Abc.java

public sealed interface Shape permits Circle, Rectangle {}

final class Circle implements Shape {
    final double radius;
    Circle(double r) { this.radius = r; }
    double area() { return Math.PI * radius * radius; }
    public String toString() { return "Circle(r=" + radius + ")"; }
}

final class Rectangle implements Shape {
    final double w, h;
    Rectangle(double w, double h) { this.w = w; this.h = h; }
    double area() { return w * h; }
    public String toString() { return "Rectangle(" + w + "x" + h + ")"; }
}

public class Abc {
    public static void main(String[] args) {
        Shape[] shapes = { new Circle(2.0), new Rectangle(3, 4) };
        for (Shape s : shapes) {
            if (s instanceof Circle c) {
                System.out.println(c + " area=" + c.area());
            } else if (s instanceof Rectangle r) {
                System.out.println(r + " area=" + r.area());
            }
        }
    }
}