A CompletableFuture can be explicitly completed using complete​(T value), completeAsync​(Supplier<? extends T> supplier), and completeAsync​(Supplier<? extends T> supplier, Executor executor). T is the value returned by get(). Here it is a method that creates CompletableFuture and returns it immediately. Another thread is responsible for executing some tax computations and completing the CompletableFuture with the corresponding result:

public static CompletableFuture<Integer> taxes() {

  CompletableFuture<Integer> completableFuture 
    = new CompletableFuture<>();

  new Thread(() -> {
    int result = new Random().nextInt(100);
    Thread.sleep(10);

    completableFuture.complete(result);
  }).start();

  return completableFuture;
}

And, let's call this method:

logger.info("Computing taxes ...");

CompletableFuture<Integer> cfTaxes = CustomerAsyncs.taxes();

while (!cfTaxes.isDone()) {
  logger.info("Still computing ...");
}

int result = cfTaxes.get();
logger.info(() -> "Result: " + result);

A possible output will be the following:

[14:09:40] [INFO ] Computing taxes ...
[14:09:40] [INFO ] Still computing ...
[14:09:40] [INFO ] Still computing ...
...
[14:09:40] [INFO ] Still computing ...
[14:09:40] [INFO ] Result: 17

If we already know the result of  CompletableFuture, then we can call completedFuture​(U value) as in the following example:

CompletableFuture<String> completableFuture 
  = CompletableFuture.completedFuture("How are you?");

String result = completableFuture.get();
logger.info(() -> "Result: " + result); // Result: How are you?