`toString` method adding null to object in Dart - dart

Question

I am working on a Flutter web app, and am using a custom class to keep track of form data. I am instantiating a new instance of the following class:
class Contact {
String name;
String relationship;
String phoneNo;
#override
String toString() {
print("""{
Name: $name,
Relation: $relationship,
Phone: $phoneNo
}""");
}
}
In my controller, once I instantiate, I am printing out the value immediately:
// Method in controller, triggered by onTap
Contact contact = Contact();
print(contact);
The output is:
{
Name: null,
Relation: null,
Phone: null
}
null
Which is causing issues later down the line, as instances of this class are being used as values of a HashMap. I have narrowed down the issue to being caused by the toString method, and when I remove it, Instance of 'Contact' is then printed out, as desired. What is the best way to handle this?

Answer 1

// Method in controller, triggered by onTap
Contact contact = Contact();
print(contact);
You created a new instance of Contact with no arguments that's why all the value of Contact will be null.
To assign a value there are two solutions:
1. Inside from class itself:
class Contact {
String name = 'John doe';
String relationship = 'Brother';
String phoneNo = '1234567890';
#override
String toString() {
print("""{
Name: $name,
Relation: $relationship,
Phone: $phoneNo
}""");
}
}
2.From outside of Class
for that, you have to initiate Constructor in your class
class Contact {
String name;
String relationship;
String phoneNo;
Contact(this.name, this.relationship, this.phoneNo);
//You can also choose between named parameters and positional parameters
// For named parameters Contact({this.name, and so on....})
#override
String toString() => """{
Name: $name,
Relation: $relationship,
Phone: $phoneNo
}""";
}
in this scenario you have to pass values where you create instance of class as shown here:
Contact contact = Contact('John Doe', 'brother', '123456789');
print(contact);