对于接口和类，'instanceof'操作符的行为不同

问题描述

我想知道在Java中 instanceof 运算符的以下行为。

I would like to know regarding following behavior of instanceof operator in Java.

interface C {}

class B {}

public class A {
    public static void main(String args[]) {
        B obj = new B();
        System.out.println(obj instanceof A);      //Gives compiler error
        System.out.println(obj instanceof C);      //Gives false as output
    }
}

为什么会这样？在接口C 和类B 之间没有关系，但它给出了false，而在 obj instanceof A 它会给编译器错误？

Why is it so? There is no relation between interface C and class B, but it gives false whereas in case of obj instanceof A it gives compiler error?

推荐答案

在编译期间已知的 obj 类型 B 的对象不能是 A 。另一方面，它可能是接口 C 的子类型，例如在这种情况下：

Because Java has no multiple class inheritance it's absolutely known during the compilation that obj object of type B cannot be subtype of A. On the other hand it possibly can be subtype of interface C, for example in this case:

interface C {}

class B {}

class D extends B implements C {}

public class A {
    public static void main(String args[]) {
        B obj = new D();
        System.out.println(obj instanceof C);      //compiles and gives true as output
    }
}

在 obj instanceof C 表达式编译器不能提前告诉它是真还是假，而是看 obj instanceof A 它知道这总是false，因此无意义，并帮助您防止错误。如果您还想在程序中进行无意义的检查，可以向对象添加显式投射：

So looking only at obj instanceof C expression compiler cannot tell in advance whether it will be true or false, but looking at obj instanceof A it knows that this is always false, thus meaningless and helps you to prevent an error. If you still want to have this meaningless check in your program, you can add an explicit casting to the Object:

System.out.println(((Object)obj) instanceof A);      //compiles fine

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