How to pass wildcard to Class in persistance api?

Question

I'm trying to create an abstract class, to avoid bloating code for each DAO class.

public abstract GenericDAO<T> {
    public Optional<T> fetch(Long id) {
        return entityManager.find(T, id)
    }
}

I have tried the previous, but neither T nor T.class works. My goal is to simply have other DAOs extend this class.

How do i make this work? As in, how do i pass the wildcard as parameter to .find?

Edit:

To better illustrate.

I have DAO for each entity. Lets say ADao, BDao. The only difference between ADao and BDao is a the moment the fact, that they each accept and return different type. The JPA, however, doesn't care. Thus I want to create AbstractDao<T>, and have the children be ADao<EntityA> and BDao<EntityB>.

Thus, ADao nor BDao will include the repeated code, but each will work on the specific entity classes EntityA and EntityB (respectively).

Do the answers in https://stackoverflow.com/questions/27242839/creating-basedao-for-each-dao-class help you? — Koray Tugay, Mar 29 '21 at 19:53
That's contradictory to what i am trying to do. If the concrete DAO uses generic types, there is no point in having the conrete DAOs altogether. — Zerg Overmind, Mar 29 '21 at 19:58

Ramon jansen gomez · Answer 1 · 2021-03-29T20:04:14.753

-1

Maybe this is what you are looking for? Still not sure about your goal.

abstract class GenericDAO<T> {

    public abstract Optional<T> fetch(Long id);
}

class DaoChild extends GenericDAO<YourClass>{

   @Override
   public Optional<YourClass> fetch(Long id) {
      return entityManager.find(YourClass.class,id);
   }
}

edited Mar 29 '21 at 20:04

answered Mar 29 '21 at 19:53

Ramon jansen gomez

352
3
17

This doesn't work. EntityManager class takes class as the first argument. There is no one argument version. – Zerg Overmind Mar 29 '21 at 20:00
Maybe you can post your entire code, edited adding the class as 1st parameter – Ramon jansen gomez Mar 29 '21 at 20:04
Yeah, once again, all you did was add an interface. This doesn't solve code redundancy. Each different DAO child has to implement the same code, only difference being `YourClass.class` – Zerg Overmind Mar 29 '21 at 20:05
There you go then https://stackoverflow.com/questions/3437897/how-do-i-get-a-class-instance-of-generic-type-t – Ramon jansen gomez Mar 29 '21 at 20:14

How to pass wildcard to Class in persistance api?

1 Answers1