为什么不能将包含泛型类型的泛型类型分配给泛型类型的泛型类型(Why can't a Generic type containing a Generic type be assigned to a Generic typed class of wildcard type)

 对不起，如果标题似乎令人困惑，但一些例子是为了。 
 
 假设我有一些具有泛型类型参数的Java类： 
 
public class GenericClass<T> {
}
 
 我可以创建一个变量类型来存储一个对象，通用参数设置为，比如一个String 。 Java也会让我把这个变量赋值给另一个变量，但通用参数设置为通配符<?>类型： 
 
GenericClass<String> stringy = ...
GenericClass<?> generic = stringy; // OK
 
 但是，在使用具有泛型参数的类时，如果将该参数的类型设置为泛型，则不能将该类的对象分配给相同类型/泛型类型，其中后者（内部/嵌套）参数是通配符类型<?> ： 
 
GenericClass<GenericClass<String>> stringy = ...
GenericClass<GenericClass<?>> generic = stringy; // Compile Error

// And just in case that is confusing, a more
// realistic example involving Collections:
List<GenericClass<String>> stringy = ...
List<GenericClass<?>> generic = stringy; // Compile Error
 
 具体的编译错误是： 
 
Type mismatch: cannot convert from List<GenericClass<String>> to List<GenericClass<?>>
 
 直觉上我会认为这个任务不应该成为问题。 那么为什么这个任务是一个问题？ 

Sorry if the title seems confusing, but some examples are in order.
 
Let's say I have some Java class with a generic type parameter:
 
public class GenericClass<T> {
}
 
I can create a variable typed to store an object, with the generic parameter set to, say a String. Java will also let me assign that variable to another variable but with the generic parameter set to the wildcard <?> type:
 
GenericClass<String> stringy = ...
GenericClass<?> generic = stringy; // OK
 
However, when working with a class with a generic parameter, if you set the type of that parameter to be generic, you can't then assign an object of that class to an identically typed/genericized type where the latter (inner/nested) parameter is of the wildcard type <?>:
 
GenericClass<GenericClass<String>> stringy = ...
GenericClass<GenericClass<?>> generic = stringy; // Compile Error

// And just in case that is confusing, a more
// realistic example involving Collections:
List<GenericClass<String>> stringy = ...
List<GenericClass<?>> generic = stringy; // Compile Error
 
The specific compile error is:
 
Type mismatch: cannot convert from List<GenericClass<String>> to List<GenericClass<?>>
 
Intuitively I would think that the assignment in question should not be a problem. So why is this assignment an problem?

原文：https://stackoverflow.com/questions/1476447