强制转换"Map< String,List< dynamic>>"到"Map< String,List< String>>" [英] Cast "Map<String, List<dynamic>>" to "Map<String, List<String>>"
问题描述
我想将 Map< String,dynamic>
转换为 Map< String,List< String>>
.
我已经在Flutter中尝试了以下方法:
I have tried the following in Flutter:
Map<String, dynamic> dynamicMap = {
'a': ['1', '2', '3'],
'b': ['1', '2', '3'],
'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap = dynamicMap;
但是出现以下错误:
type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, List<String>>'
然后我尝试:
Map<String, List<String>> typedMap = dynamicMap.cast<String, List<String>>();
这似乎可以正常工作,但是尝试使用它时会遇到相同的错误,例如 print(typedMap)
.
which seems to work initally, but you get the same error when trying to use it, such as print(typedMap)
.
感谢您的帮助.
值得一提的是以下内容:
It's worth mentioning that the following:
Map<String, dynamic> dynamicMap = {
'a': ['1', '2', '3'],
'b': ['1', '2', '3'],
'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap = dynamicMap.cast<String, List<String>>();
实际上没有做过(对问题中的错误表示歉意.)
did in fact work (apologies for the mistake in the question).
但是,当数据来自json时,就像这样:
However, when the data comes say from json like so:
String rawJson = """{
"a": ["1", "2", "3"],
"b": ["1", "2", "3"],
"c": ["1", "2", "3"]
}""";
final Map<String, dynamic> dynamicMapFromJson = json.decode(rawJson);
Map<String, List<String>> typedMap = dynamicMapFromJson.cast<String, List<String>>();
该代码不再起作用,并引发了提到的异常.
The code no longer works and raised the exception mentioned.
下面的答案帮助解决了这个问题.
The answers below helped solve this problem.
推荐答案
Map<String, dynamic> dynamicMap = {
'a': ['1', '2', '3'],
'b': ['1', '2', '3'],
'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap;
dynamicMap.forEach((key,value){
List<String> list = List.castFrom(value);
typedMap[key] = list
});
这是因为可以在运行时为动态数据类型变量分配任何数据类型.但是这里的typedMap值已经用 List< String>
的数据类型声明,除了 List< String>
之外,其他任何其他都不能赋给它.该错误是因为您试图将动态数据类型分配给 List< string>
数据类型(一个伪示例)
This is because a dynamic data type variable can be assigned with any of the data type at run time.
But values of typedMap here has been already declared with data type of List<String>
, nothing else can be assigned to it other than a List<String>
.
The error is because you're trying to asign a dynamic data type to a List<string>
data type, a pseudo example
String s = dynamic(); //impossible
dynamic s = String(); //possible
这篇关于强制转换"Map< String,List< dynamic>>"到"Map< String,List< String>>"的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!