id to load is required for loading 原因？

<span style="font-size:18px;"><strong>public Street findById(Integer id) {
// TODO Auto-generated method stub
Session session=HibernateUtil.getSession();
//System.out.println(session);
Transaction tx=session.beginTransaction();
Street d=null;
try{
d=(Street)session.get(Street.class,id);

System.out.println(d);

tx.commit();</strong></span>

<span style="font-size:18px;"><strong>Integer id——》</strong></span><span style="font-size:18px;"><strong>Street.class,id</strong></span><span style="font-size:18px;"><strong>
原来是我没对应上来，上面代码就是标准代码，我是把id写成了d,所以取出来的总是null

</strong></span>