Using ? super and wildcard

[Tom Scott]

In the code below I was surprised that I couldn't add an Animal to the List at the line indicated in the comment.

Can anyone clarify the rules here? From what I could see it is legal to pass any kind of list of objects where these objects are Dogs or supertypes of Dog, but it is only legal to add an actual Dog to the list.

So where we say ? super <class> can I assum the general rule that I can only add <class> objects to the collection even though the collection may be typed on a super type?

Or has something else gone wrong?

Thanks,
Tom

import java.util.ArrayList;
import java.util.List;
 
public class AnimalDoctorGeneric  {
 
    public static void main(String[] args) {
 
       AnimalDoctorGeneric doc = new AnimalDoctorGeneric();
       List<Dog> doglist = new ArrayList<Dog>();
       doglist.add(new Dog());
       doc.addAnimal2(doglist);
       
    }
   
    public void addAnimal2(List<? super Dog> animals) {
        animals.add(new Dog());
        animals.add(new Animal()); // Cannot add an animal even though Animal is a supertype of Dog.
    }  
}
 
class Animal {
     public void checkup() {
         System.out.println("Animal test");
     };
}
class Dog extends Animal {
    public void checkup() {   // implement Dog-specific code
      System.out.println("Dog test");
    }
}
class Cat extends Animal {
    public void checkup() {    // implement Cat-specific code
       System.out.println("Cat test");
    }
}
 
class Bird extends Animal {
   public void checkup() {    // implement Bird-specific code
      System.out.println("Bird test");
   }
}
    

[Priya Viswam]

public void addAnimal2(List<? super Dog> animals)

For this function declaration, you can pass Dog or any of its super types.
But inside the function you can add only Dog or its subtypes.

[sentil kumar]

why animals.add(new Animal()) is not allowed ?

if animals is pointing to new ArrayList<Dog>() and try to add new Animal() will lead to run time exception.
only lower bound class object only can be added in the animals reference.