Build a form validation system with custom exception classes for different validation errors. Learn to create meaningful error messages and handle multiple validation scenarios.
Build a FormValidator class that validates multiple fields
Validate: email format, password strength, username length, age range, phone number format
Collect all validation errors before throwing
Display clear error messages for each failed validation
Use try/catch to handle validation in the main program
Expected Output
Terminal
╔══════════════════════════════════╗
║ FORM VALIDATOR SYSTEM ║
╚══════════════════════════════════╝
📝 Validating form data...
✅ Email: user@example.com - Valid
✅ Username: john_doe - Valid
❌ Password: weak - Too short (min 8 characters)
❌ Age: 15 - Must be at least 18
✅ Phone: (555) 123-4567 - Valid
📊 Validation Results: 3/5 fields passed
❌ Errors found:
• Password: Password must be at least 8 characters
• Age: Age must be 18 or older
// Step 1: Custom exception hierarchy
class ValidationException implements Exception {
final String field;
final String message;
ValidationException(this.field, this.message);
@override
String toString() => 'ValidationException[$field]: $message';
}
class EmptyFieldException extends ValidationException {
EmptyFieldException(String field) : super(field, '$field cannot be empty');
}
class InvalidFormatException extends ValidationException {
InvalidFormatException(String field, String expected)
: super(field, '$field has invalid format. Expected: $expected');
}
class LengthException extends ValidationException {
LengthException(String field, int min, int max)
: super(field, '$field must be between $min and $max characters');
}
class RangeException extends ValidationException {
RangeException(String field, num min, num max)
: super(field, '$field must be between $min and $max');
}
// Step 2: Validation result class
class ValidationResult {
final String field;
final bool isValid;
final String? errorMessage;
ValidationResult(this.field, this.isValid, {this.errorMessage});
}
// Step 3: FormValidator class
class FormValidator {
final List _results = [];
void validateEmail(String field, String? value) {
if (value == null || value.isEmpty) {
_results.add(ValidationResult(field, false,
errorMessage: '$field cannot be empty'));
return;
}
final emailRegex = RegExp(r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$');
if (!emailRegex.hasMatch(value)) {
_results.add(ValidationResult(field, false,
errorMessage: '$field must be a valid email address'));
return;
}
_results.add(ValidationResult(field, true));
}
void validatePassword(String field, String? value) {
if (value == null || value.isEmpty) {
_results.add(ValidationResult(field, false,
errorMessage: '$field cannot be empty'));
return;
}
List errors = [];
if (value.length < 8) errors.add('at least 8 characters');
if (!value.contains(RegExp(r'[A-Z]'))) errors.add('one uppercase letter');
if (!value.contains(RegExp(r'[a-z]'))) errors.add('one lowercase letter');
if (!value.contains(RegExp(r'[0-9]'))) errors.add('one number');
if (errors.isNotEmpty) {
_results.add(ValidationResult(field, false,
errorMessage: 'Password must contain ${errors.join(", ")}'));
return;
}
_results.add(ValidationResult(field, true));
}
void validateLength(String field, String? value, int min, int max) {
if (value == null || value.isEmpty) {
_results.add(ValidationResult(field, false,
errorMessage: '$field cannot be empty'));
return;
}
if (value.length < min || value.length > max) {
_results.add(ValidationResult(field, false,
errorMessage: '$field must be between $min and $max characters'));
return;
}
_results.add(ValidationResult(field, true));
}
void validateRange(String field, int? value, int min, int max) {
if (value == null) {
_results.add(ValidationResult(field, false,
errorMessage: '$field must be a valid number'));
return;
}
if (value < min || value > max) {
_results.add(ValidationResult(field, false,
errorMessage: '$field must be between $min and $max'));
return;
}
_results.add(ValidationResult(field, true));
}
void validatePhone(String field, String? value) {
if (value == null || value.isEmpty) {
_results.add(ValidationResult(field, false,
errorMessage: '$field cannot be empty'));
return;
}
final phoneRegex = RegExp(r'^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$');
if (!phoneRegex.hasMatch(value)) {
_results.add(ValidationResult(field, false,
errorMessage: '$field must be a valid phone number'));
return;
}
_results.add(ValidationResult(field, true));
}
bool get isValid => _results.every((r) => r.isValid);
List get results => List.unmodifiable(_results);
List get errors =>
_results.where((r) => !r.isValid).toList();
void displayResults() {
print('📊 Validation Results: ${_results.where((r) => r.isValid).length}/${_results.length} fields passed');
if (!isValid) {
print('❌ Errors found:');
for (var error in errors) {
print(' • ${error.errorMessage}');
}
} else {
print('✅ All validations passed!');
}
}
}
// Step 4: Main program
void main() {
print('╔══════════════════════════════════╗');
print('║ FORM VALIDATOR SYSTEM ║');
print('╚══════════════════════════════════╝\n');
final validator = FormValidator();
print('📝 Validating form data...\n');
// Test data
validator.validateEmail('Email', 'user@example.com');
validator.validateLength('Username', 'john_doe', 3, 20);
validator.validatePassword('Password', 'weak');
validator.validateRange('Age', 15, 18, 120);
validator.validatePhone('Phone', '(555) 123-4567');
// Display individual results
for (var result in validator.results) {
String status = result.isValid ? '✅' : '❌';
String message = result.isValid ? 'Valid' : result.errorMessage!;
print('$status ${result.field}: $message');
}
print('');
validator.displayResults();
}