What is the difference between using `late` keyword before the variable type or using `?` mark after the variable type it in the Flutter?

user_11874694 0
Viewed 788

I think in new Dart rules the variables can not be declared/initialized as null. So we must put a late keyword before the variable type like below:

late String id;

Or a ? mark after the variable type like below:

String? id;

Are these two equal Or there are some differences?

flutter non-nullable null
3 Answers

A nullable variable does not need to be initialized before it can be used.

It is initialized as null by default:

void main() {
  String? word;
  
  print(word); // prints null
}

The keyword late can be used to mark variables that will be initialized later, i.e. not when they are declared but when they are accessed. This also means that we can have non-nullable instance fields that are initialized later:

class ExampleState extends State {
  late final String word; // non-nullable

  @override
  void initState() {
    super.initState();

    // print(word) here would throw a runtime error
    word = 'Hello';
  }
}

Accessing a word before it is initialized will throw a runtime error.

user_73879410

when you use late keyword you can't let variable uninitialized when you called it with ? allows to you let variable uninitialized when you create it and call it

user_129724300

Null safety rules doesn't mean you can't use null. You can, but you should indicate that a variable might have the value null, by add "?" to its type declaration.

By use keyword late you indicate that variable has a non-nullable type, is not initialized yet, but will be initialized later. Exception is thrown if try to acсess value of late variable before initialization.

user_143177990
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