JSON to Dart Class Generator

What is JSON to Dart Class Generator?

Flutter and Dart use a manual serialization pattern: each model class must implement a fromJson(Map<String, dynamic> json) factory constructor and a toJson() method. Unlike some frameworks that use runtime reflection, Dart compiles ahead-of-time (AOT) and disallows mirrors in Flutter, so there is no built-in JSON-to-class magic. Every field mapping must be written explicitly. Dart 2.12 introduced sound null safety, which means every type is either non-nullable (String — cannot be null) or nullable (String? — can be null). Getting this wrong crashes your Flutter app: a non-nullable field receiving null from the API will throw a Null check operator used on a null value error at runtime. This generator handles null safety correctly based on the sample data, generates fromJson/toJson for the entire class hierarchy, and optionally adds copyWith (essential for Bloc/Cubit state management) and freezed boilerplate.

How to Use

1. Set the root class name to match the Flutter model concept (e.g. "UserProfile", "ProductItem") — it becomes the outermost Dart class
2. Enable "copyWith" to generate a copyWith method on each class — required for immutable state updates in Bloc, Cubit, or Riverpod state management patterns
3. Enable "toString" to generate a human-readable toString() override for debugging
4. Enable "freezed" to generate code compatible with the freezed package — adds @freezed annotations and @JsonSerializable; run flutter pub run build_runner build after pasting to generate the .g.dart and .freezed.dart files
5. Paste your JSON (from a real API response or mock data) and click "Generate Dart Code" — copy the output into a .dart file in your Flutter project

Tips & Best Practices

• The (json["key"] as num).toDouble() pattern is important for double fields — Dart JSON parsing sometimes gives int where you expect double (e.g. a price of 10 instead of 10.0). Without this cast, the app throws a type 'int' is not a subtype of type 'double' error
• For Flutter Bloc/Cubit, always generate copyWith — bloc pattern requires creating a new state instance with modified fields, and copyWith makes this one-liner clean: emit(state.copyWith(user: updatedUser))
• The freezed package generates additional features: value equality (== compares by value, not reference), pattern matching on sealed classes, and union types — worth adding for complex models with multiple variants
• For Riverpod + freezed, add the @riverpod annotation to your provider and run build_runner once to generate everything — the generated Dart class is the same base
• Dart's json_serializable package is an alternative to manual fromJson/toJson — it uses @JsonSerializable() and code generation, producing the same result as this manual output but with less maintenance for large models

Frequently Asked Questions

Real-world Examples

{
  "productId": "prod-5512",
  "name": "Mechanical Keyboard",
  "price": 89.99,
  "rating": 4.5,
  "reviewCount": 128,
  "inStock": true,
  "imageUrl": "https://cdn.example.com/keyboards/5512.jpg",
  "category": "Electronics",
  "tags": ["keyboard", "mechanical", "gaming"]
}

class Product {
  final String productId;
  final String name;
  final double price;
  final double rating;
  final int reviewCount;
  final bool inStock;
  final String imageUrl;
  final String category;
  final List<String> tags;

  const Product({
    required this.productId,
    required this.name,
    required this.price,
    required this.rating,
    required this.reviewCount,
    required this.inStock,
    required this.imageUrl,
    required this.category,
    required this.tags,
  });

  factory Product.fromJson(Map<String, dynamic> json) => Product(
    productId: json['productId'] as String,
    name: json['name'] as String,
    price: (json['price'] as num).toDouble(),
    rating: (json['rating'] as num).toDouble(),
    reviewCount: json['reviewCount'] as int,
    inStock: json['inStock'] as bool,
    imageUrl: json['imageUrl'] as String,
    category: json['category'] as String,
    tags: (json['tags'] as List).map((e) => e as String).toList(),
  );

  Map<String, dynamic> toJson() => {
    'productId': productId,
    'name': name,
    'price': price,
    'rating': rating,
    'reviewCount': reviewCount,
    'inStock': inStock,
    'imageUrl': imageUrl,
    'category': category,
    'tags': tags,
  };

  Product copyWith({
    String? productId, String? name, double? price, double? rating,
    int? reviewCount, bool? inStock, String? imageUrl,
    String? category, List<String>? tags,
  }) => Product(
    productId: productId ?? this.productId,
    name: name ?? this.name,
    price: price ?? this.price,
    rating: rating ?? this.rating,
    reviewCount: reviewCount ?? this.reviewCount,
    inStock: inStock ?? this.inStock,
    imageUrl: imageUrl ?? this.imageUrl,
    category: category ?? this.category,
    tags: tags ?? this.tags,
  );
}

{
  "rideId": "ride_8821",
  "status": "searching",
  "estimatedArrival": null,
  "driverLocation": null,
  "pickupAddress": "123 Main St, San Francisco, CA"
}

class DriverLocation {
  const DriverLocation();
  factory DriverLocation.fromJson(Map<String, dynamic> json) =>
      const DriverLocation();
  Map<String, dynamic> toJson() => {};
}

class RideRequest {
  final String rideId;
  final String status;
  final String? estimatedArrival;
  final DriverLocation? driverLocation;
  final String pickupAddress;

  const RideRequest({
    required this.rideId,
    required this.status,
    this.estimatedArrival,
    this.driverLocation,
    required this.pickupAddress,
  });

  factory RideRequest.fromJson(Map<String, dynamic> json) => RideRequest(
    rideId: json['rideId'] as String,
    status: json['status'] as String,
    estimatedArrival: json['estimatedArrival'] as String?,
    driverLocation: json['driverLocation'] == null
        ? null
        : DriverLocation.fromJson(json['driverLocation'] as Map<String, dynamic>),
    pickupAddress: json['pickupAddress'] as String,
  );
}

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