示例如下：

@freezed
class ContactAccount with _$ContactAccount {
  const factory ContactAccount({
    required int id,
    String? accountNumber,
    String? bank,
    num? stateId,
  }) = _ContactAccount;

  factory ContactAccount.fromJson(Map<String, Object?> json) => 
      _$ContactAccountFromJson(json);
}

要了解更多关于freezed的信息，请查看以下链接：
https://pub.dev/packages/freezed