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.
To create an empty list, use [] for a growable list or List. empty for a fixed length list (or where growability is determined at run-time). The created list is fixed-length if length is provided. The list has length 0 and is growable if length is omitted.
Null safety means that a variable cannot have a null or void value. This feature improves user satisfaction by reducing errors and app crashes. Null safety ensures that all runtime null-dereference problems are shown at compile-time.
The @required annotation marks named arguments that must be passed; if not, the analyzer reports a hint. With null safety, a named argument with a non-nullable type must either have a default or be marked with the new required keyword.
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();
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
valuesWith 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.
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'); }
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