Is it possible to add multiple actions to a CompletableFuture with whenComplete(…)?

Question

The class CompletableFuture allows to add an action which is called when the future completes after calling complete(...) .

Can I use whenComplete(...) to add multiple BiConsumer actions for executing when completing the future and are all of them executed when the complete(...) method is called?

Answer 1

Yes, all BiConsumer actions are added and they are executed in reverse addition order when calling complete(...).

An example to demonstrate this might look like this:

public class Application {
    public static void main(String[] args) {
        System.out.println("My tests ...");
        CompletableFuture<String> futureString = new CompletableFuture<String>();
        futureString.whenComplete((s,e)->System.out.println("one " + s));
        futureString.whenComplete((s,e)->System.out.println("two " + s));
        futureString.whenComplete((s,e)->System.out.println("three " + s));
        System.out.println("do something else; "+ futureString.isDone());
        futureString.complete("step(s)");
        System.out.println("Done " + futureString.isDone());
    }
}

When running this program, the printed result looks this:

My tests ...
do something else; false 
three step(s)
two step(s)
one step(s)
Done true