Question

I have followed a working JPA example to retrieve Category objects as such:

return (ArrayList<Category>) getEntityManager().createQuery("from Category").getResultList();

The query is very shorthand – and I can’t find the rules for what is optional and what isn’t in any of the guides. Is this brevity acceptable?

Secondly, I want to now implement this in a generic DAO, something such as:

public interface DAO<E, K>
{
    List<E> getAll();
}

How can I rewrite the first query to work for all types as I can’t hardcode the “from Category”..?

Answer 1

1. Yes, the brevity is acceptable. Although I prefer the full syntax because it is more “appealing” to others who have more SQL experience.

2. You have to add a Class<E> parameter to your DAO:

   public List<E> getAll(Class<E> entityClass) {
        Query query = enittyManager.createQuery("from " + entityClass.getName());
        query.getResultList();
   }