code: private static CompletionStage<?> toCompletionStage(Object obj) {
          if (obj instanceof CompletionStage) {
              return (CompletionStage<?>) obj;
          }
          return CompletableFuture.completedFuture(obj);
      }
docs: Wraps the specified {@code obj} with {@link CompletableFuture} if it is not an instance of
  {@link CompletionStage}.