java中泛型层次结构中的运行时类型比较

Question

我无法从完整的参考资料中理解本文 代码是这样的

public class Generic
{
    public static void main(String[] args)
    {
        SuperClass<Integer> s1=new SuperClass<>(135);
        SubClass<Double> s2=new SubClass<>(1.35);
        if(s1 instanceof SuperClass<Integer>)
        {
            System.out.println("I am instance of SuperClass");
        }
    }
}

class SuperClass<T>
{
    T y;
    SuperClass(T ob)
    {
        y=ob;
    }
    T ret()
    {
        return(y);
    }
}

class SubClass<T> extends SuperClass<T>
{
    T x;
    SubClass(T y)
    {
        super(y);
        x=y;
    }
}

根据文本

if(s1 instanceof SuperClass<Integer>) this line can’t be compiled because it attempts to compare s1 with a specific type of SuperClass,in this case, SuperClass<Integer>. Remember, there is no generic type information available at run time. Therefore, there is no way for instanceof to know if s1 is an instance of SuperClass<Integer> or not.

有人能给我解释一下这些台词到底是什么意思吗

Answer 1

在引擎盖下面，当使用泛型时，会发生一种称为类型擦除的情况。这意味着在将程序编译成字节码的过程中，泛型类型将转换为原始（非泛型）形式。所有类型参数都将成为对象，所有指定给类型变量的值都将转换为对象。这就是“运行时没有可用的泛型类型信息”的意思

一行，如

if(s1 instanceof SuperClass<Integer>)

变成

if(s1 instanceof SuperClass)

编译后。在参数化类型上使用instanceof的问题是，在运行时，Java VM无法区分SuperClass<Integer>和SuperClass<Double>等类型。如果编译器允许instanceof与参数化类型一起使用，那么前一行也会为SuperClass<Double>返回true，这可能会产生未定义的行为

如果你想知道更多，甲骨文的网站有一个更全面的解释：https://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#cannotCast

Answer 2

当你的。java文件被编译成。类文件，也称为字节码，SuperClass<Integer>这段代码被转换为SuperClass。这称为类型擦除。所以在运行时没有关于泛型类型的信息

Answer 3

泛型信息是编译时特性。它在运行时被擦除。错误在this section of the Java Language Specification中解释：

It is a compile-time error if the ReferenceType mentioned after the instanceof operator does not denote a reference type that is reifiable (§4.7).

一个可重新定义的类型是defined，如下所示：

Because some type information is erased during compilation, not all types are available at run time. Types that are completely available at run time are known as reifiable types.

A type is reifiable if and only if one of the following holds:

• It refers to a non-generic class or interface type declaration.

• It is a parameterized type in which all type arguments are unbounded wildcards (§4.5.1).

• It is a raw type (§4.8).

• It is a primitive type (§4.2).

• It is an array type (§10.1) whose element type is reifiable.

• It is a nested type where, for each type T separated by a ".", T itself is reifiable.

因此，不允许像SuperClass<Integer>这样的参数化类型作为instanceof的引用类型。另一方面，允许SuperClass<?>