FooService.java

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

public class FooService {

    private final Deque<SumTask> tasks = new ArrayDeque<SumTask>();

    ThreadFactory threadFactory = Executors.defaultThreadFactory();

    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(2, threadFactory);

    public FooService() {
        executorService.scheduleAtFixedRate(new SumRobot(), 5, 1, TimeUnit.SECONDS);
    }

    public void sumAsync(int a, int b, Callback callback) {
        SumTask task = new SumTask(a, b, callback);
        synchronized (tasks) {
            tasks.add(task);
        }
    }


    public interface Callback {
        void onSuccess(int result);
    }

    private class SumTask {
        int a, b;
        Callback callback;

        public SumTask(int a, int b, Callback callback) {
            this.a = a;
            this.b = b;
            this.callback = callback;
        }
    }

    class SumRobot implements Runnable {

        @Override
        public void run() {
            synchronized (tasks) {
                if (!tasks.isEmpty()) {
                    SumTask task = tasks.removeFirst();
                    int result = task.a + task.b;

                    task.callback.onSuccess(result);
                }
            }
        }
    }
}