list.execute();

    // Verify that all of the runnables execute in a reasonable amount of time.
    assertTrue(countDownLatch.await(1L, SECONDS));
  }

  public void testExecute_idempotent() {
    AtomicInteger runCalled = new AtomicInteger();
    list.add(runCalled::getAndIncrement, directExecutor());
    list.execute();
    assertEquals(1, runCalled.get());
    list.execute();

    list.execute();

    // Verify that all of the runnables execute in a reasonable amount of time.
    assertTrue(countDownLatch.await(1L, SECONDS));
  }

  public void testExecute_idempotent() {
    AtomicInteger runCalled = new AtomicInteger();
    list.add(runCalled::getAndIncrement, directExecutor());
    list.execute();
    assertEquals(1, runCalled.get());
    list.execute();