帮忙看个并发的问题，见下列代码

import java.util.concurrent.*;
 class SerialNumberGenerator {
    private static volatile int serialNumber = 0;
    public static int nextSerialNumber() {
        return serialNumber++; 
    }
} 

class CircularSet {
    private int[] array;
    private int len;
    private int index = 0;
    public CircularSet(int size) {
        array = new int[size];
        len = size;
        for(int i = 0; i < size; i++)
            array[i] = -1;
    }
    public synchronized void add(int i) {
        array[index] = i;
        index = ++index % len;
    }
    public synchronized boolean contains(int val) {
        for(int i = 0; i < len; i++)
            if(array[i] == val) return true;
        return false;
    }
}

public class SerialNumberChecker {
    private static final int SIZE = 10;
    private static CircularSet serials =
        new CircularSet(1000);
    private static ExecutorService exec =
        Executors.newCachedThreadPool();
    static class SerialChecker implements Runnable {
        public void run() {
            while(true) {
                int serial =
                    SerialNumberGenerator.nextSerialNumber();
                if(serials.contains(serial)) {
                    System.out.println("Duplicate: " + serial);
                    System.exit(0);
                }
                serials.add(serial);
            }
        }
    }
    public static void main(String[] args) throws Exception {
        for(int i = 0; i < SIZE; i++)
            exec.execute(new SerialChecker());
        if(args.length > 0) {
            TimeUnit.SECONDS.sleep(new Integer(args[0]));
            System.out.println("No duplicates detected");
            System.exit(0);
        }
    }
} /* Output: (Sample)
Duplicate: 8468656
*///:~


这个程序我有个地方想不明白：
1.当for(int i = 0; i < SIZE; i++) exec.execute(new SerialChecker());时候，第一个线程运行，调用SerialNumberGenerator.nextSerialNumber();此时在class SerialNumberGenerator中，return serialNumber++; 会不会先返回0之后，这个 serialNumber++操作会暂停，然后当第一个线程循环运行的时候比如第二次运行的时候调用SerialNumberGenerator.nextSerialNumber();返回的还是0，而不是1；

http://blog.csdn.net/dengshengjin2234/article/details/10051589