JCH
Java Community | Help. Code. Learn.•6mo ago
Thorinwasher

Merging completable future results - Good practices

Is this a bad idea? 
public static <T> CompletableFuture<List<T>> mergeFutures(List<CompletableFuture<T>> completableFutureList) {
        if (completableFutureList.isEmpty()) {
            return CompletableFuture.completedFuture(List.of());
        }
        T[] unwrappedFutures = (T[]) new Object[completableFutureList.size()];
        CompletableFuture<?>[] completableFutures = new CompletableFuture<?>[completableFutureList.size()];
        for (int i = 0; i < completableFutureList.size(); i++) {
            final int iFinal = i;
            completableFutures[i] = completableFutureList.get(i).thenAcceptAsync(t -> unwrappedFutures[iFinal] = t);
        }
        return CompletableFuture.allOf(completableFutures)
                .thenApplyAsync(ignored -> Arrays.asList(unwrappedFutures));
    }
public static <T> CompletableFuture<List<T>> mergeFutures(List<CompletableFuture<T>> completableFutureList) {
        if (completableFutureList.isEmpty()) {
            return CompletableFuture.completedFuture(List.of());
        }
        T[] unwrappedFutures = (T[]) new Object[completableFutureList.size()];
        CompletableFuture<?>[] completableFutures = new CompletableFuture<?>[completableFutureList.size()];
        for (int i = 0; i < completableFutureList.size(); i++) {
            final int iFinal = i;
            completableFutures[i] = completableFutureList.get(i).thenAcceptAsync(t -> unwrappedFutures[iFinal] = t);
        }
        return CompletableFuture.allOf(completableFutures)
                .thenApplyAsync(ignored -> Arrays.asList(unwrappedFutures));
    }
Will there be race conditions here? I tested 100000 times and found no issues, but is this theoretically a bad idea?
6 Replies
JavaBot
JavaBot•6mo ago
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0x150
0x150•6mo ago
what's your goal here? do you just want to have a future that resolves when all futures are done?
Thorinwasher
ThorinwasherOP•6mo ago
I want this to be able to merge multiple completable futures into the same future and retrieve all items of those completable futures. I can for example do array operations in that merged future.
0x150
0x150•6mo ago
@SafeVarargs
public static <T> CompletableFuture<List<? extends T>> merge(CompletableFuture<? extends T>... futures) {
    return CompletableFuture.allOf(futures).thenApply(unused -> Arrays.stream(futures).map(CompletableFuture::join).toList());
}
@SafeVarargs
public static <T> CompletableFuture<List<? extends T>> merge(CompletableFuture<? extends T>... futures) {
    return CompletableFuture.allOf(futures).thenApply(unused -> Arrays.stream(futures).map(CompletableFuture::join).toList());
}
Thorinwasher
ThorinwasherOP•6mo ago
Damn that looks a lot simpler 😅 Also looks more thread safe
JavaBot
JavaBot•6mo ago
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