对于接口和类，“ instanceof”操作符的行为不同

我想知道关于以下 instanceof操作符在 Java 中的行为。

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？interface C和 class B之间没有关系，但是它给出 false，而在 obj instanceof A的情况下，它给出编译器错误？

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最佳答案

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
}
}

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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By using the final modifier in the class declaration below, it is guaranteed that there couldn't be a subclass of Test, which may implement the interface Foobar. In this case, it is obvious that Test and Foobar are not compatible with each other:

public final class Test {


public static void main(String[] args) {
Test test = new Test();
System.out.println(test instanceof Foobar); // Compiler error: incompatible types
}
}


interface Foobar {
}

Otherwise, if Test is not declared final, it might be possible that a subclass of Test implements the interface. And that's why the compiler would allow the statement test instanceof Foobar in this case.


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