));
Future<Person> oldest2 =
                CompletableFuture.supplyAsync(() -> people.stream().reduce((p1, p2) -> p1.getAge() > p2.getAge() ? p1 : p2).get());

?

Answer 1

For me this doesn't make much sense with an array list as stream source.

If the stream source is a big file that you process with

StreamForker<Person> forker = new StreamForker<>(
    java.nio.file.Files.lines(Paths.get("somepath"))
        .map(Person::new))
    .fork(...)

then it could prove beneficial since you would process the whole file only once, whereas with three seperat calls to Files.lines(...) you would read the file three times.