Map< String，Class&lt ;?声明的问题扩展Serializable>> [英] Issue with declaration of Map<String,Class<? extends Serializable>>

问题描述

Java为我提供了<扩展类> 一种过滤可用于
的java类的方法，例如在此例中构建新的HashMap：

<我可以这样做：

 地图< String ,?扩展Serializable> map1 = new HashMap< String，String>（）; 
  

这是正确的，因为String实现了Serializable，所以编译器让我这么做。

但是当我尝试这样做时：

  Map< String，GenericClass<扩展Serializable>> map2 = new HashMap< String，GenericClass< String>>（）; 
  

因为它是GenericClass：

  public class GenericClass< T> 
 {
。 
。 
。 
} 
  

编译器会抛出一个错误：

 类型不匹配：无法从HashMap转换< String，GenericClass< String>>映射< String，GenericClass<？扩展Serializable>> 
  

我想知道，发生了什么事情？

也许编译器无法检测到扩展类是泛型类型的一部分。 您需要使用以下内容：

 地图< String，？扩展GenericClass< ;?扩展Serializable>> map2 = 
 new HashMap< String，GenericClass< String>>（）; 
  

嵌套通配符与顶级通配符有很大不同 - 只有后者执行 通配符捕获 。因此， HashMap< String，GenericClass< String>> 被认为不可转换为 Map< String，GenericClass<扩展Serializable>> ，因为 GenericClass< ;? extends Serializable> 是一个具体的类型参数（因为泛型

有关嵌套通配符的更多信息，请参阅以下文章：多种通配符使Java编译器（和我！）非常困惑

Java provides me by <? extends class> a way of filtering the java classes that you can use to build in this case the new HashMap, for example:

I can do that:

Map<String,? extends Serializable> map1 = new HashMap<String,String>();

It is correct, because String implements Serializable, so the compiler let me do that.

But when i try to do it:

Map<String,GenericClass<? extends Serializable>> map2 = new HashMap<String, GenericClass<String>>();

Being the GenericClass as it:

public class GenericClass<T>
{
.
.
.
}

The compiler throw an error saying:

Type mismatch: cannot convert from HashMap<String,GenericClass<String>> to Map<String,GenericClass<? extends Serializable>>

I would like to know, what is happen?

Maybe the compiler cannot detect the extends class being part of a generic type.

解决方案

You would need to use the following:

Map<String, ? extends GenericClass<? extends Serializable>> map2 =
        new HashMap<String, GenericClass<String>>();

Nested wildcards are much different from top-level wildcards - only the latter perform wildcard capture. As a result, HashMap<String, GenericClass<String>> is considered inconvertible to Map<String, GenericClass<? extends Serializable>>, because GenericClass<? extends Serializable> is a concrete type argument (and because generics aren't covariant).

See this post for further information on nested wildcards: Multiple wildcards on a generic methods makes Java compiler (and me!) very confused

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