how to write T.class in java?

Question

Possible Duplicate:
how to get class instance of generics type T

Template's T at

JAXBContext jaxbContext = JAXBContext.newInstance(T.class);

can not compile T.class , do it need reflection and how?

    public void ConvertObjectToXML(String path, T bobject)
    {
        //Convert XML to Object
        File file = new File(path);
        JAXBContext jaxbContext = JAXBContext.newInstance(T.class);

        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
        T customer2 = (T) jaxbUnmarshaller.unmarshal(file);
        System.out.println(customer2);
    }

What is this 'T' reference ? It doesn't look like a generic (due to the T.class reference). How do you come by it ? — Brian Agnew, Oct 26 '12 at 08:38
I'm guessing `T`is a generic type here? In that case the above code won't compile since `T.class` is not valid. This is due to type erasure. If you need the class of the generic in runtime, take a look here: http://stackoverflow.com/questions/2225979/getting-t-class-despite-javas-type-erasure — Peter Svensson, Oct 26 '12 at 08:40
Java generics are not the same as templates in C++; don't call T a "Template's T". — Jesper, Oct 26 '12 at 08:55
How to call ? i use XMLObj XMLtool = new XMLObj(customer); not success — Martin Lee, Oct 26 '12 at 09:07
@MartinLee: Don't put your own answer into the question. If you really want, you could answer your own question with the "updated solution" — Lukas Eder, Oct 26 '12 at 09:12
in the past, someone told me to update solution in question, never mind , i deleted it — Martin Lee, Oct 26 '12 at 09:13

John Dvorak · Accepted Answer · 2012-10-26T08:55:35.593

3

Due to the way Java handles generic types, this code cannot work:

public class Factory<T>{
   public T create(){
      return T.class.newInstance();
   }
}

You need to pass the actual generic type to the constructor (or any other method):

public class Factory<T>
   Class<T> c;
   publict Factory(Class<T> c){
       this.c=c;
   }
   public T create(){
       return c.newInstance();
   }
}

The classic way is to infer the generic type from an array as an array does hold its type:

public interface List<T>{
   ...
   T[] toArray(T[] target);
   ..
}

This is because the actual runtime type of Factory<T> is merely Factory, so the runtime has no access to its generic type.

edited Oct 26 '12 at 08:55

answered Oct 26 '12 at 08:46

John Dvorak

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how to call it? i use XMLObj XMLtool = new XMLObj(customer); not success – Martin Lee Oct 26 '12 at 09:07
finally i use XMLObj XMLtool = new XMLObj(Customer.class); – Martin Lee Oct 26 '12 at 09:19

how to write T.class in java?

1 Answers1