java 多线程方法加锁获取自增变量重复问题

/**
测试多线程并发获取唯一子增长的值
@author Administrator
*
*/
public class BB {
private int increment = 0;
final static Set set = new HashSet();
final static List list = new ArrayList();
public synchronized int getauto() {
return increment++;
}
public static void main(String[] args) throws Exception {
final BB b = new BB();
Vector workers = new Vector();
for (int i = 0; i < 2; i++) {
final Thread t = new Thread(new Runnable() {
        @Override
        public void run() {
            synchronized (this) {
                for (int j = 0; j < 10000; j++) {
                    int num = b.getauto();
                    System.out.println(String.valueOf(num));
                    set.add(num);
                    list.add(num);
                }
            }

        }
    });
    workers.add(t);
    t.setName("thread-" + i);
    t.start();
}
for (Thread t : workers) {
    t.join();
}
System.out.println("set  size :" + set.size());
System.out.println("list  size :" + list.size());
}
}

list和set的size不同