Execute a for loop in parallel using CompletableFuture in Java and log the execution

mohitmayank :

I have a for loop which I am trying to parallelize using CompletableFuture.

for (int i = 0; i < 10000; i++) {
    doSomething();
    doSomethingElse();
}

What I have till now is:

for (int i = 0; i < 10000; i++) {
    CompletableFuture.runAsync(() -> doSomething());
    CompletableFuture.runAsync(() -> doSomethingElse());
}

I guess this serves the purpose but there is a requirement to print log just before the start and end of all the processing. If I do this:

log("Started doing things");
for (int i = 0; i < 10000; i++) {
    CompletableFuture.runAsync(() -> doSomething());
    CompletableFuture.runAsync(() -> doSomethingElse());
}
log("Ended doing things");

Does this guarantee that the second log statement will be printed once all the for loop is over since that is executing in a separate thread? If not, is there a way to do this without blocking the main thread?

kairaedsch :

You have to collect all CompletableFutures and wait for their complete:

log("Started doing things");
List<CompletableFuture> futures = new ArrayList();
for (int i = 0; i < 10000; i++) {
    futures.add(CompletableFuture.runAsync(() -> doSomething()));
    futures.add(CompletableFuture.runAsync(() -> doSomethingElse()));
}
CompletableFuture.allOf(futures.toArray(new CompletableFuture[0]))
                 .thenRunAsync(() -> log("Ended doing things"));

Or when you use the ExecutorService:

CompletableFuture.runAsync(() -> {
    try {
        executorService.invokeAll(tasks);
    } catch (InterruptedException) {
        e.printStackTrace();
    }
    log("Ended doing things");
});