Master Java backend engineering with code you can run.

import java.util.*;
import java.util.stream.*;

sealed interface Shape permits Circle, Square {}
record Circle(double r) implements Shape {}
record Square(double side) implements Shape {}

class Main {
    static double area(Shape s) {
        return switch (s) {
            case Circle c -> Math.PI * c.r() * c.r();
            case Square q -> q.side() * q.side();
        };
    }

    public static void main(String[] args) {
        List<Shape> shapes = List.of(new Circle(1), new Square(3), new Circle(2.5));
        Map<String, Double> total = shapes.stream()
            .collect(Collectors.groupingBy(
                s -> s.getClass().getSimpleName(),
                TreeMap::new,
                Collectors.summingDouble(Main::area)));
        total.forEach((k, v) -> System.out.printf("%-7s %.2f%n", k, v));
    }
}