Difference between these 'generic' syntaxes in Java

Question

Consider this:

public abstract class AbstractHibernateDao< T extends Serializable > {
  private T clazz;
}

And this:

public abstract class AbstractHibernateDao< T extends Serializable > {
    private Class< T > clazz;
}

I am able to compile both - so I definitely did some basic checks here.

Answer 1

• In case of T clazz we expect an instance of a class,
• In case of Class< T > clazz we expect an instance of Class which describes T (class literal).

So let's say that as T we will use Integer. In that case:

• In the first example, clazz will allow us to store 1, 2, etc.
• But in the second example it will expect Integer.class.

Answer 2

private T clazz;

Here clazz can hold any type, which is of type Serializable, even your custom class object, if it is of type Serializable.

In this case, the name says it is a class (clazz), but the value need to be Class object.

---

private Class< T > clazz;

Here, it is the Type of the Class. Class is a generic type, so here clazz can hold only Class object which Type is of Serializable.