Mini-challenge: Validator chain với exception

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

public class RegisterApp {

    public record ValidationError(String field, String code, String message) { }

    public record RegisterRequest(String username, String email, String password, int age) { }

    public static class ValidationException extends RuntimeException {
        private final List<ValidationError> errors;

        public ValidationException(List<ValidationError> errors) {
            super(buildMessage(errors));
            this.errors = List.copyOf(errors);
        }

        private static String buildMessage(List<ValidationError> errors) {
            return errors.stream()
                .map(e -> e.field() + " [" + e.code() + "]: " + e.message())
                .collect(Collectors.joining("\n  - ", "Validation failed:\n  - ", ""));
        }

        public List<ValidationError> getErrors() { return errors; }
    }

    public static class RegisterValidator {
        public void validate(RegisterRequest req) {
            List<ValidationError> errors = new ArrayList<>();

            validateUsername(req.username(), errors);
            validateEmail(req.email(), errors);
            validatePassword(req.password(), errors);
            validateAge(req.age(), errors);

            if (!errors.isEmpty()) {
                throw new ValidationException(errors);
            }
        }

        private void validateUsername(String username, List<ValidationError> errors) {
            if (username == null || username.isBlank()) {
                errors.add(new ValidationError("username", "REQUIRED", "username is required"));
                return;
            }
            if (username.length() < 3 || username.length() > 20) {
                errors.add(new ValidationError("username", "LENGTH",
                    "username must be 3-20 chars, got " + username.length()));
            }
        }

        private void validateEmail(String email, List<ValidationError> errors) {
            if (email == null || !email.contains("@") || !email.contains(".")) {
                errors.add(new ValidationError("email", "FORMAT",
                    "email must contain '@' and '.'"));
            }
        }

        private void validatePassword(String password, List<ValidationError> errors) {
            if (password == null || password.length() < 8) {
                int len = password == null ? 0 : password.length();
                errors.add(new ValidationError("password", "LENGTH",
                    "password must be >= 8 chars, got " + len));
                return;
            }
            if (password.chars().noneMatch(Character::isDigit)) {
                errors.add(new ValidationError("password", "COMPLEXITY",
                    "password must contain at least 1 digit"));
            }
        }

        private void validateAge(int age, List<ValidationError> errors) {
            if (age < 18) {
                errors.add(new ValidationError("age", "MIN",
                    "age must be >= 18, got " + age));
            }
        }
    }

    public static class RegisterController {
        private final RegisterValidator validator = new RegisterValidator();

        public String register(RegisterRequest req) {
            try {
                validator.validate(req);
                return "OK";
            } catch (ValidationException e) {
                System.out.println(e.getMessage());
                String errorList = e.getErrors().stream()
                    .map(err -> String.format(
                        "{\"field\":\"%s\",\"code\":\"%s\",\"message\":\"%s\"}",
                        err.field(), err.code(), err.message()))
                    .collect(Collectors.joining(",", "[", "]"));
                return "{\"status\":\"error\",\"errors\":" + errorList + "}";
            }
        }
    }

    public static void main(String[] args) {
        RegisterController controller = new RegisterController();

        RegisterRequest bad = new RegisterRequest("ab", "bad", "short", 15);
        System.out.println("Validating: " + bad);
        System.out.println("Controller response: " + controller.register(bad));

        System.out.println();

        RegisterRequest good = new RegisterRequest("alice", "[email protected]", "pwd12345", 25);
        System.out.println("Validating: " + good);
        System.out.println("Controller response: " + controller.register(good));
    }
}