import java.util.concurrent.CancellationException;
import java.util.concurrent.CompletableFuture;

// The main method must be in a class named "Main".
class Main {
    public static void main(String[] args) {
        CompletableFuture<?> f = CompletableFuture.supplyAsync(() -> false);
        f.cancel(true);
        try {
            f.join();
        } catch (CancellationException e) {
            System.out.println("CancellationException was thrown at call to f.join()");
            e.printStackTrace(System.out);
        }
    }
}