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 [英] Why can't a Generic type containing a Generic type be assigned to a Generic typed class of wildcard type

问题描述

比方说，我有一些具有泛型类型参数的Java类：

  public class GenericClass< T> {
} 
  

我可以创建一个类型变量来存储一个对象，设置为，比如 String 。 Java也允许我将该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; //确定
  

但是，在使用泛型参数的类时，如果将类型该参数是通用的，则不能将该类的对象分配给相同类型/通用化类型，其中后者（内部/嵌套）参数是通配符类型<？> ：

  GenericClass< GenericClass< String>> stringy = ... 
 GenericClass< GenericClass<>> generic = stringy; //编译错误
 
 //为了防止混淆，还需要一个
 //逼真的例子：
 List< GenericClass< String>> stringy = ... 
 List< GenericClass<>> generic = stringy; //编译错误
  

具体的编译错误是：

 类型不匹配：无法从List< GenericClass< String>>转换列出< GenericClass<>> 
  

直觉上来说，我认为有问题的分配不应该成为问题。那么，为什么这个任务是一个问题呢？

解决方案

您的面临的问题被命名为 Covariance 。

  List< GenericClass<字符串>> stringy = ... 
 List< GenericClass<>> generic = stringy; 
 generic.add（new GenericClass< Integer>（））; 
  

如果这不是编译错误，那么代码的最后一行是可能的。 p>

您可以这样做来避免错误：

  List< ;?扩展了GenericClass<>>> generic = stringy; 
  

但您也不能使用 add ，因为你真的不知道什么？扩展GenericClass<？> is（Covariance再一次）。在这种情况下，您只能枚举列表并期望 GenericClass<> 。

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?

解决方案

The problem your facing is denominated Covariance.

List<GenericClass<String>> stringy = ...
List<GenericClass<?>> generic = stringy;
generic.add(new GenericClass<Integer>());

If this wasn't a compile error, then the last line of code would be possible.

You could get around the error by doing this:

 List<? extends GenericClass<?>> generic = stringy;

but you can not use add also because you don't really know what ? extends GenericClass<?> is (Covariance once again). In this case you can only enumerate through the List and expect GenericClass<?>.

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