[英]Dart/Flutter How to initialize a List inside a Map?
How can I initialize a list inside a map?如何初始化地图内的列表?
Map<String,Map<int,List<String>>> myMapList = Map();
I get an error :我收到一个错误:
The method '[]' was called on null.
I/flutter (16433): Receiver: null
I/flutter (16433): Tried calling: [](9)
By just adding the elements when you create the map, or later by add
ing them.通过在创建地图时添加元素,或者稍后通过
add
它们。 For example:例如:
var myMapList = <String, Map<int, List<String>>>{
'someKey': <int, List<String>>{
0: <String>['foo', 'bar'],
},
};
or或者
var m2 = <String, Map<int, List<String>>>{}; // empty map
m2['otherKey'] = <int, List<String>>{}; // add an empty map at the top level
m2['otherKey'][2] = <String>[]; // and an empty list to that map
m2['otherKey'][2].add('baz'); // and a value to that list
print(m2);
prints {otherKey: {2: [baz]}}
打印
{otherKey: {2: [baz]}}
Try initializing using {}
尝试使用
{}
初始化
Map<String,Map<int,List<String>>> myMapList = {}; // just add {}
it will create an empty map and works perfectly fine.它将创建一个空地图并且工作得很好。
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