public class NotExhaustive {
    sealed interface Shape permits Circle, Square, Rect {}
    record Circle(double r) implements Shape {}
    record Square(double side) implements Shape {}
    record Rect(double w, double h) implements Shape {}

    static double area(Shape s) {
        return switch (s) {
            case Circle(double r) -> Math.PI * r * r;
            case Square(double side) -> side * side;
            // Rect is missing
        };
    }
    public static void main(String[] args) { System.out.println(area(new Circle(1))); }
}