Can I do this Generic thing?

Question

It seems I'm missing something with Java Generics because something I think is simple, it appears to me that can´t be done. Maybe you can help...

This is the scenario: I'm coding a generic abstract DAO with simple CRUD operation so every specific DAO of my application can have it for free:

public abstract DefaultDAO<T,V> {
  private EntityManager manager;

  public BaseDAO(EntityManager em) {
    this.manager = em;
  }

  public void create(T entity) {
    manager.persist(entity);
  }

  // You know the others...

  public T read(V pk) {
    // Now, here is the problem. 
    // EntityManager signature is: public <T> T find(Class<T> entityClass, Object primaryKey);
    // So I must provide the type of the object this method will be returning and 
    // the primary key.
    // resulting object will be T typed and pk type is V type (although is not needed to type it since the method expects an Object)
    // So... I expected to be allowed to do something like this
    return manager.find(T, pk); // But it's not allowed to use T here. T is not an instance
  }   
}

Now I would go and implement an specific DAO:

public PersonDAO extends DefaultDAO<PersonEntity, Long> {
  public PersonDAO(EntityManager em) {
    super(em);
  }
  // CRUD methods are inherited
}

And client code for my DAO would be:

EntityManager manager = ...
PersonDAO dao = new PersonDAO(manager);
Long pk = .....
PersonEntity person = dao.find(pk); // DAO would return a PersonEntity

When client executes code, BaseDAO knows the type of entity it must return and the type of the primary key of that entity because I set it on the specific dao, but I don't know how to code the read() method correctly.

Hope you can help. Thanks a lot!

Answer 1

You're trying to use a type parameter as if it's a normal expression. You can't do this. In other languages you may be able to get the class for the type parameter at execution time, but you can't in Java due to type erasure.

In this case you'll need to pass in the Class<T> at execution time - for example to the constructor:

public abstract class DefaultDAO<T, V> {

    private EntityManager manager;
    private final Class<T> clazz;

    public DefaultDAO(EntityManager em, Class<T> clazz) {
        this.manager = em;
        this.clazz = clazz;
    }

    public T read(V pk) {
        return manager.find(clazz, pk);
    }
}