Instanceof operator, please help!

Question

I am reading the codes written by others. I am confused on instanceof operator here.
(A instanceof B) where A and B are interfaces. I found a definition in a book such that the
instanceof operator returns true or false based on whether the object is an instance of the named
class or any of that class's subclass. However, I didn't find the relationship between A and B is
established in the codes. In this case, I wonder if instanceof operator can still be used because I am thinking that it will always evaluate to be false.
Thank you for your help!
Cathy

Answer 1

The instanceof operator can always be used - a return value of false is still a valid return value!
However, when it comes to interfaces it is possible for a class to implement both interfaces even though there may be no direct relationship between those interfaces.
Have a look at this:
public interface MyInterface
public interface YourInterface
public class MyClass implements MyInterface, YourInterface
}There is no relationship between the two interfaces but I can still do this:
public void check(MyInterface myObj)
  if(obj instanceof YourInterface)
    YourInterface yourObj = (YourInterface)myObj;
    System.out.println("Woo hoo - an instance of MyInterface that's also an instance of YourInterface!");
}Essentially the instanceof operator is asking if the value passed can be cast to something else for the method to use it more specifically.
There could be classes other than MyClass that implement both interfaces, particularly if a system is designed to be flexible.
I hope this makes sense!