CompletableFutureExample.java

import java.util.Optional;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionException;
import java.util.concurrent.CancellationException;


public class CompletableFutureExample {

  private static CompletableFuture<Optional<String>> applyTaskAsync(String s) {
    return CompletableFuture.completedFuture(s)
      .thenApplyAsync(Main::taskA)
      .thenApplyAsync(Main::taskB)
      .handle((v,ex) -> Optional.ofNullable(v));
  }
  
  public static void main(String[] args) {
    try {
      CompletableFuture<Optional<String>> f = applyTaskAsync("xyz");

      // f.cancel(true or false):
      //   cancel Future object returned from handle() method,
      //   so taskA and taskB will continue their tasks.

      System.out.println(Thread.currentThread().getId()+"join");
      Optional<String> result = f.join();
      System.out.println(Thread.currentThread().getId()+"result=" + result.orElse("(fail)"));
    } catch (CompletionException ex) {
      System.out.println("catch:" + ex.getCause());
    } catch (CancellationException ex) {
      System.out.println("cancelled");
    }
  }

  private static String taskA(String s) {
    sleep(100);
    System.out.println(Thread.currentThread().getId() + "TaskA");
    if (s == null || s.isEmpty())
      throw new RuntimeException("FAIL");
    return "{" + s + "}";
  }
  private static String taskB(String s) {
    sleep(100);
    System.out.println(Thread.currentThread().getId() + "TaskB");
    return "(" + s + ")";
  }

  private static void sleep(long n) {
    try { Thread.sleep(n); } catch (InterruptedException ex) {}
  }
}