[英]Issue with interface as Map key & value
In a library project, I have : 在一个图书馆项目中,我有:
public interface InterfaceA {
}
public interface InterfaceB {
}
public void myMethod(Map<? extends InterfaceA, List<? extends InterfaceB>> map) {
//do something
}
Then I have another project (having this library as a dependency) that contains two object implementing these interfaces : 然后,我有另一个项目(具有此库作为依赖项),其中包含两个实现这些接口的对象:
public class ObjectA implements InterfaceA {
}
public class ObjectB implements InterfaceB {
}
When I try to call the library method myMethod
like this : 当我尝试像这样调用库方法myMethod
:
HashMap<ObjectA, List<ObjectB>> hashMap = new HashMap<>();
//populate hashmap
myMethod(hashMap);
I get a compilation warning saying there is an argument mismatch. 我收到一个编译警告,提示参数不匹配。
What am I missing here ? 我在这里想念什么? Does it have something to do with the map ? 它和地图有关吗?
EDIT : The exact error (it's not a warning actually) is : 编辑:确切的错误(实际上不是警告)是:
incompatible types: HashMap<ObjectA,List<ObjectB>> cannot be converted to Map<? extends InterfaceA,List<? extends InterfaceB>>
Generics are invariant. 泛型是不变的。
If your method declares: 如果您的方法声明:
Map<? extends InterfaceA, List<? extends InterfaceB>>
Then the second type parameter has to be exactly List<? extends InterfaceB>
然后,第二个类型参数必须恰好是 List<? extends InterfaceB>
List<? extends InterfaceB>
. List<? extends InterfaceB>
。
You can fix it by using: 您可以使用以下方法修复它:
Map<? extends InterfaceA, ? extends List<? extends InterfaceB>>
Instead. 代替。
You either modify your Hashmap creation for this: 您可以为此修改Hashmap创建:
Map<? extends InterfaceA, List<? extends InterfaceB>> hashMap = new HashMap<>();
or modify your method definition for this: 或为此修改方法定义:
public <A extends InterfaceA, B extends InterfaceB> void myMethod(Map<A, List<B>> map) {
//do something
}
将您的地图声明为
HashMap<ObjectA, List<? extends InterfaceB>> hashMap = new HashMap<ObjectA, List<? extends InterfaceB>>();
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