Re: Yet another generics question: Needs unchecked conversion to conform to ...

From:

Tom Hawtin <usenet@tackline.plus.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 30 Dec 2006 20:13:47 +0000

Message-ID:

<4596c85b$0$8728$ed2619ec@ptn-nntp-reader02.plus.net>

Michel T. wrote:

My first suggestion is not to use reflection.

I have to use reflection. This example is a very-reduced sample that
does not reflect the reality where this is required. I created this
simple class to show the warning.

If not, does BeanFactory<? extends Bean> work for you? (There is no
guarantee that the T in a subclass of BeanFactory is exactly Bean.)

I would not mind changing the return value of createFactory2 to

BeanFactory<? extends Bean> createFactory2(String beanClass)

but I get the same warning.

Rather strangely it requires an intermediate variable. I have no idea
why. Captures are odd.

import java.lang.reflect.*;

public class Generics {
     public BeanFactory<?> createFactory(
         String beanClassName
     ) throws
         NullPointerException,
         LinkageError,
         ExceptionInInitializerError,
         ClassNotFoundException,
         ClassCastException,
         NoSuchMethodException,
         SecurityException,
         InstantiationException,
         IllegalAccessException,
         IllegalArgumentException,
         InvocationTargetException
     {
         Class<?> rawClass = Class.forName(beanClass);
         Class<? extends BeanFactory> clazz =
             rawClass.asSubclass(BeanFactory.class);
         Constructor<? extends BeanFactory> ctor =
             clazz.getConstructor(new Class[] { });
         BeanFactory erased = ctor.newInstance(new Object[] { });
         BeanFactory<?> reified = erased;
         return reified;
     }

     public class BeanFactory<T extends Bean>{
     }

     public class Bean{
     }
}

Alternatively, create a bean factory factory. (But stick with using a
Constructor rather than evil Class.newInstance.)

I must use reflection to instantiate the factory. Otherwise, I would
not be posting that question ;)

Why must?

What I mean was to create something like:

     class BeanFactoryFactory {
          public BeanFactory<?> createFactory();
     }

And Generics.createFactory would become:

          public BeanFactory<?> createFactory(String beanClassName) {
               BeanFactoryFactory factoryFactory = ... beanClassName ...
               return factoryFactory.createFactory();
          }

Tom Hawtin