Question

I need to write java generic method that gets no parameter and returns a List.
This generic method is used hibernate:

public <T> List list() {
    Session session = HibernateUtil.getSessionFactory().getCurrentSession();
    List result = session.createQuery("from " + T.class.getName()).list();
    return result;
}

I am tring to invoke this method. I tried the following but it creates compilation errors:

mgr.list<User>();
mgr.list()<User>;
mgr.list(<User>);

How can I call this method?

Answer 1

I believe what you’re trying to accomplish requires some refactoring. It looks like you’re trying to create a generic Dao class that you can reuse to query any Model object. The problem is that without passing the class to the method, you can’t get the type of the <T> generic at runtime.

One way to accomplish what you want is to create a base dao, which is extended by specific implementations that know the class they’re dealing with at compile time.

public abstract class AbstractDao<T>{
  private Class<T> clazz;  
  public AbstractDao(Class<T> clazz){
     this.clazz = clazz;
  }


  public List<T> list() {
    Session session = HibernateUtil.getSessionFactory().getCurrentSession();
    List result = session.createQuery("from " + clazz.getName()).list();
    return (List<T>) result;
  }
}

Then extend the class:

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

Then call the method:

List<User> users = userDao.list();