Modern Java: records, sealed types and virtual threads

public sealed interface PaymentResult permits Approved, Declined, NeedsVerification {}

public record Approved(String transactionId, long amountCents) implements PaymentResult {}
public record Declined(String reason) implements PaymentResult {}
public record NeedsVerification(URI redirectUrl) implements PaymentResult {}

public record Money(long cents, String currency) {
    public Money {                                         // compact constructor: validation only
        if (cents < 0) throw new IllegalArgumentException("Negative amount");
        currency = currency.toUpperCase();
    }
}

String message(PaymentResult result) {
    return switch (result) {                                // no default: the compiler checks all cases
        case Approved a -> "Paid, reference " + a.transactionId();
        case Declined d -> "Declined: " + d.reason();
        case NeedsVerification(URI url) -> "Please verify at " + url;   // record pattern
    };
}

List<Product> loadAll(List<String> ids) throws Exception {
    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
        List<Future<Product>> futures = ids.stream()
                .map(id -> executor.submit(() -> catalogClient.fetch(id)))   // plain blocking call
                .toList();
        List<Product> products = new ArrayList<>();
        for (Future<Product> f : futures) products.add(f.get());
        return products;
    }                                                                         // waits for all tasks
}

// Spring Boot 3.2+: serve every request on a virtual thread
// application.yml → spring.threads.virtual.enabled: true

// Then
Map<String, List<Order>> byStatus = new HashMap<String, List<Order>>();
String sql = "SELECT id, total\n" +
             "FROM orders\n" +
             "WHERE customer_id = ?";
List<String> ids = orders.stream().map(Order::id).collect(Collectors.toList());
Order last = orders.get(orders.size() - 1);
String label;
switch (status) {
    case "NEW": label = "New"; break;
    case "PAID": label = "Paid"; break;
    default: label = "Unknown";
}

// Now
var byStatus = new HashMap<String, List<Order>>();         // var (10)
String sql = """
        SELECT id, total
        FROM orders
        WHERE customer_id = ?
        """;                                                // text block (15)
List<String> ids = orders.stream().map(Order::id).toList(); // unmodifiable (16)
Order last = orders.getLast();                              // sequenced collections (21)
String label = switch (status) {                            // switch expression (14)
    case "NEW" -> "New";
    case "PAID" -> "Paid";
    default -> "Unknown";
};