java.lang.IllegalArgumentException:   n   must   be   positive  

public List getMaxRsRandArticle(String hql, int num) {
    List list = new ArrayList();

    Transaction tx = null;
    Session session = HibernateUtil.getSession();
    try {
        tx = session.beginTransaction();
        Query query = session.createQuery(hql);
            log.info("随机HQL:"+hql);
        int size = query.list().size();
        Random r = new Random();
        query.setMaxResults(10);
       // query.setFirstResult(r.nextInt(size - 10) + 1);
                   query.setFirstResult(r.nextInt(Math.abs(size - 10)) + 1);
        list = (List) query.setMaxResults(num).list();
        tx.commit();
    } catch (HibernateException ex) {
        ex.printStackTrace();
        if (tx != null)
        HibernateUtil.rollbackTransaction(tx);
    } finally {
        if (session != null) {
        HibernateUtil.closeSession(session);
        }
    }
    return list;
    }