列表< List<？>>和List< List>是java中不兼容的类型 [英] List<List<?>> and List<List> are incompatible types in java

问题描述

我没有得到这个代码以任何方式编译：

  List< List& a = new ArrayList（）; 
 List< List<？>> b = new ArrayList（）; 
 
 a = b; //不兼容类型
 b = a; //不兼容的类型
  

看来java不会考虑 List 和列表<？> 在泛型时是相同的类型。

就是它？

上下文

有一个具有以下签名的库函数： public< T>设置< Class< ;?延伸T> getSubTypesOf（final Class< T> type）。这对于作为参数传递的简单类型工作正常，但在泛型的情况下，结果不是使用通配符参数化，导致javac抱怨原始类型。我想把结果传播到我的应用程序的其余部分 Set< Class< ;?扩展了GenericTypeHere<？>>> ，但是简单的强制类型无法正常工作。

EDIT：Solution

$感谢您的答案，以下是我如何使它工作到最后：

  @SuppressWarnings（{rawtypes，unchecked}）
 private static Set< Class< ;? extends GenericTypeHere<？>>>> factoryTypes（）{
 return（set）new Reflections（...）。getSubTypesOf（GenericTypeHere.class）; 
} 
  

解决方案

到微妙的语义差异。

 列表
  

$ b b

这是 List 的原始类型，等于 T 的类型为 Object 。所以它就像说：

  List< Object> 
  

现在，编译器知道一个事实，无论发生什么，这是类型 Object 。

  List myList = new ArrayList（）; 
 myList.add（new Object（））; 
  

它会正常工作！这是因为 Object 是相同的或者它是类型的一些派生。

 列表<？> 
  

这是字面上的未知列表（ Java文档）。我们甚至不知道这里的东西的子类是 Object 类型。事实上，？类型是一个未知类型。它与 Object 无关！这是为什么当你尝试做。

 列表<？> myList = new ArrayList<？>（）; 
 myList.add（new Object（））; 
  

你得到一个编译时错误！

I did not get this code to compile either way:

List<List> a = new ArrayList();
List<List<?>> b = new ArrayList();

a = b; // incompatible types
b = a; // incompatible types

It seems that java does not consider List and List<?> to be the same type when it comes to generics.

Why is that? And is there some nice way out?

Context

There is a library function with following signature: public <T> Set<Class<? extends T>> getSubTypesOf(final Class<T> type). This works fine for simple types passed as argument but in case of generics the result is not parametrized with wildcard causing javac to complain about raw type. I would like to propagate the result to the rest of my application as Set<Class<? extends GenericTypeHere<?>>> but simple cast does not work as I expect.

EDIT: Solution

Thanks for the answers, here is how I get it working in the end:

@SuppressWarnings({"rawtypes", "unchecked"})
private static Set<Class<? extends GenericTypeHere<?>>> factoryTypes() {
    return (Set) new Reflections("...").getSubTypesOf(GenericTypeHere.class);
}

解决方案

Okay, so this is due to a subtle semantic difference.

List

This is the raw type of List, which equates to T being of type Object. So it's the same as saying:

List<Object>

Now, the Compiler knows for a fact, that whatever happens, this is a subclass of type Object. And if you do..

List myList = new ArrayList();
myList.add(new Object());

It will work fine! This is because Object is the same or it is some derivation of the type.

List<?>

This is literally a list of unknown (Java Docs). We don't even know that the subclass of the things in here are of type Object. In fact, the ? type is an unknown type all on its own. It has nothing to do with Object! This is why when you try and do..

List<?> myList = new ArrayList<?>();
myList.add(new Object());

You get a compile time error!

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