Example of converting from/to a custom Dart class to JSON

main.dart

// Conversion from/to JSON to String in Dart
// Also  simple DIY conversion to a custom class object
import 'dart:convert';

class dto{
  String name;
  int age;
  dto(this.name, this.age);
  
  factory dto.fromJson(Map<String, dynamic>? jmap) {
    if(jmap == null) throw FormatException('Input is NULL!');
    return dto(jmap['name'], jmap['age']);
  }
  @override
  String toString(){
    return('Your name is $name and you are $age years old.');
  }
  Map<String, dynamic> toJson(){
    Map<String, dynamic> retval = {};
    retval['name'] = name;
    retval['age'] = age;
    return (retval);
  }
}

void main() {
  String jstr = '{"name":"Randy","age":44}';   
  dynamic jmap = jsonDecode (jstr);   
  print(jmap['name']);
  print(jmap['age']);
  
  jmap['name'] = 'Appu';
  jmap['age'] = 33;
  print(jmap);
  String newstr = jsonEncode(jmap);
  print(newstr);
  
  dto mydto = dto.fromJson(jmap);
  print(mydto);
  Map<String, dynamic> jdto = mydto.toJson();
  print(jdto);
  
  Map<String, dynamic>? jm;
  try {
    dto mydto2 = dto.fromJson(jm);
  }
  on FormatException catch(e) {
    print ('conversion failed:');
    print (e.toString());
  }
}