java - java 通用层次结构中的运行时类型比较

Question

我无法理解完整引用中的这段文字代码是这样的，

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;
    }
}

根据正文，

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.

有人可以解释一下这些行到底是什么意思吗？

Answer 1

当您的 .java 文件被编译为 .class 文件(也称为字节码)时，SuperClass<Integer>这段代码被转换为 SuperClass 。这称为类型删除。所以在运行时没有关于泛型类型的信息。