CODEWITHSUNDEEP
dartflutter
Naveen
Naveen

Reputation: 1523

Flutter: Default assignment of List parameter in a constructor

Is it possible to assign a constant value to an optional parameter of datatype List while defining a constructor. for example,

`class sample{
  final int x;
  final List<String> y;
  sample({this.x = 0; this.y = ["y","y","y","y"]});
 }`

the above code throws an error focused at y assignment saying Default values of an optional parameter must be constant

what is the reason for this error? how else can I assign a default value to the List?

Upvotes: 40

Views: 52986

Answers (2)

G&#252;nter Z&#246;chbauer
G&#252;nter Z&#246;chbauer

Reputation: 657168

Default values currently need to be const. This might change in the future.

If your default value can be const, adding const would be enough

class sample{
  final int x;
  final List<String> y;
  sample({this.x = 0, this.y = const ["y","y","y","y"]});
}

Dart usually just assumes const when const is required, but for default values this was omitted to not break existing code in case the constraint is actually removed.

If you want a default value that can't be const because it's calculated at runtime you can set it in the initializer list

class sample{
  final int x;
  final List<String> y;
  sample({this.x = 0; List<String> y}) : y = y ?? ["y","y","y","y"];
}

Upvotes: 81

ehusmann
ehusmann

Reputation: 37

Very old question, but as it can still be found with Google, people like me can still find it. Flutter now has a new constructor for the List class to do just this:

final List<String> y = List.filled(4, 'y');

This will create a List with four elements, all set to a value of 'y'.

Upvotes: 1

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