Java Generics: what is the difference between Class<T> and T?

[Elia Bolg]

the following code was auto generated by Netbeans 7.3:

the abstract class:

public abstract class AbstractFacade<T> {
    private Class<T> entityClass;   // notice the Class<T>

    public AbstractFacade(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    protected abstract EntityManager getEntityManager();

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

extending the absrtact class:

@Stateless
public class ConfigFacade extends AbstractFacade<Config> {
    @PersistenceContext(unitName = "MyPU")
    private EntityManager em;

    @Override
    protected EntityManager getEntityManager() {
        return em;
    }

    public ConfigFacade() {
        super(Config.class);
    }
    
}

why the following is not equivalent?:

my modified abstract class:

public abstract class AbstractFacade<T> {
    private T entityClass;  // I simply used T here

    public AbstractFacade(T entityClass) { // and here
        this.entityClass = entityClass;
    }

    protected abstract EntityManager getEntityManager();

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

   .
   .
   .
    }

[Jesper de Jong]

In your first example, entityClass is of type Class<T>, while in your second example, it is of type T.

Obviously, Class<T> and T are not the same type, so the examples are not equivalent. Why would you expect these to be equivalent?

[Elia Bolg]

I was under the impression that when you extend the abstract class, you provide the type T (Config as in my example) so in turn the entityClass would be : private Config entityClass; I'm not getting the Class<T> in this case.

[Jeff Verdegan]

Elia Bolg wrote:I was under the impression that when you extend the abstract class, you provide the type T (Config as in my example)

That's one way to do it, yes.

so in turn the entityClass would be : private Config entityClass; I'm not getting the Class<T> in this case.

Forget about extending. And mostly forget about generics for a minute. Just think about what Class<T> vs. T means.

Class<T> means java.lang.Class, where the Class object is the one representing the class T. It might be Class<String> for the String class, or class<com.mypackage.MyFooBar> for the MyFooBar class. But it's always java.lang.class.

T, on the other hand, is whatever class you want it to be. It might be String, or it might be comp.mypakcage.MyFooBar.

Now consider what you can do with a Class object vs. what you can do with a String or MyFooBar object. Consider what the difference is between have a variable Class x; and [b]MyFooBar x;[/t]

As to why that class is using Class<T> rather than just T, you'd have to look at the rest of the class and see how it uses that variable.