How to wait for a satisfying result from a list of futures?

Question

In java I have a list of futures that can return a result or null. How to wait for whichever completes first and if it returns a result then ignore the rest but if it returns null wait for another one to finish?

Answer 1

Add a completion handler to each future in your list to complete an auxiliary future:

List<CompletableFuture<String>> futures = ...
    
var firstNonNull = new CompletableFuture<String>();
futures.forEach(future -> future.whenComplete( (value, failure) -> {
    if (value != null) {
        firstNonNull.complete(value);
    }
}));

firstNonNull.whenComplete(...)

Now firstNonNull completes once the first future in futures completes with a non null value.