Null Safety Codes and examples

creating_vars.dart

void main (){
  // In null-safe Dart, none of these can ever be null.
  var i =  42; // Inferred to be int.
  String name = getFileName(); // getFileName() should return string, never null.
  final b = Foo(); // Inferred to be Foo.
  
  // Nullable var declaration just needs an [?] after the type.
  int? aNullableInt = null;
  var x; // Inferred nullable
  
  // We can define a non-nullable var without his value but it
  //  need to be initialized before using
  int y; // non-nullable int.
  print(y); // Analyzer Error.
  y = 5;
  print(y); // OK
}

map_considerations.dart

void main (){
  var myMap = <String, int> {'one': 1};
  
  // Because 'two' doesn't exist, the map will return null
  var valueOfTwo = myMap['two']; // Inferred int?
  
  // Because the lookup could return null we can't assign it to a non-nullable
  int noNullableInt = myMap['one']; // Analyzer Error
  int? nullableInt = myMap['one']; // OK
  
  // we could use [!] if we are shure a key exist
  int prettySureNoNull = myMap['one']!; // OK
  
  // But it's allways preferible to use [if] ternary [? :] or null check [??]
  int valueOfOne = myMap['one'] ?? 0;
  
}

using_late.dart

void main (){
  late int x; // non-nullable int.
  print(x); // Analizer Error.
  
  final myMeal = Meal();
  print(myMeal.description); // LateInitializationError
  myMeal.setDescription('Feijoada!'); // Initialize [description]
  print(myMeal.description); // OK
}

class Meal {
  late String description; // Class member that will be defined later.
  
  void setDescription(String str) {
    description = str;
  }
}