Why name of object2 is being showned null? - dart

Question

class MyMain{
String name;
int mobile;
String gender;
String city;
MyMain({this.name , this.mobile , this.gender , this.city});
}
class MySmall extends MyMain{
int age;
String std;
MySmall({this.age , this.std , name , mobile , gender , city})
: super();
}
main(){
MyMain object1 = MyMain(name:"object1" , mobile: [phone] , gender: "M" , city: "xyz");
print(object1.name);
MySmall object2 = MySmall( name: "object2" ,age: 20 , std: "First" , mobile: [phone], gender: "M" , city: "XYZ");
print(object2.name);
}
I am trying to create child class and when I retrieve the name of child class it shows null , why so ? Any Solution ?

Answer 1

Your problem is that MySmall does not call the constructor of the class it extends from (MyMain) with the parameters it needs.
The reasons this happens here is because your named arguments is optional and defaults to null if not specified. So when you do super() you are just setting all parameters in the MyMain constructor to null.
I can guess you are running without Dart null-safety (introduced with Dart 2.12) since it would have been complained about the possible assignment of null values on non-nullable types.
You can fix your code by changing your MySmall constructor to:
MySmall(
{this.age, this.std, String name, int mobile, String gender, String city})
: super(name: name, mobile: mobile, gender: gender, city: city);
If we convert your code to be null-safe (and therefore compatible with Dart 2.12.0 and later) it would look like this. The required keyword was introduce to make it possible to have named arguments which is required to be set:
class MyMain {
String name;
int mobile;
String gender;
String city;
MyMain({
required this.name,
required this.mobile,
required this.gender,
required this.city,
});
}
class MySmall extends MyMain {
int age;
String std;
MySmall({
required this.age,
required this.std,
required String name,
required int mobile,
required String gender,
required String city,
}) : super(name: name, mobile: mobile, gender: gender, city: city);
}
void main() {
MyMain object1 = MyMain(
name: "object1",
mobile: [phone],
gender: "M",
city: "xyz",
);
print(object1.name); // object1
MySmall object2 = MySmall(
name: "object2",
age: 20,
std: "First",
mobile: [phone],
gender: "M",
city: "XYZ",
);
print(object2.name); // object2
}