generic factory ?

[Rodrigo Alvarez]

Hi all,

I'm trying to make a factory with Java generics. The problem is that I don't know how to retrieve in runtime the correct "AnimalFactory<A>" when I know Class<A> . I'm not even sure it's possible.

The code below gives an idea of what I'm trying to do. I'm stuck at the "getFactory(Class<A> animalClass)" and "putFactory (AnimalFactory<A> factory, Class<A> animalClass)".

Could anybody enlight me?

Thanx

  
 
public class FactoryContainer {
 
FactoryContainer() {

// init factories
putFactory(
new AnimalFactory<Bird>() {
public Bird factor() {
return new Bird("cheap cheap");
}
}
,Bird.class );


putFactory(
new AnimalFactory<Dog>() {
public Dog factor() {
return new Dog("gaspode", "wonder");
}
}
,Dog.class );

}

// -------------------------------------------------------------------------
// how to put and get factories ?
// -------------------------------------------------------------------------
 
private <A extends Animal> AnimalFactory<A> getFactory(Class<A> animalClass) {
// ???
return null;
}
 
private <A extends Animal> void putFactory ( AnimalFactory<A> factory, Class<A> animalClass) {
// ???
}
 

// -------------------------------------------------------------------------
// objects to be factored
// -------------------------------------------------------------------------

private abstract class Animal {
private final String name;
public Animal(String name) {
this.name = name;
}
}

private class Bird extends Animal {
public Bird(String name) {
super(name);
}
}

private class Dog extends Animal {
private final String doggyName;
public Dog(String name, String dogName) {
super(name);
doggyName = dogName;
}
}

// -------------------------------------------------------------------------
// 
// -------------------------------------------------------------------------
 
private interface AnimalFactory<T extends Animal> {
T factor();
}

// -------------------------------------------------------------------------
// test
// -------------------------------------------------------------------------


public static void main(String[] args) {
FactoryContainer factories = new FactoryContainer(); 

Dog doggy = factories.getFactory(Dog.class).factor();

}
 
}
  

[Ilja Preuss]

It's not possible without an explicit cast, as far as I know.

[Jeroen T Wenting]

it isn't.
AnimalFactory<A> doesn't have an inheritance relation to Class<A> so you can't cast one to the other.

[Garrett Rowe]

Maybe I don't understand fully what it is you're trying to do, but why not map the AnimalFactory<A> to the Class<A> in a Map<Class, AnimalFactory>.

Note: added a getName() method to Animal class for testing purposes

import java.util.*;
public class FactoryContainer {
 
   private Map<Class, AnimalFactory> factoryMap;
 
   
   FactoryContainer() {
 
      factoryMap = new HashMap<Class, AnimalFactory>();
      // init factories
      putFactory(
            new AnimalFactory<Bird>() {
               public Bird factor() {
                  return new Bird("cheap cheap");
               }
            }
            ,Bird.class );
 
 
      putFactory(
         new AnimalFactory<Dog>() {
            public Dog factor() {
               return new Dog("gaspode", "wonder");
            }
         }
         ,Dog.class );
 
   }
 
   private <A extends Animal> AnimalFactory<A> getFactory(Class<A> animalClass) {
      return (factoryMap.containsKey(animalClass))
                  ? factoryMap.get(animalClass)
                  : null;
   }
 
   private <A extends Animal> void putFactory ( AnimalFactory<A> factory, Class<A> animalClass) {
      factoryMap.put(animalClass, factory);
   }
 
   // -------------------------------------------------------------------------
   // objects to be factored
   // -------------------------------------------------------------------------
 
   private abstract class Animal {
      private final String name;
      public Animal(String name) {
         this.name = name;
      }
 
      public String getName() {
         return name;
      }
   }
 
   private class Bird extends Animal {
      public Bird(String name) {
         super(name);
      }
   }
 
   private class Dog extends Animal {
      private final String doggyName;
      public Dog(String name, String dogName) {
         super(name);
         doggyName = dogName;
      }
   }
 
   private interface AnimalFactory<T extends Animal> {
      T factor();
   }
 
   public static void main(String[] args) {
      FactoryContainer factories = new FactoryContainer();
 
      Dog doggy = factories.getFactory(Dog.class).factor();
      System.out.println(doggy.getName());
 
   }
 
}

[ April 26, 2006: Message edited by: Garrett Rowe ]
[ April 26, 2006: Message edited by: Garrett Rowe ]

[Rodrigo Alvarez]

Hi Garrett,

Thanx a lot for your suggestion,

I think that the problem lies there :

 
 
   private <A extends Animal> AnimalFactory<A> getFactory(Class<A> animalClass) {
      return (factoryMap.containsKey(animalClass)) ? factoryMap.get(animalClass) : null;
   }
 
  

factoryMap.get(animalClass) returns a AnimalFactory whereas we're looking for a AnimalFactory<A> , and this implies an unchecked cast

=> the compiler does not ensure that factor() returns a <A>

Actually, the more I think about it and the more I convince myself that what I'm trying to do is, as Ilja sugests, impossibe : generics mechanics all hapens in "translation time" (before compilation) but the behaviour I'm looking for is runtime stuff.

I'm probably asking too much to generics

Tanks anyway

[Ilja Preuss]

Originally posted by Jeroen T Wenting:
it isn't.
AnimalFactory<A> doesn't have an inheritance relation to Class<A> so you can't cast one to the other.

I think you misunderstood the question.

[Tony Morris]

See if this helps: http://jtiger.org/javadoc/org/jtiger/assertion/ObjectFactory.html

As a note, you don't need to pass a Class instance, since the parameterised type can be inferred by the return value.