What is CompletableFuture's equivalent of flatMap?

Question

I have this weird type CompletableFuture<CompletableFuture<byte[]>> but i want CompletableFuture<byte[]> . Is this possible?

public Future<byte[]> convert(byte[] htmlBytes) {
    PhantomPdfMessage htmlMessage = new PhantomPdfMessage();
    htmlMessage.setId(UUID.randomUUID());
    htmlMessage.setTimestamp(new Date());
    htmlMessage.setEncodedContent(Base64.getEncoder().encodeToString(htmlBytes));

    CompletableFuture<CompletableFuture<byte[]>> thenApply = CompletableFuture.supplyAsync(this::getPhantom, threadPool).thenApply(
        worker -> worker.convert(htmlMessage).thenApply(
            pdfMessage -> Base64.getDecoder().decode(pdfMessage.getEncodedContent())
        )
    );

}

Answer 1

There's a bug in its documentation, but the CompletableFuture#thenCompose family of methods is the equivalent of a flatMap . Its declaration should also give you some clues

public <U> CompletableFuture<U> thenCompose(Function<? super T,? extends CompletionStage<U>> fn)

thenCompose takes the result of the receiver CompletableFuture (call it 1 ) and passes it to the Function you provide, which must return its own CompletableFuture (call it 2 ). The CompletableFuture (call it 3 ) returned by thenCompose will be completed when 2 completes.

In your example

CompletableFuture<Worker> one = CompletableFuture.supplyAsync(this::getPhantom, threadPool);
CompletableFuture<PdfMessage /* whatever */> two = one.thenCompose(worker -> worker.convert(htmlMessage));
CompletableFuture<byte[]> result = two.thenApply(pdfMessage -> Base64.getDecoder().decode(pdfMessage.getEncodedContent()));