stream_zip.java

    public static void main(String[] args) {
        final Stream s1 = Stream.generate(new Supplier<String>() {
            private final AtomicInteger cnt = new AtomicInteger();
            @Override
            public String get() {
                sleep();
                return "A" + String.valueOf(cnt.incrementAndGet());
            }
        });

        final Stream s2 = Stream.generate(new Supplier<String>() {
            private final AtomicInteger cnt = new AtomicInteger();
            @Override
            public String get() {
                sleep();
                return "B" + String.valueOf(cnt.incrementAndGet());
            }
        });

        final Iterator i1 = s1.iterator();
        final Iterator i2 = s2.iterator();

        StreamSupport.stream(Spliterators.spliteratorUnknownSize(
                new Iterator<String>() {
                    @Override
                    public boolean hasNext() {
                        final long t1 = System.currentTimeMillis();

                        final CompletableFuture<Boolean> cf1 = CompletableFuture.supplyAsync(i1::hasNext);
                        final CompletableFuture<Boolean> cf2 = CompletableFuture.supplyAsync(i2::hasNext);
                        CompletableFuture.allOf(cf1, cf2).join();
                        final boolean has1 = cf1.join();
                        final boolean has2 = cf2.join();

                        final long t2 = System.currentTimeMillis();
                        System.out.println("Wait = " + (t2-t1) + " ms");

                        return has1 && has2;
                    }

                    @Override
                    public String next() {
                        final Object o1 = i1.next();
                        final Object o2 = i2.next();
                        return o1 + "_" + o2;
                    }
                }, Spliterator.ORDERED
        ), false).forEach(System.out::println);
    }

    private static void sleep() {
        try {
            Thread.sleep(1_000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }