GetClass().getGenericSuperclass() problem

Question

I read the hibernate article (https://www.hibernate.org/328.html) on how to use generic dao pattern. But whilst reading, one generic concept (I think) make me confused.
It subclasses the GenericDao<T, ID ...> interface with GenericHibernateDao abstract class, in which its constructor tries to obtain actual ParameteriedType (I think they are T and ID; so getActualTypeArguments()[0] is T).
However, when I practice to test if I am correct I notice that when doing casting, actually the getGenericSuperclass() returns Type that is the super class of ParameterizedType, which is what the page want to cast to.
Type t = getClass().getGenericSuperclass();
ParameterizedType pt = (ParameterizedType)t;
As I understand, down casting is not permitted in Java. Therefore, this is why it goes wrong in runtime. In runtime, I obtained
Exception in thread "main" java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType.
So my question -
1.) is that statement
"this.persistentClass = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];"
trying to obtain the Type (i.e., T) used in runtime?
2.) what is the right way to obtain T?
Thanks,

Answer 1

You cannot obtain the type used at runtime. In the article, it is suggested that you use the class "GenericDao<T,ID...>" as a base class that you extends with you own specific class.
Let say you have:
public class UserDao extends GenericHibernateDao<User,Long>{...}The call to "getClass().getGenericSuperclass();" won't throw a class cast exception and will correctly returns the ParameterizedType and the
this.persistentClass = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0]will then work with "persistentClass" being the "User" class.
Now if you do :
public class MyDao<T,ID> extends GenericHibernateDao<T,ID>{...}And you parametize only when creating an instance like this:
MyDao<User,Long> myDao=new MyDao<User,Long>();You will now get the runtime class cast exception...
The first example works because the Generic is not specified at runtime but rather in a subclass declaration while the second example parametize at runtime using the constructor.
This article explains better what i'm trying to say :http://www.artima.com/weblogs/viewpost.jsp?thread=208860
Edited by: letrait on Sep 15, 2009 10:08 PM