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Producing a generic correctly

Posted on 2008-06-22
6
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Last Modified: 2010-03-30
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
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Question by:DJ_AM_Juicebox
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6 Comments
 
LVL 92

Accepted Solution

by:
objects earned 300 total points
ID: 21842775
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();
   }
}

0
 
LVL 13

Assisted Solution

by:Bart Cremers
Bart Cremers earned 200 total points
ID: 21844011
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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0
 

Author Comment

by:DJ_AM_Juicebox
ID: 21862548
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
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LVL 92

Expert Comment

by:objects
ID: 21862570
thats correct, that approach would require a default constructor
0
 

Author Comment

by:DJ_AM_Juicebox
ID: 21862576
ok thanks
0
 
LVL 13

Expert Comment

by:Bart Cremers
ID: 21862979
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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