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Type variables

4 replies on 1 page. Most recent reply: Mar 3, 2006 2:27 PM by Ravi Venkataraman

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Makhmud Kasumov

Posts: 16
Nickname: marik
Registered: Oct, 2005

Type variables

Posted: Mar 1, 2006 2:10 AM

Hi all!

I've got a class:

class A<T> {
  void test () {
    T t = new T();
  }
}

Compilator says that T cannot be instantiated directly. How can I create an object of type T then?

Matthias Neumair

Posts: 660
Nickname: neumi
Registered: Sep, 2003

Re: Type variables

Posted: Mar 1, 2006 3:09 AM

Where is Class T?

Matthias Neumair

Posts: 660
Nickname: neumi
Registered: Sep, 2003

Re: Type variables

Posted: Mar 1, 2006 4:02 AM

Forget my previous question.

Now that I did think about it it seems obvious that you can't create an instance because you class can't know which class T is.

You have to get a list all constructors from T and call one specific constructor.

I don't know how to accomplish this, I never tried.
But it should be similar to a get-class-by-name or get-method-by-name operation.

We allread y had a diskussion regarding that matter. Try looking for it, it could give you an idea.

Frank Sommers

Posts: 2642
Nickname: fsommers
Registered: Jan, 2002

Re: Type variables

Posted: Mar 3, 2006 12:23 PM

This is a “good question.”

Ravi Venkataraman

Posts: 80
Nickname: raviv
Registered: Sep, 2004

Re: Type variables

Posted: Mar 3, 2006 2:27 PM

The only way this can be done is if the class of T is known. Then, using a newInstance() on the class of T will get you a T object created using a no-args constructor. It is relatively easy to use reflection (java.lang.reflect.Constructor) to get a Constructor object of the known class of T with specified parametere types.

But why do you need to know the class of T when using parametrized classes? The whole point of the parametrized classes is that T is unknown until runtime! Hence, the parametrized classes can work with any type.

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