An example of composing CompletableFutures where the underlying future can fail.

HelloCompletableFuturesWithFailures.java

import java.time.Duration;
import java.time.temporal.ChronoUnit;
import java.util.concurrent.CompletableFuture;

import static java.lang.System.err;
import static java.lang.System.out;

public class HelloCompletableFuturesWithFailures {
    public static void main(String[] args) {
        CompletableFuture<String> future = new Async().thinkOfSomething();
        future.thenApply(String::toUpperCase)
                .handle((ok, ex) -> {
                    if (ok != null) {
                        out.println(ok);
                        return ok;
                    } else {
                        err.println("Got an exception: " + ex.getMessage());
                        return null;
                    }
                });
    }
}

class Async {
    CompletableFuture<String> thinkOfSomething() {
        CompletableFuture<String> future = new CompletableFuture<>();
        new Thread(new Task(future)).start();
        return future;
    }

    private class Task implements Runnable {
        private final CompletableFuture<String> future;

        Task(CompletableFuture<String> future) {
            this.future = future;
        }

        public void run() {
            try {
                Thread.sleep(Duration.of(3, ChronoUnit.SECONDS).toMillis());
                future.completeExceptionally(new RuntimeException("The future is bleak..."));
            } catch (InterruptedException e) {
                future.completeExceptionally(e);
            }

        }
    }
}