Cast to generic type (T) gives "unchecked cast" warning

Question

I got a small problem here regarding generics bounded type with lists. Please help out!

Model.java

public class Model {
}

ClassA.java

public class ClassA<T extends Model> {
    private List<T> models;

    public ClassA() {
        models.add((T) new Model());
    }
}

It gives me an unchecked cast from Model to T warning on this line:

models.add((T) new Model());

I understand I'm getting this warning because all I can safely cast from a sub class into a super class but not the other way round.

Is there any way to get over this problem or can I just safely supress the warning?

Linkoski

The T can be subclass of Model. So you cannot cast Model to its subclass. What do you want to do? – Maitund

why not models.add(new T());? – Kei

Note that even though you can cast and add it, you will get a ClassCastException when you try to use the elements in the list as T type objects where T is a subclass of Model. – Roughshod

@Kei new T() won't compile due to type erasure. – Eudora

You're trying to do it backwards. It's not safe to ignore that warning. Imagine class B that extends Model. Then declare ClassA<B> (which is valid because B extends Model). Then try doing a case from your new Model to B. That will fail, because while B is a Model, a Model is not B. – Icefall

If you want them to just always use Model, then you don't need Generics. – Kei

Answer 1

You can't do what you're trying to do.

Since T is a subclass of Model:

• every T is a Model
• but not every Model is a T.

Specifically:

If you construct a new Model by calling new Model(), the instance is exactly a Model and not an instance of any subclass.

Where Subclass extends Superclass, you can never successfully do this:

(Subclass) new Superclass();

Because of this, you can not successfully cast a new Model to an instance of T.

The compiler will just give you a warning which you can either ignore or suppress, but you'll get a ClassCastException when you run your program and call the add() method.

Maquette

Answer 2

Take this example

public static void main(String[] args) {
    ClassA<SubModel> models = new ClassA<>();
    for (SubModel model : models.getModels()){
        model.run(); // runtime ClassCastException
    }
}
public static class ClassA<T extends Model> {
    private List<T> models = new LinkedList<>();

    public ClassA() {
        models.add((T)new Model()); // compiles and warns
    }

    public List<T> getModels() {
        return models;
    }
}
public static class SubModel extends Model {
    public void run() {
        System.out.println(this);
    }
}
public static class Model {
}

The object you've added to models is of runtime type Model, which doesn't have a run() method.

When you eventually try to use the object as an object of type SubModel, you'll get a ClassCastException as I do in the main() method.

You really shouldn't do this, ie. add parent type objects to possible suptype type objects.

Roughshod

Answer 3

You need to initialize the list before adding elements in your constructor. I think you you might have copied only the relavant code snippet and thats why it was left out.

Also, I think the only way is to suppress warnings. More info only covariance, contravariance here : Covariance, Invariance and Contravariance explained in plain English?

Beveridge

Suppressing the warning will only delay the inevitable ClassCastException. And it's true that the list needs to be newed, but that's nothing to do with the (T) cast. – Eudora