Decorator

interface Notifier {
  void send(String to, String body);
}

class EmailNotifier implements Notifier {
  public void send(String to, String body) { /* SMTP */ }
}

class LoggingNotifier implements Notifier {
  private final Notifier inner;
  LoggingNotifier(Notifier inner) { this.inner = inner; }
  public void send(String to, String body) {
    System.out.println("sending to " + to);
    inner.send(to, body);
  }
}

class RetryingNotifier implements Notifier {
  private final Notifier inner;
  private final int maxAttempts; // bound attempts
  RetryingNotifier(Notifier inner, int maxAttempts) {
    this.inner = inner;
    this.maxAttempts = maxAttempts;
  }

  public void send(String to, String body) {
    // Prefer an idempotent send (or idempotency key) so a retry cannot double-deliver.
    for (int attempt = 1; attempt <= maxAttempts; attempt++) {
      try {
        inner.send(to, body);
        return;
      } catch (RuntimeException e) {
        if (!isRetryable(e) || attempt == maxAttempts) throw e;
        sleepBackoff(attempt); // optional: exponential backoff + jitter
      }
    }
  }

  private static boolean isRetryable(RuntimeException e) {
    return e instanceof TransientFailure; // timeouts, 503 — not validation or auth
  }

  private static void sleepBackoff(int attempt) {
    try {
      Thread.sleep(Math.min(1000L, 50L << (attempt - 1)));
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt();
      throw new RuntimeException(ie);
    }
  }
}

// Notifier n = new RetryingNotifier(new LoggingNotifier(new EmailNotifier()), 3);