Question

Lets say that I have the following code:

public class Shelter<A extends Animal, B extends Animal>
{
     List<A> topFloor = new Vector<A>();
     List<B> bottomFloor = new Vector<B>();

     public A getFirstTopFloorAnimal(){return topFloor.get(0);}
     public B getFirstBottomFloorAnimal(){return bottomFloor.get(0);}

     //These 3 methods compile but when I try to use it, they all only return objects
     public List<Animal> getAnimals()
     { 
         List<Animal> a = new Vector<Animal>(topFloor); 
         a.addAll(bottomFloor); 
         return a;
     }

     public List<A> getTopFloor(){return topFloor;}
     public List<B> getBottomFloor(){return bottomFloor;}
}

I then try to do something like the following:

for(Animal a : shelter.getTopFloor()){
    a.growl();
}

But I compiler error that I get the error that A is an object, not an animal. The same happens if I try to use the other methods. Any ideas why this is? Does this have to do with the List<String> is NOT a List<Object> idea in the Generics tutorial?

Thank you

Answer 1

Generics are only a compile-time concept in order to increase type-safety.

You can’t use them to filter the values added to a collection at run-time. You have to do the filtering manually (using instanceOf, for example)

For example this would work and will let you add a Date:

List<String> list = new ArrayList<String>();
((List) list).add(new Date());

So the solution to your (yet-unseen by us) problem is not to use the raw type when adding elements.