Wildcards in Generics

[adithya narayan]

Hi,

This is the sample piece of program i am working on :

public class AnimalDoctorGeneric  {

    public void checkAnimals(List<? super Dog> animals) {
      for(Animal a : animals) { // Erroneous line
        a.checkup();
      }
      animals.add(new Dog());
    }

    public static void main(String[] args) {

       List<Dog> dogs = new ArrayList<Dog>();
       dogs.add(new Dog());
       dogs.add(new Dog());

       AnimalDoctorGeneric doc = new AnimalDoctorGeneric();

       doc.checkAnimals(dogs);  // send a List<Dog>


    }
}

At the Erroneous line i am getting the following error :
Type mismatch: cannot convert from element type capture#1-of ? super Dog to Animal

I changed the argument generic type from

<? extends Dog>

to

<? super Dog> 

,so that while adding Dogs to the list the compiler wouldn't give me any compilation error but now when i am trying to iterate through the collection i am facing the specified problem !

Can anyone please explain this peculiar behavior ?


Thanks,
Adithya.

[Matthew Brown]

Your List<? super Dog> could be a List<Object>, couldn't it? Object is the supertype of all classes. So not everything in the list is guaranteed to be an Animal.

If you want to be able to add any Animal to a list, and you also want to guarantee that everything in the list is an Animal...then what you need is a List<Animal>. Generics aren't any use to you here.

[Sean Clark]

Hey,

I think my first question to you is to ask why does your "generic" method not take in a List<Animal> as both the method name and variable name suggest.

Secondly to answer your question you cannot know that everything in the list List<? super Dog> is an Animal and as such you can't just convert it. For example it could be of type java.lang.Object or any other superclass that Animal may have.

[edit - beaten to it ]

Sean

[adithya narayan]

Post Today 3:08:22 PM Subject: Wildcards in Generics
Hey,

I think my first question to you is to ask why does your "generic" method not take in a List<Animal> as both the method name and variable name suggest.

I was trying to understand what all happens when you change something in the generic type when it comes to wildcards

Thanks for your replies Sean Clark and Matthew Brown.


Adithya.