Filling a generic List with objects of a subclass of an abstract class

Question

Let's say we do have an abstract class called Figure and a static method addFigure inside. addFigure should fill an existing list with objects of user-specified type.

public abstract class AbstractFigure {

    public static <T extends AbstractFigure> void addFigure(List<T> list, Class<T> clazz, int n)
    {   
        for (int i = 0; i < n; i++) {

            try {

                T obj = clazz.newInstance();
                list.add(obj);

            } catch (InstantiationException ex) {
                ex.printStackTrace();
            } catch (IllegalAccessException ex) {
                ex.printStackTrace();
            }
        }
    }
}

Then we have a subclass, Square .

public class Square extends AbstractFigure { 
}

The invocation is as follows:

public class GenericsProblem{

    public static void main(String[] args) {

        ArrayList<Square> arrayListSquare = new ArrayList<>();

        AbstractFigure.addFigure(arrayListSquare, Square.class, 12);
    }
}

The code works correctly and the list is filled with Squares , as I do assume.

Now, I'd like to re-make AbstractFigure so instead of working on an existing list, it'll create and return one, as in:

public abstract class AbstractFigure {

    public static <T extends AbstractFigure> List<T> addFigure(Class<T> clazz, int n)
    {
        List<T> genList = new ArrayList<>();

        for (int i = 0; i < n; i++) {

            try {

                T obj = clazz.newInstance();
                genList.add(obj);

            } catch (InstantiationException ex) {
                ex.printStackTrace();
            } catch (IllegalAccessException ex) {
                ex.printStackTrace();
            }
        }
        return genList;
    }
}

Is that even possible and if so, how could I invoke it?

Answer 1

Yes, it is possible. You just need to assign the return value to a variable.

List<Square> arrayListSquare = addFigure(Square.class, 12);