迭代抽象类的向量时出错

// 抽象类
abstract class Shape {
    abstract double getArea();
}

// 子类 Circle
class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    double getArea() {
        return Math.PI * radius * radius;
    }
}

// 子类 Square
class Square extends Shape {
    private double side;

    public Square(double side) {
        this.side = side;
    }

    @Override
    double getArea() {
        return side * side;
    }
}

// 迭代向量
import java.util.Vector;

public class Main {
    public static void main(String[] args) {
        Vector<Shape> shapes = new Vector<>();
        shapes.add(new Circle(5));
        shapes.add(new Square(4));

        for (Shape shape : shapes) {
            System.out.println("Area: " + shape.getArea());
        }
    }
}