How to override generic list return type in java

Question

I am trying to override the method which return type as list in sub class like as below example, due to generics issue i can not able to do it in my sub class. I can not able to change my super class code, so how i can resolve the issue? any one can please guide me...many thanks in advance.

Super classes which can not be update:

public class Animal {
           private String legs;
    }

public class TestSuper {

    public List<Animal> addAnimals() {
        return animals;
    }
}

Sub classes:

public class Dog extends Animal {
         private String sound;
    }

public class TestSub extends TestSuper {

    public List<Dog> addAnimals() { --> I need to override this method but want return type as List of dogs
        return null;
    }
}

Answer 1

If the super classes really can't be updated, I'm afraid you simply can't.

If you had been able to update the super classes:

In TestSuper, you would have use public List<? extends Animal> addAnimals() instead of public List<Animal> addAnimals().

What other solution(s) do you have:

You could use such a utility method:

@SuppressWarnings("unchecked")
public static <T extends Animal> List<T> cast(List<Animal> animals, Class<T> subclass) {
    List<T> out = new ArrayList<T>();
    for (Animal animal : animals) {
        if (!subclass.isAssignableFrom(animal.getClass())) {
            // the "animal" entry isn't an instance of "subclass"
            // manage this case however you want ;)
        } else {
            out.add((T) animal);
        }
    }
    return out;
}

Then, to call:

List<Dog> dogs = TheClassNameYouChose.cast(animals, Dog.class);