User problem: Fetch the order of a certain customer and print it.

Typically, a callback that doesn't return a result acts as a terminal action of an asynchronous pipeline.

This behavior can be obtained via the thenAccept() method. It takes  Consumer<T> and returns CompletableFuture<Void>. This method can process and transform the result of CompletableFuture, but doesn't return a result. So, it can take an order, which is the result of  CompletableFuture, and print it as shown in the following snippet of code:

public static void fetchAndPrintOrder() {

  CompletableFuture<String> cfFetchOrder 
      = CompletableFuture.supplyAsync(() -> {

    logger.info(() -> "Fetch order by: "
      + Thread.currentThread().getName());
    Thread.sleep(500);

    return "Order #1024";
  });

  CompletableFuture<Void> cfPrintOrder = cfFetchOrder.thenAccept(
    o -> logger.info(() -> "Printing order " + o +
      " by: " + Thread.currentThread().getName()));

  cfPrintOrder.get();
  logger.info("Order was fetched and printed 
");
}

Or, it can be more compact as follows:

public static void fetchAndPrintOrder() {

  CompletableFuture<Void> cfFetchAndPrintOrder 
      = CompletableFuture.supplyAsync(() -> {

    logger.info(() -> "Fetch order by: "
      + Thread.currentThread().getName());
    Thread.sleep(500);

    return "Order #1024";
  }).thenAccept(
      o -> logger.info(() -> "Printing order " + o + " by: "
        + Thread.currentThread().getName()));

  cfFetchAndPrintOrder.get();
  logger.info("Order was fetched and printed 
");
}