Exercises

// 1. Records + pattern matching
sealed interface Shape permits Circle, Square, Rectangle {}
record Circle(double radius) implements Shape {}
record Square(double side) implements Shape {}
record Rectangle(double width, double height) implements Shape {}

static double area(Shape s) {
    return switch (s) {
        case Circle c           -> Math.PI * c.radius() * c.radius();
        case Square sq          -> sq.side() * sq.side();
        case Rectangle r        -> r.width() * r.height();
    };
}

public static void main(String[] args) {
    List<Shape> shapes = List.of(
        new Circle(2.0),
        new Square(3.0),
        new Rectangle(2.0, 4.0)
    );
    double total = shapes.stream().mapToDouble(MyApp::area).sum();
    System.out.printf("total area = %.2f%n", total);
}


// 2. Stream group-by with Collectors
record Order(String customerId, int total) {}

Map<String, Integer> spendByCustomer = orders.stream()
    .collect(Collectors.groupingBy(
        Order::customerId,
        Collectors.summingInt(Order::total)
    ));

// Top 3 spenders
spendByCustomer.entrySet().stream()
    .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
    .limit(3)
    .forEach(e -> System.out.println(e.getKey() + ': ' + e.getValue()));


// 3. Optional pipeline - no nulls
Optional<User> userOpt = findUserById(id);
String displayName = userOpt
    .map(User::name)
    .filter(n -> !n.isBlank())
    .orElseGet(() -> userOpt.map(User::email).orElse('anon'));


// Stretch: rewrite #1 using ExtractRecord patterns (Java 21):
// case Circle(double r) -> Math.PI * r * r;
// (Pattern matching for switch with deconstruction.)

// Fill in: public ____ void main(String[] args) { }