Dart test

example.dart

class A {
  String method() => "value";
}

class B {
  A? field;
  
  B([this.field]);   // Using [] to denote optional parameter.
  
  Map<String, String?> toJson() {
    return <String, String?> {
      'field': field?.method(),  // If field is null, this returns null.
    };
  }
}

main() {
  
  var b1 = B();
  var b2 = B(A());
  
  print(b1.toJson());
  print(b2.toJson());
}