Converts List<CompletableFuture<X>> to a CompletableFuture<List<X>>

CompletableFutureSequence.java

static<T> CompletableFuture<List<T>> sequence(List<CompletableFuture<T>> com) {
    return CompletableFuture.allOf(com.toArray(new CompletableFuture[com.size()]))
            .thenApply(v -> com.stream()
                .map(CompletableFuture::join)
                .collect(toList())
            );
}

This is not true non-blocking since join blocks and also might throw an exception which is not handled.
Have a look at this implementation: https://gist.github.com/diversit/add7524def84fe20f9cfd73db5728ff2

Just a clarification for diversit :
join will never block here, "CompletableFuture.allOf" waits for all futures to be done before calling the lambda passed to thenApply. If "join" throws an exception, the returned completableFuture will complete exceptionally, as with all methods returning completableFutures, the caller will have to decide how to handle the exceptional behavior.