Instanceof不会显示带有接口的编译错误，但是带有抽象类会显示

Question

I think the title is self explanatory. So suppose I have this code:

interface A { }

abstract class B { }

class C { }

C c = new C();

System.out.println(c instanceof A); //fine
System.out.println(c instanceof B); // compile error

In a comment from the question I read this:

The compiler can never know whether a given type doesn't implement an interface because a potential subclass could implement it.

So if for interface this works, why it should not work for an abstract class ? It also should be extended by some other class, as it can't exist by it's own. Can someone clarify this?

Update Compile message:

Error:(22, 28) java: incompatible types: C cannot be converted to B

Answer 1

It is simple: C extends Object. No subclass of C could possible extend B. You can't add another base class, because Java doesn't support multiple inheritance.

Whereas a subclass of C can very well implement that additional interface. But there is simply no way how a C object could also be a B instance.

So:

D extends C implements B // obviously all fine

whereas

D extends B extends C 

is impossible. Because B is already defined to not extend anything but Object. Of course, the "trick" here is that both classes B, C are both known, and as said: C isn't extending B.

Answer 2

Take for example

class D extends C implements A{}

C c = new D();

The compiler can immediately tell that c can never refer to an instance of B because if a class extends C it cannot extend B also. As the above example shows, the same cannot be said of interfaces.

Answer 3

That is because it's strictly impossible to create instances of an abstract class in java. The operator instanceof is called from class Object and cannot be called if there is no instance of a class that revoked it (object).