Make a `Callable` of my `Runnable` in Java

I have some tasks defined as Runnable objects. I want to invoke those tasks by using the invokeAll or invokeAny methods on ExecutorService.

The problem is that those methods take a collection of Callable objects (tasks that return a result) rather than a collection of Runnable objects (tasks that do not return a result).

Can I convert or wrap my Runnable objects to act as Callable objects without having to rewrite my task class?

Executors.callable

Easy to solve.

The Executors utility class offers a pair of callable methods that transform a Runnable into a Callable.

One method produces null as the result of the task. The other returns your specified object as the result.

Example code

Let's start with some Runnable objects.

Collection<Runnable> runnables =
        List.of(
                new MyRunnable(),
                new MyRunnable(),
                new MyRunnable()
        );

Convert each Runnable object into a Callable object that returns null as the result obtained via a Future object.

We can use streams to write a one-liner for converting Runnable objects to Callable objects.

Collection<Callable<Object>> tasks = runnables.stream().map(Executors::callable).toList();

If you are not comfortable with streams, use for loop.

Collection<Callable<Object>> tasks = new ArrayList<>();
for (Runnable runnable : runnables) {
  Callable<Object> callable = Executors.callable(runnable);
  tasks.add(callable);
}

Let's execute those Callable task objects.

Here we use try-with-resources syntax to automatically close the ExecutorService after it completes all submitted tasks.

try (ExecutorService executorService = Executors.newCachedThreadPool()) {
  try {
    List<Future<Object>> futures = executorService.invokeAll(tasks);
  } catch (InterruptedException e) {
    throw new RuntimeException(e);
  }
}

When run:

Running `run` method of `MyRunnable` id: bbc1b37a-dfc9-43da-8245-6a451b1d5517 at 2024-06-03T23:32:04.505167Z
Running `run` method of `MyRunnable` id: fec26a2f-3b11-413b-a6c3-3bc802874302 at 2024-06-03T23:32:04.505167Z
Running `run` method of `MyRunnable` id: d71fefe3-3387-4743-9c7d-20c338b84a33 at 2024-06-03T23:32:04.505163Z

IF we were to loop through the futures collection, and call get, we would receive a null from each of them.

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By the way, notice how output from System.out.println does not necessarily appear on the console in chronological order when called across threads.