ExecutorService executorService = Executors.newSingleThreadExecutor();

        Future<String> future = executorService.submit(() -> {
            Thread.sleep(2000);
            return "Hello from Callable";
        });

        while(!future.isDone()) {
            System.out.println("Task is still not done...");
            Thread.sleep(200);
        }

        System.out.println("Task completed! Retrieving the result");
        String result = future.get();
        System.out.println(result);

        executorService.shutdown();

Runnable r = () -> {
            for (int i = 0; i < 5; i++) {
                System.out.println(i);
            }
        };

        Callable<String> callable = () -> {
            // Perform some computation
            new Thread(r).start();
            Thread.sleep(1000);
            return "Hello from Callable";
        };