Create a generic array with toArray() and Array.newInstance

GenericArray.java

Map<String, CompletableFuture<JsonNode>> completableFutures = 
    this.createCompletableFutures(myRequest);

Collection<CompletableFuture<JsonNode>> values = completableFutures.values();

CompletableFuture<JsonNode>[] completableFuturesArray = 
values.toArray((CompletableFuture<JsonNode>[])Array.newInstance(CompletableFuture.class,completableFutures.size()));
CompletableFuture<Void> combinedFutures = CompletableFuture.allOf(completableFuturesArray);

combinedFutures.get(100, TimeUnit.MILLISECONDS);