How to initialize List/Map as a default argument?

问题

I know how to initialize other variables such as int or String in constructor but I have no clue how to do it for List and Map.

class StackOverFlowQuestion{
StackOverFlowQuestion({this.test='', this.map=?, this.list=?});
String test;
Map map;
List list;
}

What shall I replace the question marks with?

Thanks.

回答1:

The answer depends on whether your default list and map are constant. Let's assume your list is, but map is not.

You'd write:

class StackOverFlowQuestion {
  StackOverFlowQuestion({
    this.test = '',
    Map map,
    this.list = const [1, 2, 3],
  }) {
    this.map = map ?? {};
  }

  String test;
  Map map;
  List list;
}

If the list optional parameter is omitted, then it will be initialized to [1, 2, 3]. If the map is omitted or null, it will be initialized with a mutable empty map.

来源：https://stackoverflow.com/questions/62418736/how-to-initialize-list-map-as-a-default-argument