java - 如何转换收藏> 到 CompletableFuture> 无需调用加入或获取？

Question

给定一组期货

final Collection<CompletableFuture<X>> futures = ...;
final CompletableFuture<Collection<X>> joined = ???

如何在不调用的情况下为所有值组合在一起的集合创造未来join？

标签： javaconcurrencycompletable-future

Answer 1

解决方案是

1. 转换Collection<CompletableFuture<X>>为Collection<CompletableFuture<Collection<X>>>
2. CompletableFuture.completedFuture(Collections.emptyList()) 使用作为初始值减少此集合。

final CompletableFuture<Collection<X>> joined = 
                futures
                .stream()
                .map(f -> f.thenApply(value -> (Collection<X>)Collections.singletonList(value)))
                .reduce(
                        CompletableFuture.completedFuture(Collections.emptyList()), 
                        (f, g) -> 
                            f.thenCompose(
                                xs -> g.thenApply(
                                     ys -> Stream
                                               .of(xs, ys)
                                               .flatMap(Collection::stream)
                                               .collect(Collectors.toList()))
                                )
                            );