Callable.java

// new single thread created and managed by executor service
ExecutorService executorService = Executors.newSingleThreadExecutor(); 

Callable<String> hello = () -> {
    Thread.sleep(2000L);
    return "Hello";
};

Future<String> submit = executorService.submit(hello); // callable object executed and falls asleep for 2 sec
System.out.println(submit.isDone()); // main thread keep running - 1
System.out.println("Started"); // - 2

submit.get(); // To get the result, it joins the main thread - 3

System.out.println(submit.isDone()); // once done, the main thread operates the remaining tasks - 4
System.out.println("End!!");
executorService.shutdown(); // executors should be shut down manually, otherwise it keeps waiting for another task forever,