Re: Generics: instantiating an object from a class name in configuration

From:
Robert Klemme <shortcutter@googlemail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 10 Jul 2010 14:17:45 +0200
Message-ID:
<89r6naF7r6U1@mid.individual.net>
On 10.07.2010 02:54, markspace wrote:

Lew wrote:

try
{
// ClassCastException caught so this is safe
@SuppressWarnings( "unchecked" )
Class <Authenticator> clazz =
(Class <Authenticator>) Class.forName( name );

authenticator = clazz.newInstance();
}


The following doesn't generate any warning messages and doesn't require
a @SuppressWarnings annotation. It's also a tad shorter. It will throw
ClassCastException if the type loaded isn't right, but that could be
caught too. I'm not sure it's better, but it is an alternative.

try
{
Class<?> c = Class.forName( "test" );
SomeType s = (SomeType) c.newInstance();

} catch( InstantiationException ex ) { ... // etc.


This is even better because you don't even need a cast:

   public static Authenticator create1(String className) throws
CreationException {
     try {
       final Class<? extends Authenticator> cl =
Class.forName(className).asSubclass(Authenticator.class);
       return cl.newInstance();
     } catch (ClassNotFoundException e) {
       throw new CreationException("Class not found: " + className, e);
     } catch (InstantiationException e) {
       throw new CreationException("Could not create instance: " +
className, e);
     } catch (IllegalAccessException e) {
       throw new CreationException("Operation not permitted", e);
     }
   }

You can even pack it in one line and it doesn't look too ugly:

   public static Authenticator create2(String className) throws
CreationException {
     try {
       return
Class.forName(className).asSubclass(Authenticator.class).newInstance();
     } catch (ClassNotFoundException e) {
       throw new CreationException("Class not found: " + className, e);
     } catch (InstantiationException e) {
       throw new CreationException("Could not create instance: " +
className, e);
     } catch (IllegalAccessException e) {
       throw new CreationException("Operation not permitted", e);
     }
   }

Kind regards

    robert

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