无法将通用列表添加到通用地图 [英] can't add generic list to generic map
问题描述
我有一张通用地图
Map<String, List<A<?>>> map = new HashMap<>();
然后我有一个像这样的列表:
then I have a list like:
List<A<Integer>> list1 = getData();
List<A<String>> list2 = getData2();
我正在尝试将它们添加到地图中,但出现错误,指出参数错误.
I'm trying to add them to the map, but I'm getting an error saying the argument is wrong.
map.put("a", list1);
map.put("b", list2);
//doesn't work
使地图像
Map<String, List<A>> map = HashMap<>();
也不起作用.如何更改地图"以能够添加这两个列表?
doesn't work either. How can I change "map" to be able to add those 2 lists?
推荐答案
对泛型不进行递归评估.通配符(<?>
)在此处未求值.
Generics are not evaluated recursively. The wildcard (<?>
) does not get evaluated here.
如果您有 Map< String,List< A<?>>>
,那么您必须添加 List< A<?>>
值; List< A< Integer>>
不符合 List< A<?<?>
的资格. List< A<?>>
表示具有未知类型的A实例的列表,因此您必须传递具有未知类型的A实例的列表.
If you have a Map<String, List<A<?>>>
then you must add List<A<?>>
values to it; List<A<Integer>>
does not qualify as a List<A<?>>
. List<A<?>>
means a List of A instances with unknown types, so you must pass a List of A instances with unknown types.
您可以做的是,显式地创建一个这样的列表,并向其中添加键入的A列表的所有元素:
What you can do is create such a List, explicitly, and add all the elements of your typed-A List to it:
List<A<Integer>> list1 = getData();
List<A<String>> list2 = getData2();
List<A<?>> list1Unknown = new ArrayList<>();
list1.forEach(list1Unknown::add);
List<A<?>> list2Unknown = new ArrayList<>();
list2.forEach(list2Unknown::add);
map.put("a", list1Unknown);
map.put("b", list2Unknown);
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