The default 'List' constructor isn't available when null safety is enabled. Try using a list literal, 'List.filled' or 'List.generate'

Question

Why List() constructor is not accessible after Dart's null safety?

// Compile time error: 'List' is deprecated and shouldn't be used.
// The default 'List' constructor isn't available when null safety is enabled. 
// Try using a list literal, 'List.filled' or 'List.generate'.
List<int> foo = List(); 

However, you can still do:

List<int> foo = []; // No error

So, what's the difference between the two? Either both of them should show the error or none of them.

Answer 1

Short answer:

Instead of the pre-null-safety operations

var foo = List<int>();  // Now error
var bar = List<int>(n); // Now error
var baz = List<int>(0); // Now error

use the following:

var foo = <int>[];           // Always the recommended way.
var bar = List.filled(1, 0); // Not filled with `null`s.
var baz = List<int>.empty();

Long answer:

The List constructor had two uses:

• new List() to create an empty growable list, equivalent to [].
• new List(n) to create a fixed-length list of length n filled with null values

With null safety, the second use was unsound most of the time, and there was no good way to fix it. It's possible to force a type argument to be non-nullable, but List<T>(4) only works when T is nullable. There is no way to enforce that.

So, the List(n) mode needed to go (replaced by List.filled(n, value) which forces you to provide a fill-value). That left List(), which doesn't really carry its own weight. You can just use [] instead (and you should!), so it was decided to remove the constructor entirely - all uses of it was either unsafe or useless. (Also, it was a weird constructor already, because if we wanted to properly make it null safe, it would have an optional parameter with a non-nullable type and no default value.)

By removing it completely, it makes it possible to, potentially, introduce a new List constructor in the future, perhaps as a shorter alias for List.filled. One can hope.

Answer 2

Apart from what @lrn sir mentioned, you can also create a list using:

List<int> foo = List<int>.empty(growable: true); // []

Answer 3

This is because the the default List() element was deprecated how about you try using List.filled() element as shown below

void display() {
    var fixedList = new List<int>.filled(5, 0, growable: false);
    fixedList[0] = 0;
    fixedList[1] = 10;
    fixedList[2] = 20;
    fixedList[3] = 30;
    fixedList[4] = 40;
    print('Elements in the list are as follows: $fixedList');
  }
} 

While for the Growable Length List you can try doing as shown below:

void main() {
    var growableList = new List<int>.filled(0,0, growable:true);
    growableList = [0, 10, 20, 30, 40, 50, 60, 70, 80, 90];
    print('The elements in the growable list include: $growableList');
  }

Answer 4

_items = List<DropdownMenuItem<String>>(); can be written in null safty as

_items = List<DropdownMenuItem<String>>.from(<DropdownMenuItem<String>>[]);