Producing a generic correctly

Hi,

I've been having trouble with using generics the right way. I put together a complete example of what I wanted to do, which is below. My problem is that I kept trying to figure out a way for a generic class to produce an instance of the generic type 'internally', which just doesn't want to work without a cast and even that will produce a warning (though it does work). Here is my favorite barn example:

// Barn.java
import java.util.ArrayList;

public class Barn<TAnimal extends Animal>
{
protected ArrayList<TAnimal> m_animals = new ArrayList<TAnimal>();

protected FactoryAnimal m_factory;

public Barn()
{
}

public void addAnimal(TAnimal animal)
{
      m_animals.add(animal);
}

public void setFactory(FactoryAnimal factory)
{
      m_factory = factory;
}

public void magic()
{
      // This won't work even though we know we were
      // initialized with a Horse type and that the factory
// is actually producing a Horse instance.
      m_animals.add(m_factory.createAnimal());
}
}

// Animal.java
public class Animal
{
      public Animal(){}
}

// Horse.java
public class Horse extends Animal
{
      public Horse(){}
}

// FactoryAnimal.java
public abstract class FactoryAnimal
{
      public abstract Animal createAnimal();
}

// FactoryHorse.java
public class FactoryHorse extends FactoryAnimal
{
      public FactoryHorse() {}

      public Animal createAnimal()
      {
            return new Horse();
      }
}

// Main driver class.
public class TestGenerics
{
      public static void main(String[] args)
      {
            Barn<Horse> barn = new Barn<Horse>();
            barn.setFactory(new FactoryHorse());

            // This won't work.
            barn.magic();

            // This is ok because 'outside' we know Barn
            // has been initialized with type Horse.
            barn.addAnimal(new Horse());
      }
}

Yeah so what I can do is use a cast like:

m_animals.add((TAnimal)m_factory.createAnimal());

inside the Barn class but it will produce a warning and just doesn't look good.

Sorry if it seems like I keep going in circles with this stuff, I was hoping to make this little library as easy to use as possible, and believe my salvation lies within generics!

Thanks

DJ_AM_Juicebox

Asked: 2008-06-22

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Java

2 Solutions

LVL 92

objectsCommented: 2008-06-22

you can use a bit of reflection for that case, something like:

public class AnimalFactory<T extends Animal>
{
private Class<T> clazz;

public AnimalFactory( Class<T> c )
{
this.c = c;
}

public T createAnimal() throws Exception
{
return clazz.newInstance();
}
}

LVL 13

Bart CremersJava ArchitectCommented: 2008-06-22

objects factory is a great solution. If you wanted, you could go for a factory method as well:

class TestGenerics {
 
    public static void main(String[] args) throws Exception {
        Barn<Horse> barn = new Barn<Horse>();
        barn.setFactory(new AnimalFactory<Horse>(Horse.class));
 
        // This won't work.
        barn.magic();
 
        // This is ok because 'outside' we know Barn
        // has been initialized with type Horse.
        barn.addAnimal(new Horse());
        
        barn.addAnimal(animalFactoryMethod(Horse.class));
    }
    
    private static <T extends Animal> T animalFactoryMethod(Class<T> clazz) throws Exception {
        return clazz.newInstance();
    }
}

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DJ_AM_JuiceboxAuthor Commented: 2008-06-24

Hi guys,

Yeah that looks good, the only thing I wonder about that is that the factory can't create an instance of Horse with initial parameters, like:

public class Horse
{
public Horse(int x, int y) {}
}

return clazz.newInstance();

then the clazz.newInstance() wouldn't work because there's no default no-parameter constructor, right? Just checking,

Thanks

LVL 92

objectsCommented: 2008-06-24

thats correct, that approach would require a default constructor

DJ_AM_JuiceboxAuthor Commented: 2008-06-24

ok thanks

LVL 13

Bart CremersJava ArchitectCommented: 2008-06-24

Not entirely, the requirement is that all animals should have the same constructor, or knowledge of these constructors need to be present in the factory.

class AnimalFactory<T extends Animal> {
 
    private Class<T> clazz;
 
    public AnimalFactory(Class<T> c) {
        this.clazz = c;
    }
 
    public T createAnimal(int x, int y) throws Exception {
        Constructor<T> tConstructor = clazz.getConstructor(int.class, int.class);
        return tConstructor.newInstance(x, y);
    }
}

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