Java CompletableFuture applyToEither 方法对第一个完成的未来进行操作还是从两个中随机选择一个？

Question

From Oracle document: https://docs.oracle.com/javase/10/docs/api/java/util/concurrent/CompletionStage.html#applyToEither(java.util.concurrent.CompletionStage,java.util.function.Function)

applyToEither

<U> CompletionStage<U> applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn)

Returns a new CompletionStage that, when either this or the other given stage complete normally, is executed with the corresponding result as argument to the supplied function. See the CompletionStage documentation for rules covering exceptional completion.

It says it applies to "either this or the other", which I think is not clear. I wonder whether it chooses the first completed future or just randomly picks up one from the two?

Answer 1

It chooses the first completed future.

CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> task("Peter", 100)); // Generate "Peter", sleep 100ms
CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> task("John", 200)); // Generate "John", sleep 200ms

CompletableFuture<Void> future3 = future2.acceptEither(future1, (text) -> {
  Truth.assertThat(text).isEqualTo("Peter"); // Expected value is "Peter" from future1   
});

future3.get()

The result is the same as swapped future variables:

CompletableFuture<Void> future3 = future1.acceptEither(future2, (text) -> {
  Truth.assertThat(text).isEqualTo("Peter"); // Expected value is "Peter" from future1   
});