CompletableFutures.java

package com.pivovarit.allof;

import java.util.Collection;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.stream.Collectors;

import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toList;

public final class CompletableFutures {

    private CompletableFutures() {
    }

    public static <T> CompletableFuture<T> either(CompletableFuture<T> f1, CompletableFuture<T> f2) {
        CompletableFuture<T> result = new CompletableFuture<>();
        CompletableFuture.allOf(f1, f2).whenComplete((__, throwable) -> {
            if (f1.isCompletedExceptionally() && f2.isCompletedExceptionally()) {
                result.completeExceptionally(throwable);
            }
        });

        f1.thenAccept(result::complete);
        f2.thenAccept(result::complete);
        return result;
    }

    public static <T> CompletableFuture<List<T>> allOf(Collection<CompletableFuture<T>> futures) {
        return CompletableFuture.allOf(futures.toArray(new CompletableFuture[0]))
            .thenApply(__ -> futures.stream()
              .map(CompletableFuture::join)
              .collect(Collectors.toList()));
    }

    public static <T> CompletableFuture<List<T>> allOfOrException(Collection<CompletableFuture<T>> futures) {
        CompletableFuture<List<T>> result = allOf(futures);

        for (CompletableFuture<?> f : futures) {
            f.handle((__, ex) -> ex == null || result.completeExceptionally(ex));
        }

        return result;
    }

    public static <T> CompletableFuture<T> anyOf(List<CompletableFuture<T>> cfs) {
        return CompletableFuture.anyOf(cfs.toArray(new CompletableFuture[0])).thenApply(o -> (T) o);
    }
}