futures.add(
          executor.submit(
              new Callable<@Nullable Void>() {
                @Override
                public @Nullable Void call() throws Exception {
                  barrier.await();
                  Integer first = network.nodes().iterator().next();
                  for (Integer node : network.nodes()) {
                    Set<Integer> unused = network.successors(node);
                  }

      futures.add(
          executor.submit(
              new Callable<@Nullable Void>() {
                @Override
                public @Nullable Void call() throws Exception {
                  barrier.await();
                  Integer first = network.nodes().iterator().next();
                  for (Integer node : network.nodes()) {
                    Set<Integer> unused = network.successors(node);
                  }