如何用有效的泛型声明Class.class [英] how to declare Class.class with valid generics

问题描述

我想知道是否有方法来声明 Class Class 具有有效类型参数的对象：

  Class cc1 = Class.class; //原始类型
 Class< Class> cc2 = Class.class; //现在参数是原始类型
 Class< Class<>> cc3 = Class.class; //编译错误：不可兑换类型
  

如果 Class 和 Class <？> 是可以互换的，为什么 Class< Class> 和 Class< ; Class<>> not？

编辑：这个问题可以概括为嵌套的原始类型参数。例如：

  ArrayList< ArrayList<>> lst = new ArrayList< ArrayList>（）; //相同的编译错误
  

EDIT2：我应该改写一下这个问题：我知道

  Class<？> c = Class.class; 
  

是有效的，但我想知道为什么 Class< Class> 与 Class >

不同

解决方案

泛型有一些非常严重的限制。在这种情况下，您不能将类型分配给 Class< Class> 的内部类型，因为您实际上引用了原始类型，而不是原始类型的实现。它会给你一个警告，但你无法解决这个警告。

Class< Class<>> 本身并不是不可转换的类型，因为它没有类型 Class< Class< T>>< / $ c，所以你不能直接指定类$ c>，它有类型 Class< T> 。

想想另一种方式;试试列表< List< String>> 。要创建它，你需要创建一个带有字符串列表的List。这是可行的，因为列表可以包含列表。

类更像是一个原始数据对象，所以我不认为有可能创建一个类这是类别的其他类型。

编辑：关于 ArrayList< ArrayList<> > 是您对 Class< Class<>> 的不可转换类型问题的一个更明显的例子。

Note: purely out of curiosity and not for any actual use case.

I'm wondering if there is a way to declare the Class Class object with valid type parameters:

Class cc1 = Class.class; //raw type
Class<Class> cc2 = Class.class; //now parameter is raw type
Class<Class<?>> cc3 = Class.class; //compile error: inconvertible types

If Class and Class<?> are interchangeable, why are Class<Class> and Class<Class<?>> not?

EDIT: the question can be generalized to an issue of nested raw type parameters. For example:

ArrayList<ArrayList<?>> lst = new ArrayList<ArrayList>(); //same compile error

EDIT2: I should rephrase the question a little: I know that

Class<?> c = Class.class;

is valid but I'm wondering why Class<Class> is not the same as Class<Class<?>>

解决方案

Generics have some pretty serious limitations. In this case you can't assign a type to the inner type of Class<Class> because you're actually referencing the raw type, not an implementation of the raw type. It will give you a warning, but you have no way to fix that warning.

Class<Class<?>> by itself isn't an inconvertible type, you just can't assign a class directly to it because it doesn't have the type Class<Class<T>>, it has the type Class<T>.

Think of it another way; try List<List<String>>. To create that, you need to create a List that takes a List of Strings. This works because lists can contain lists.

A Class is more like a primitive than a data object, so I don't think it'd be possible to create a Class that is of type Class of something else.

Edit: your extra question about ArrayList<ArrayList<?>> is a more obvious example of the inconvertible type issue you're having with Class<Class<?>>.

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