In FLUTTER / DART, why do we sometimes add a question mark to "String" when declaring a variable?

Question

In the demo app, we find an instance of "final String? title;" - > Why do we add this "?" after the type String ?

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, this.title}) : super(key: key);

  **final String? title;**

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

In the same way, why when using it, we add a "!" ?

return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: **Text(widget.title!),**
      ),
      body: Center(

Answer 1

variable_type ? name_of_variable; means that name_of_variable can be null.

variable_type name_of_variable1; means that name_of_variable1 cannot be null and you should initialize it immediately.

late variable_type name_of_variable2; means that name_of_variable2 cannot be null and you can initialize it later.

late variable_type name_of_variable3;
variable_type ? name_of_variable4;
name_of_variable4=some_data;
name_of_variable3=name_of_variable4!;// name_of_variable4! means that you are sure 100% it never will be null

Real example with int type:

int ? a=null; // OK
int  b=null; // b cannot be equal null
late int c =null; // c cannot be equal null
late int d;
d=5; //OK

late int d; // if you never initialize it, you ll got exception

int e=5;
int? f=4;
int ? z;
e=f!; // OK, You are sure that f never are null

e=z!;// You get exception because z is equal null