为什么“t instanceof T”在T是类型参数且t是变量的情况下不允许？ [英] Why "t instanceof T" is not allowed where T is a type parameter and t is a variable?

问题描述

我同意，在运行时，没有类型信息保留。但考虑以下类的通用声明：

  class SomeClass< T> {
 T t; 
 SomeClass（Object o）{
 System.out.println（o instanceof T）; //非法
} 
} 
  

在运行时，不会存在T ！但是，如果我实例化这类Integer类，那么相应的对象将有一个Integer类型的字段t。

然后，为什么我不能检查一个类型的a T可以用Integer在运行时替换。实际上我会做类似o instanceof Integer。

在这种情况下，允许instanceof使用类型参数会导致麻烦，但是如果我实例化这类Integer类，那么
对应的对象将会有一个Integer类型的字段t

不，它不会。它将有一个Object类型的字段。

请考虑以下代码：

  SomeClass的<整数> c = new SomeClass< Integer>（）; 
 SomeClass untyped =（SomeClass）c; //是哪种类型？ 
 SomeClass< String> stringTyped =（SomeClass< String>）无类型; //现在它是STRING？ 
  

作品。给你一堆编译器警告，但工作。因为字段T实际上是Object类型，可以转换为任何东西。

Eclipse says that the instanceof operation is not allowed with Type Parameter due to generic type eraser.

I agree that at runtime, no type information stays. But consider the following generic declaration of class :

class SomeClass<T>{
    T t;
    SomeClass(Object o){
        System.out.println(o instanceof T);   // Illegal
    }   
}        

At runtime, no T would be present! But if I instantiate this class of type Integer, then the corresponding object will have a field t of type Integer.

Then, why can't I check the type of a variable with T which can be replaced by Integer at runtime. And I would be actually doing something like "o instanceof Integer".

Under which cases, allowing instanceof with a Type Parameter can cause trouble so that it is prohibited?

解决方案

But if I instantiate this class of type Integer, then the corresponding object will have a field t of type Integer

No, it won't. It will have a field of type Object. Just everytime you access it, it will be cast to an Integer.

Consider the following code:

SomeClass<Integer> c = new SomeClass<Integer>();
SomeClass untyped = (SomeClass)c; // Which type was it?
SomeClass<String> stringTyped = (SomeClass<String>)untyped; // Now it's STRING??

Works. Gives you a bunch of compiler warnings, but works. Because the field T is actually of type Object and can be cast to anything.

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