在 Java 中使用 instanceof 时，实现 class 的接口返回值为真

Question

I'm self-taught Java and I'm a bit confused about using "instanceof" in Java. Basically I know to use instanceof, however I'm having a hard time with this case. I write an interface, then use a class to implement that interface. Next I want to check if the interface has instanceof with the implementing class. I tried but the return value is only "false" but I want it to return "true" how should I write it? Can I achieve that goal?Is there any sample code or suggestions for me? I would be very grateful and I would appreciate it. Sorry I'm a newbie, thanks.

interface Demo {
}

class A implement Demo {
}   

public class Main {
  public static void main(String[] args){
   Demo demo = new Demo() {};
   boolean result = demo instanceof A ;  // I want result is "true";
   System.out.println(result);
  }
  }    

Answer 1

You can not create object of interface because interface is basically a complete abstract class. That means Interface only have deceleration of method not their implementation. So if we don't have any implementation of a method then that means if we create object of that interface and call that method it compile nothing as there is no code to compile

interface Demo {
}

class A implements Demo {
}

public class Main
{
    public static void main(String[] args){
        Demo demo = new A();
        boolean result = demo instanceof A ;
        System.out.println(result);
    }
}

Answer 2

A implement Demo and so: object with type A instanceof Demo will be true , but not vice versa.

Ideally, Demo knows nothing about A .

And you shouldn't create an instance of interface.

Answer 3

The code you gave runs perfectly fine. result is false, because demo is not an instance of A . In fact, it is the other way around. Because A implements Demo all instances of A would return true when they are checked to be a Demo .

You can define demo to be an instance of A and then check if it is an instance of Demo :

A demo = new A();

boolean result = demo instanceof Demo; // now it is true
System.out.println(result);