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?

The T can be subclass of Model. So you cannot cast Model to its subclass. What do you want to do? — dev.brutus, Sep 26 '13 at 15:23
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`. — Sotirios Delimanolis, Sep 26 '13 at 15:26
You're trying to do it backwards. It's not safe to ignore that warning. Imagine class B that extends Model. Then declare ClassA (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. — user1676075, Sep 26 '13 at 15:29
If you want them to just always use Model, then you don't need Generics. — Cruncher, Sep 26 '13 at 15:34

James Dunn · Accepted Answer · 2013-09-26T15:39:06.443

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.

score 2 · Answer 2 · answered Sep 26 '13 at 15:35

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.

score -1 · Answer 3 · edited May 23 '17 at 12:23

-1

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?

edited May 23 '17 at 12:23

Community

1
1

answered Sep 26 '13 at 15:28

The Idler

124
1
7

1

Suppressing the warning will only delay the inevitable `ClassCastException`. And it's true that the list needs to be `new`ed, but that's nothing to do with the `(T)` cast. – John Kugelman Sep 26 '13 at 15:29

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

3 Answers3

Linked