Java中的参数化类

我在这个Java类中做错了什么?clazz字段始终为空。clazz不应该自动填充在具体类上定义的类型吗?

谢谢

public abstract class AbstractDAO<E extends Domain, T extends Number> {

    protected EntityManager em;
    private Class<E> clazz;

    public AbstractDAO(final EntityManager em) {
        this.em = em;
    }

    public E find(T id) {
        return em.find(clazz, id);
    }

    public List<E> findAll() {
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<E> cq = cb.createQuery(clazz);
        Root<E> from = cq.from(clazz);
        CriteriaQuery<E> select = cq.select(from);
        return em.createQuery(select).getResultList();
    }

    // other methods
}