code: @Override
      public void onComplete(Consumer<Try<T>> c) {
          Validator.requireNonNull(c, "Null is not a valid consumer");
          eventHandlers.add(new EventHandler(c));
          // invoke all new handlers in case this Future is already completed
          notifyHandlers();
      }
docs: /*
  (non-Javadoc)
  
  @see javascalautils.concurrent.Future#onComplete(java.util.function.Consumer)