Java generics why doesn't this work?

Question

为什么不能使用以下泛型创建地图？

Map<Class<K extends Item>, K> classMap;

Answer 1

Because Map is already generified - your job when creating a reference is to fill in the type parameter. Unless this is inside a method parameterized with K , the compiler will have no idea what K should be replaced with (and if it were inside a parameterized method, you couldn't have K extends Item in the body - K either already extends Item , or it doesn't).

New type parameters can go in the signatures of classes and methods with the implicit promise that they will be filled in later. They cannot go inside declarations.

Answer 2

Here you are making an instance of Map , not defining it. K needs to be a specific class.