泛型投射问题 [英] Generics cast issue

问题描述

以下是我的问题：给出这些类

  class A {} 
 class B extends A {} 
  

此代码编译：

 列表< Class< ;?延伸A>> list = Arrays.asList（B.class，A.class）; 
  

这并不是：

  List< Class< ;?延伸A>> anotherList = Arrays.asList（B.class）; 
  

给出了什么？

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更新：这段代码在Java 8中编译。显然，由于'改进类型推断'。

在第一个例子中， Arrays.asList（）调用的推断类型是 List< Class< ;?扩展A>> ，这显然可分配给相同类型的变量。在第二个示例中，右侧的类型边是列表< Class< B>> 。 Class 可分配给 Class <？对< code>><>< / c $ c>< code>列表< ;类< ;?扩展A>> 。它可以分配给 List<扩展类<？这个原因与为什么 List< B> / code>不能分配给列表< A> 。如果是这样，它会使以下（非类型安全）代码成为可能：

  List< Class< B>> bb = new ArrayList< B>（）; 
列表< Class< ;?延伸A>> aa = bb; 
 aa.add（A.class）; 
  

Here's my issue: given these classes

class A {}
class B extends A {}

This code compiles:

    List<Class<? extends A>> list = Arrays.asList(B.class, A.class);

And this does not:

    List<Class<? extends A>> anotherList = Arrays.asList(B.class);

What gives?

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UPDATE: This code compiles in Java 8. Apparently, due to 'Improved Type Inference'.

解决方案

In the first example, the inferred type of the Arrays.asList() call is List<Class<? extends A>>, which is obviously assignable to a variable of the same type.

In the second example, the type of the right side is List<Class<B>>. While Class<B> is assignable to Class<? extends A>, List<Class<B>> is not assignable to List<Class<? extends A>>. It would be assignable to List<? extends Class<? extends A>>.

The reason for this is the same one as why a List<B> isn't assignable to List<A>. If it was, it would make the following (not-typesafe) code possible:

List<Class<B>> bb = new ArrayList<B>();
List<Class<? extends A>> aa = bb;
aa.add(A.class);

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