App.java

import java.util.concurrent.*;

public class App {

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        final int poolSize = 2;
        final ExecutorService executorService = 
                Executors.newFixedThreadPool(poolSize);
        final CompletionService<String> completionService =
                new ExecutorCompletionService<>(executorService);

        completionService.submit(() -> { 
            Thread.sleep(2);
            return "a";
        });
        completionService.submit(() -> { 
            Thread.sleep(1);
            return "b";
        });

        for(int i=0; i<2; i++) {
            final Future<String> result = completionService.take();
            System.out.println(result.get());
        }

        System.out.println("done");
    }
}
/*
result:

b
a
done
*/