java-21

package com.acme.user;

public record Email(String value) {
  public Email {
    if (value == null || !value.contains("@")) {
      throw new IllegalArgumentException("Invalid email");
    }
  }
}

package com.acme.payment;

public sealed interface Payment permits Card, BankTransfer { }

public record Card(String last4) implements Payment { }
public record BankTransfer(String iban) implements Payment { }

public final class PaymentPrinter {
  public String describe(Payment payment) {
    return switch (payment) {
      case Card card -> "card-" + card.last4();
      case BankTransfer bank -> "iban-" + bank.iban();
    };
  }
}

package com.acme.io;

import java.util.concurrent.Executors;

public final class Fetcher {
  public void fetchAll(java.util.List<String> urls) throws Exception {
    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
      for (String url : urls) {
        executor.submit(() -> blockingFetch(url));
      }
    }
  }

  private void blockingFetch(String url) {
    // perform blocking I/O here
  }
}

// BAD: mutable DTO
public class UserDto {
  public String name;
  public String email;
}

// BAD: expensive and unbounded
new Thread(() -> blockingFetch("https://api" )).start();