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.