使用泛型类型执行转换时出现警告 [英] Warning when performing cast with generic types

问题描述

  ...我不明白为什么我会收到警告（未经检查的转换）。 
地图< ;? estends SomeType，SomeOtherType>地图; 
 ... 
映射< SomeType，SomeOtherType> castedMap =（地图< SomeType，SomeOtherType>）地图; 
 ... 
  

我的意思是将castedMap发布到外部代码的危险是什么？
这两种操作都可以在运行时完美运行：
$ b

• 使用SomeType类型的键获取castedMap中的元素
  
• 使用SomeType类型的键将元素放在castedMap中。

我会简单地用@SuppressWarnings（unchecked）取消警告。 。

解决方案

无聊的答案可能是：当有警告时，它不是类型安全的。就是这样。

为什么它不是类型安全的，可以在这个例子中看到：

  import java.util.HashMap; 
 import java.util.Map; 
 
类SomeType {} 
类SomeSubType extends SomeType {} $ b $ class SomeOtherType {} 
 
公共类CastWarning 
 {
 public static void main（String [] args）
 {
 Map&SomeSubType，SomeOtherType> originalMap = new HashMap< SomeSubType，SomeOtherType>（）; 
地图< ;?扩展SomeType，SomeOtherType> map = originalMap; 
 Map< SomeType，SomeOtherType> castedMap =（地图< SomeType，SomeOtherType>）地图; 
 
 //因铸造而有效：映射的
 //键不是SomeType，而是SomeSubType
 //的信息已被抛弃。 .. 
 SomeType someType = new SomeType（）; 
 SomeOtherType someOtherType = new SomeOtherType（）; 
 castedMap.put（someType，someOtherType）; 
 
 //对于自身有效，但由于未经检查的映射转换导致ClassCastException 
 // $ b $ SomeSubType someSubType = originalMap.keySet（）。iterator（）。next （）; 
} 
} 
  

I do not understand why I get a warning (unchecked cast) when I try to perform this:

...
Map<? estends SomeType, SomeOtherType> map;
...
Map<SomeType, SomeOtherType> castedMap = (Map<SomeType, SomeOtherType>) map;
...

I mean what is the danger of publishing castedMap to an outside code? Both opperations will work perfectly at runtime:

• getting elements from castedMap using key of type SomeType
• putting elements in castedMap using key of type SomeType.

I would simply suppress warnings using @SuppressWarnings("unchecked").

解决方案

As boring as the answer may be: When there is a warning, then it's not type-safe. That's it.

Why it is not type safe can be seen in this example:

import java.util.HashMap;
import java.util.Map;

class SomeType {}
class SomeSubType extends SomeType {}
class SomeOtherType {}

public class CastWarning
{
    public static void main(String[] args)
    {
        Map<SomeSubType, SomeOtherType> originalMap = new HashMap<SomeSubType, SomeOtherType>();
        Map<? extends SomeType, SomeOtherType> map = originalMap;
        Map<SomeType, SomeOtherType> castedMap = (Map<SomeType, SomeOtherType>) map;        

        // Valid because of the cast: The information that the
        // key of the map is not "SomeType" but "SomeSubType"
        // has been cast away...
        SomeType someType = new SomeType();
        SomeOtherType someOtherType = new SomeOtherType();
        castedMap.put(someType, someOtherType);

        // Valid for itself, but causes a ClassCastException
        // due to the unchecked cast of the map
        SomeSubType someSubType = originalMap.keySet().iterator().next();
    }
}

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