java - instanceof 与自定义/核心类

Question

为什么我可以使用 instanceof 针对我的自定义接口(interface)测试我的自定义类，而我无法对 java 核心类执行相同的操作？

interface A{}
class B{}
public class Tmp {
    public static void main(String [] args) {
        String s = "";
        Integer i = 1;
        B b = new B();
        System.out.println(s instanceof A); //1
        System.out.println(i instanceof A); //2
        System.out.println(b instanceof A); //3
    }
}

第 1 行和第 2 行无法编译。第 3 行将编译(并打印 false)。有什么区别？

Answer 1

如果编译器可以检测到 instanceof 操作永远不会为真，它将生成一个编译器错误。它通过遵循与类型转换相同的规则来确定这一点。具体规则详见Section 15.20.2 JLS 的:

If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.