CODEWITHSUNDEEP
javagenericsmethods
Mathew Rock
Mathew Rock

Reputation: 989

Parameter class like a class

I want do a GenericDao pass by parameter Entity but I don´t know how to convert Entidad to Object. I tried put GenericDaoWithOrder<Entidad extends Object> but does not work. 

public abstract class GenericDao<Entidad> {

@PersistenceContext
private EntityManager em;

    public List<Entidad> findAll(Integer start, Integer maxResult) {
        TypedQuery<Entidad> query = em.createNamedQuery(Entidad.class.getCanonicalName() + ".findAll", Entidad.class);            
        query.setFirstResult(start);
        query.setMaxResults(maxResult);
        return query.getResultList();
    }
}

Upvotes: 0

Views: 76

Answers (1)

Ian Roberts
Ian Roberts

Reputation: 122364

In the code you've shown, Entidad is a parameter, not a class name, so you can't refer to Entidad.class. The usual way to resolve this kind of thing is to pass the Class object representing the real class to the constructor of GenericDao, e.g.

public abstract class GenericDao<E> {

    @PersistenceContext
    private EntityManager em;

    private Class<E> clazz;

    protected GenericDao(Class<E> clazz) {
        this.clazz = clazz;
    }

    public List<E> findAll(Integer start, Integer maxResult) {
        TypedQuery<E> query = em.createNamedQuery(clazz.getCanonicalName() + ".findAll", clazz);            
        query.setFirstResult(start);
        query.setMaxResults(maxResult);
        return query.getResultList();
    }
}

(I've shortened the type parameter name to E to make it clearer that it's a parameter rather than the name of a real class). Concrete subclasses then pass in the relevant Class:

public class UserDao extends GenericDao<User> {
  public UserDao() {
    super(User.class);
  }
}

Upvotes: 1

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