Test for next method

next.java

    @Test
    public void correctValuesForConcurrentNext() throws ExecutionException, InterruptedException {

        for (int i = 0; i < 5000; i++) {
            final Iterator<String> syncIterator = new SyncIterator<>(
                new ListOf<>("a", "b")
                    .iterator());
            final CountDownLatch startSignal = new CountDownLatch(1);
            final ExecutorService executors = Executors.newFixedThreadPool(2);
            final Future<String> letter = executors.submit(() -> {
                startSignal.await();
                return syncIterator.next();
            });

            final Future<String> letter2 = executors.submit(() -> {
                startSignal.await();
                return syncIterator.next();
            });

            startSignal.countDown();

            MatcherAssert.assertThat(
                "Two threads have read the same value (race condition)",
                letter.get(),
                Matchers.not(letter2.get())
            );
        }
    }