what is Future<void>?

I have following code snippet.

import io.vertx.core.Future;
public void start(Future<void> fut){

  /*
  some code
  */
  fut.complete()
}

Why is the **Future** used here?

Future<Void> is a future result of an execution that returns no value.

That would be typically the result of invoking the run method of a Runnable.

The normal void call looks like (see r.run()):

Runnable r = () -> System.out.println("running");
r.run();

When such a call is done asynchronously, such as via an executor service or a completable future, it turns into a future:

Future<Void> future = CompletableService.runAsync(r);

It's just a future of an execution that returns no result. This "future" contains information about the execution, even though it has no "return" value (such as what Future<Object> would have).

You can get information about the asynchronous execution. Some examples of information it hods:

boolean finished = future.isDone(); //Check if the async execution has completed
future.get(10L, TimeUnit.SECONDS); //Wait for completion with timeout
future.cancel(true); //cancel the execution

java.lang.Void is a reference type for void (a placeholder that doesn't get instantiated). So you can look at Future<Void> the same way you look at Future<Object>, just keeping in mind what you know about void not returning any value.

You can read more about these types here:

• Future
• Void
• CompletableFuture